i

Integrated management of Plutella xylostella (Lepidoptera: Plutellidae) in

vegetable growth tunnels

G Oberholzer

orcid.org 0000-0002-2257-879X

Dissertation accepted in fulfilment of the requirements for the degree Master of Science in Environmental Sciences with Integrated Pest Management at the North-West University

Supervisor: Prof MJ Du Plessis

Co-supervisor: Prof J Van den Berg

Graduation October 2019

24164763

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ACKNOWLEDGEMENTS

Dit is met ʼn hart vol dankbaarheid dat ek die volgende mense wil bedank vir al die

ondersteuning liefde en geduld deur my studies:

Aan Prof. Hannalene du Plessis en Prof. Johnnie Van den Berg baie dankie vir die

geleentheid en al die ondersteuning en raad deur die afgelope paar jaar dit word opreg

waardeer. Dankie dat julle ʼn liefde het vir wat julle doen en dat julle my daardie liefde

geleer het.

Aan my ouers, Nico en Renata Oberholzer, baie dankie vir die geleentheid wat julle vir

my gegee het ek sal julle ewig dankbaar wees. Julle is die definisie van perfekte ouers

en ek hoop ek kan eendag vir my kinders doen wat julle vir my gedoen het.

Aan my liewe sussie en swarrie baie dankie vir die ondersteuning, raad en laat aande

se werk. Sussie jy is iets besonders en ek is verby geseënd om jou in my lewe te kan

hê.

Aan Melana my engel dankie dat jy altyd daar was wanneer die werk te veel was en

wanneer ek net ʼn oor nodig gehad het. Jy is onbeskryflik, baie dankie vir die omgee liefde

baie koffie en moed in praat my lief dit is alles wat ek kon voor vra.

My vriende: Vir al die Caput manne wat altyd daar was vir my deur die koshuis dae dankie

vir die mense wat julle is ek waardeer elke liewe een se bydrae, veral Jako van Rooyen

my goeie vriend jy was altyd daar vir moed in praat en laat aande se werk dankie my

maat.

My vriende en kolegas by ECO–Rehab baie dankie vir julle ondersteuning en bydraes

deur my studies dit is onvervangbaar en baie spesiaal. Charlene Scott baie dankie vir jou

bydrae as vriendin en swot buddy vir meer as 6 jaar jy is n vriendin duisend.

Aan Pico–Gro en Erika Oberholzer baie dankie vir al die bystand van die nodige plante,

sade, populasies en nog vele meer dit word opreg waardeer.

Laastens my Hemelse Vader wat my die vermoë gegee het en al die mense in my lewe

geplaas het om my droom van ʼn meesters graad ʼn werklikheid te maak sonder U is niks

moontlik nie. Joh. 14:1 “Laat julle hart nie ontsteld word nie; glo in God, glo ook in My”.

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ABSTRACT

Plutella xylostella (L.) (Lepidoptera: Plutellidae) is a major pest on all Brassicaceae in

South Africa. Insecticide application is the main control strategy worldwide and concerns

are raised about the susceptibility of P. xylostella populations to insecticides in South

Africa. This concern sparked the investigation into other feasible control strategies and

measures mainly focused cultural control with trap crops. The objectives of this study

were to assess the following: feeding damage and percentage growth of P. xylostella on

selected host plants in no choice tests, the feeding preference of P. xylostella larvae on

selected host plants in two choice tests, the development and oviposition of P. xylostella

on selected host plants for possible use in a Push–Pull strategy and the susceptibility of

P. xylostella to selected insecticides. Five known P. xylostella host plant species were

selected, viz. pak–choi (Brassica rapa var. chinensis), cabbage (Brassica oleracea),

tatsoi (Brassica narinosa), kale (Brassica oleracea) and mustard (Brassica juncea). In the

two–choice tests conducted mustard was preferred for oviposition as well as feeding

preference by P. xylostella larvae. The fact that mustard was the preferred choice over

the other selected crops for oviposition and feeding preference makes it a viable

candidate for a trap crop for P. xylostella. Plutella xylostella populations were sampled

from two localities, Nelspruit (Mpumalanga province) and Bapsfontein (Gauteng

province). The toxicity of three insecticides used for control of P. xylostella in South Africa,

viz. lambda–cyhalothrin, flubendiamide and spinosad was estimated for both these

populations using an IRAC approved bioassay method. When comparing the

susceptibility of the two populations (Nelspruit (N2018–01) and Bapsfontein (B2018–01))

for lambda–cyhalothrin the LC80 values are higher (N2018–01: 9.35 and B2018–01 24.87)

than the recommended dosage of 8 ppm. Conversely, the selected populations (Nelspruit

and Bapsfontein) are highly susceptible to flubendiamide and spinosad with high mortality

levels > 80% with low dosages of 3 and 2 ppm respectively compared to the

recommended dosages of 48 and 216 ppm. The integration of trap crops and application

of insecticides with high efficacy of control of P. xylostella can contribute to reduce the

number of insecticide applications per season. It can also prolong the effective use of

these insecticides for control of P. xylostella in South Africa.

Keywords: Plutella xylostella, Brassicaceae, susceptibility, lambda–cyhalothrin, mustard

(Brassica juncea), bioassay, oviposition, feeding preference

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iv

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ................................................................................................ i

ABSTRACT ......................................................................................................................ii

Chapter 1: Introduction and literature review ................................................................... 1

1.1 Introduction ............................................................................................................ 1

1.2 Biology of Plutella xylostella .................................................................................. 2

1.3 Damage symptoms................................................................................................ 3

1.4 Distribution of Plutella xylostella ............................................................................ 3

1.5 Host preference ..................................................................................................... 5

1.6 Economic impact ................................................................................................... 6

1.7 Control of Plutella xylostella .................................................................................. 7

1.7.1 Cultural control .................................................................................................... 7

1.7.2 Biological control ................................................................................................. 9

1.7.3 Chemical control ................................................................................................ 10

1.8 Insecticide resistance .......................................................................................... 12

1.9 Resistance mechanisms in Plutella xylostella ..................................................... 18

1.10 Integrated Pest Management (IPM) ................................................................. 19

1.11 Aims and objectives ......................................................................................... 20

1.11.1 Aims ................................................................................................................ 20

1.11.2 Objectives........................................................................................................ 20

1.12 References ....................................................................................................... 21

Chapter 2: Cultural control of Plutella xylostella ............................................................ 42

2.1 Abstract ............................................................................................................... 42

2.2 Introduction ............................................................................................................. 43

2.2.1 Trap cropping ................................................................................................ 43

2.2.2 Pico–Gro test site .......................................................................................... 45

2.3 Material and methods .......................................................................................... 49

2.3.1 Plutella xylostella stock colony .......................................................................... 49

2.3.2 Larval feeding preference for selected Brassicas .............................................. 49

2.3.3 Larval feeding choice tests ................................................................................ 50

2.3.3.1 No–choice tests ........................................................................................... 50

2.3.3.2 Two–choice tests ......................................................................................... 51

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2.3.4 Oviposition preference tests .............................................................................. 52

2.3.5 Development time ............................................................................................. 52

2.3.6 Data Analysis .................................................................................................... 53

2.4 Results ................................................................................................................ 53

Choice tests .................................................................................................................. 53

2.4.1. No–choice tests ................................................................................................ 53

2.4.2 Two–choice tests ............................................................................................... 56

2.4.2.2 Mean percentage damage........................................................................... 58

2.4.2.3 Oviposition preference ................................................................................ 59

2.5 Discussion ........................................................................................................... 60

2.6 References .......................................................................................................... 63

Chapter 3: Susceptibility of Plutella xylostella populations to selected insecticides ...... 69

3.1 Abstract ............................................................................................................... 69

3.2 Introduction ............................................................................................................. 70

3.2.1 Insecticide resistance ........................................................................................ 71

3.2.2 Research and case studies ............................................................................... 72

3.3 Material and methods .......................................................................................... 75

3.3.1 Plutella xylostella sampling................................................................................ 75

3.3.2 Experimental design .......................................................................................... 75

3.3.3 Data Analysis .................................................................................................... 77

3.4 Results ................................................................................................................ 78

3.4.1 Spinosad ........................................................................................................... 78

3.4.2 Lambda–cyhalothrin .......................................................................................... 78

3.4.3 Flubendiamide ................................................................................................... 79

3.5 Discussion ........................................................................................................... 81

3.6 References .............................................................................................................. 83

Chapter 4: Conclusion and recommendations .............................................................. 88

4.1 Recommendations .............................................................................................. 90

4.2 References .............................................................................................................. 90

1

Chapter 1: Introduction and literature review

1.1 Introduction

Brassicaceae is among the top 10 economically important plant families and includes a

diverse selection of 350 genera with over 3500 species (Warwick et al., 2003). Brassica

is a genus of plants in the mustard family and the members of the genus are collectively

known as cruciferous vegetables, cabbages, mustards and sometimes cole crops.

Brassicaceae are cultivated globally and includes Brassica oleracea (broccoli, cabbage,

kale, kohlrabi, Brussel sprouts and cauliflower), B. napus (rutabaga and rapeseed), B.

rapa (Chinese mustard, bok choy, turnip and Chinese cabbage), Raphanus sativus

(radish) and B. juncea (green mustard) (Warwick et al., 2003). Common types of brassica

used for food include cabbage, cauliflower, broccoli and Brussels sprouts (Warwick et al.,

2003). The main Brassicaceae crops that are grown in southern and eastern Africa

(Kenya, Uganda, Tanzania, Ethiopia, Mozambique, Zambia, Malawi, Zimbabwe and

South Africa) are cabbage, cauliflower, rape, kale and Chinese cabbage (Löhr and Gathu,

2002). Vegetables are grown mainly for local markets and supply vital minerals and

vitamins to mainly maize–based diets (Löhr and Gathu, 2002).

Between 1993 and 2009 the global area planted with Brassica vegetable crops increased

by 39% and in 2009 an estimated 3.4 million hectares were planted worldwide (FAO,

2012). From the cabbage (Brassica oleracea var. capitata L.) and other Brassica crops

cultivated in South Africa, an estimated 149 495 t was harvested in 2017 (FAO, 2017).

Associated with the global growth of Brassica vegetables was an intensification of farming

practices, with cabbage yields increasing by 27%, and Brassica vegetables were reported

to contribute more than US$26 billion to the world economy in 2012 (FAO, 2012).

Production constraints of Brassicaceae in Africa include insect pests, with the

Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) the most damaging

(Nyambo and Pekke, 1995). It is one of the most destructive and widespread pests of

Brassica vegetables globally, with annual control costs estimated to be billions of dollars

(Talekar, 1992; Shelton, 2004; Grzywacz et al., 2010; Zalucki et al., 2012). The earliest

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reports of P. xylostella in South Africa was in the early 1900’s (Gunn, 1917), with

economic losses to cruciferous crops recorded in Australia and South Africa (Muggeridge,

1930). The vast distribution of the pest is indicated by records on cruciferous crops

reported in at least 128 countries by 1972 (Salinas, 1972).

It may have its origin in Europe (Hardy, 1938), but Kfir (1998) speculated based on the

origin of its biocontrol agents (14 species of parasitoids) and host plants (175 species)

that it originated in South Africa. Several of the parasitoids of P. xylostella are known to

be endemic to South Africa, indicating a relationship between the diamondback moth and

the parasitoid species in this country (Kfir, 1998). The possibility that the diamondback

moth evolved on indigenous Brassicaceae in South Africa is highly possible. The large

number of indigenous Brassicaceae, the large diversity of parasitoids and the presence

of the well–known P. xylostella parasitoid, Diadegma collaris (Hymenoptera:

Ichneumonidae) all contribute to the notion that this pest may have originated in South

Africa (Kfir, 1998). Liu et al. (2000) used the same principles to argue that P. xylostella

originated in China but the number of parasitoids was not as significant as in South Africa.

1.2 Biology of Plutella xylostella

Plutella xylostella is multivoltine with four to 20 generations per year in temperate and

tropical regions, respectively (Harcourt, 1986; Vickers et al., 2004). This pest can also

reproduce in extreme climates due to its wide ecological tolerance (Chua and Lim, 1977).

A single P. xylostella female can lay more than 200 small oval, yellow eggs. The eggs are

0.5 mm in diameter and are laid singly or in clusters of up to four eggs mainly on the upper

leaf surface (Talekar et al., 1994; Justus et al., 2000). Eggs hatch after 4–8 days and first

instar larvae usually tunnel into the spongy mesophyll tissues of leaves (Harcourt, 1957).

Second, third and fourth instar larvae are surface feeders and consume leaves, buds,

flowers, siliques, the green outer layer of stems and also developing seeds within older

siliques (Harcourt, 1957). The fourth and fifth instar larvae are active feeders and the

mature larva spins a delicate cocoon that is attached at each end to the leaf. Inside the

cocoon a prepupal phase precedes pupation and the green larva turns brown (Harcourt,

1957). A moth emerges from the pupa in the cocoon. Successful development can occur

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at constant temperatures from 8 to 32 °C and under alternating regimes between 4 °C to

38 °C (Liu et al., 2002). Development from egg to adult takes approximately 14 days in

the summer under field conditions in South Africa, with 4–6 generations per year

(Harcourt, 1957). Plutella xylostella is active year–round in South Africa with development

slowing down in winter (Annecke and Moran, 1982). There is no diapause or dormancy

period in areas where year–round reproduction occurs (Robertson, 1939).

1.3 Damage symptoms

Plutella xylostella larvae feed on the leaf surface leaving only the epidermis intact where

low to medium infestation levels occur (Talekar and Shelton, 1993). The larvae and pupae

can be found near damaged leaves especially in tight spaces where leaves and veins

join. In heavily infested plants, entire leaves can be destroyed leaving only the veins

behind (Talekar and Shelton, 1993). Damage is usually first seen on the edges of fields

and the infested plants look stunted and die if left untreated (Talekar and Shelton, 1993).

1.4 Distribution of Plutella xylostella

Plutella xylostella is present wherever its host plants occur and it is considered the most

widely distributed of all lepidopteran species (Shelton, 2004). Despite its pest status and

distribution, little is known about its abundance and global distribution (Zalucki and

Furlong, 2011). Based on how P. xylostella responds to climate, Zalucki and Furlong

(2011) published a bioclimatic model and a map indicating the areas suitable for

permanent occupation as well as for seasonal reproduction and population growth (Figure

1.1). Infestations are usually most severe in the tropics and sub–tropics where Brassicas

are cultivated year–round (Shelton, 2004). These are also the areas indicated by the

abovementioned model to be climatically the most suitable for P. xylostella persistence.

The infestation levels do, however, vary between seasons and locations depending on

the natural enemies present, migration, overwintering populations and preventative

control strategies (Shelton, 2004).

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Figure 1.1: Climate suitability for Plutella xylostella. The Ecoclimatic Index (EI) describes

the potential suitability for persistence and the growth index (GI), the suitability for

seasonal reproduction and population growth (Zalucki and Furlong, 2011).

The dispersal ability of P. xylostella affects and enhances its global distribution and pest

status. The moths disperse through air convection currents over national borders

(Bretherton, 1982). Seasonal moth migrations were reported in Scotland, Canada and

China (Talekar and Shelton, 1993; Chapman et al., 2002). Immature stages such as eggs

and pupae can be dispersed by the wind (Chapman et al., 2002) and the pest is

transported from one region to another with Brassicaceae seedlings containing larvae

and eggs. Plant material left behind after harvest and moved to a refuse area can also

contain larvae and eggs that can infest other plants nearby (Chapman et al., 2002).

Plutella xylostella moths move within crops by short flights between plants (Mo et al.,

2003). Dispersal of male and female moths are similar with 90% of them traveling less

than 100 m within a week from release (Mo et al., 2003). Less than 1% of individuals

move vast distances (> 1km) to neighbouring fields for reproduction (Mo et al., 2003).

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1.5 Host preference

The host plant range for P. xylostella is largely restricted to Brassicaceae (Sarfraz et al.,

2006). However, Zhang et al. (2007) reported oviposition on non–host plants not

belonging to the Brassicaceae. The only example of non–host oviposition is on snow pea,

Pisum sativum var. saccharatum (Fabales) and sugar–snap pea P. sativum var.

macrocarpon (Fabales) (Rossbach et al., 2006) in Kenya. Survival on P. sativum was

enabled by certain alleles but they were not fixed, which indicated genetic variance of P.

xylostella and not a host shift (Löhr and Rossbach, 2004; Henniges–Jassen et al., 2011).

Plutella xylostella moths use olfactory cues rather than vision as the main means to locate

host plants at short distances (Pivnick et al., 1994). Chemical cues are used upon landing

to establish host plant and oviposition sites (Justus and Mitchell, 1996). The distribution

within a crop field is usually irregular for vegetable crops (Waters et al., 2009). The

reasons behind the sporadic distribution are nutrient levels of hosts, field size, variation

in plant morphology and chemistry and feeding damage by other pests (Waters et al.,

2009; Hambäck et al., 2010; Sarfraz et al., 2010; Kos et al., 2011).

Brassicaceae species have developed diverse defence mechanisms against herbivory

which includes chemical and mechanical traits (Karowe and Grubb, 2011). Thick cuticles

enable defence against herbivores in some species of mustard such as Arabidopsis

thaliana and Lepidium sativum (Karowe and Grubb, 2011). Brassica oleracea reacts to

herbivory with the production of glucosinolates, which is regulated by jasmonic acid levels

(Tytgat et al., 2013). Brassica nigra (black mustard) developed enlarged trichomes, and

higher concentrations of glucosinolates in more valuable plant parts such as flowers

(Traw and Feeny, 2008). Glucosinolates are non–volatile composites that control host

plant interactions (Tang et al., 1997). Plutella xylostella is able to overcome both saponins

and glucosinolates, defence compounds against insects (Ratzka et al., 2002; Himanen et

al., 2009). Glucosinolates are limited in their biological activity and only play a role when

plant material is damaged (Hopkins et al., 2009). The damaged plant material causes

glucosinolates to be exposed to myrosinase, resulting in the production of hydrolysis

products, which is toxic to herbivorous insects (Li et al., 2000). These toxic products

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include isothiocyanates, a wide spectrum toxin for insects, including P. xylostella (Li et

al., 2000). The production of isothiocyanates is avoided in P. xylostella by a glucosinolate

sulfatase in the lumen of the larval gut, which eliminates sulfur from glucosinolate

molecules, causing myrosinase hydrolysis and preventing the formation of toxins (Ratzka

et al., 2002). The high amounts of glucosinolate sulphatase in the P. xylostella gut causes

the toxic compounds to be ineffective by converting glucosinolates into desulpho–

glucosinolates (Ratzka et al., 2002). Plutella xylostella ingests glucosinolates with minimal

effect on their performance (Ratzka et al., 2002). Isothiocyanates can function as both

oviposition stimulants and attractants (Renwick et al., 2006). Plutella xylostella also rely

on these compounds for host location (Sarfraz et al., 2006). Some Barbarea species

contain glucosinolates in their surface waxes, which explains their attractiveness for P.

xylostella females for oviposition (Shelton and Badenes–Perez, 2008). Although

glucosinolates attract P. xylostella, these compounds are also related to lower weight and

growth rates at different life stages (Li et al., 2000; Sun et al., 2009) and affect fitness

(Badenes–Perez et al., 2014).

The incidence of P. xylostella infestation in cruciferous vegetables is determined by the

number of larvae present in a pre–defined area (Berry, 2000). Various traps can also be

used to aid in the detection of P. xylostella in fields. The economic threshold for P.

xylostella on cabbage crops is 37 larvae/10 plants (Jusoh et al., 1982).

1.6 Economic impact

Plutella xylostella is regarded as the most destructive pest of Brassicas worldwide with

crop losses up to 90% (Verkek and Wright, 1996). The global economic impact of P.

xylostella is difficult to establish due to the diverse range of farm sizes as well as the

range of cruciferous production areas worldwide (Zalucki et al., 2012). Schelhorn et al.

(2008) estimated the global direct loss spent on control of P. xylostella to be US$ 1 billion.

A more recent estimate amounts to US$4–5 billion, if yield losses and control cost for all

Brassicas are taken into account (Zalucki et al., 2012; Bradshaw et al., 2016). Crop loss

in smallholder cabbage production systems caused by P. xylostella is estimated at 31%

(Macharia et al., 2013), but if this pest is left uncontrolled crop losses can reach 100%

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(Ayalew, 2006). Small–scale farmers in southern Africa experience up to 90% defoliation

of their crops regardless of the growth stage of Brassicas (Machekano et al., 2017).

South Africa exports Brassicas to various countries in Africa including Zambia, Lesotho,

Swaziland, Namibia, Angola, Botswana and Mozambique (South African Department of

Agriculture, 2014). Global demands for organic vegetables have significantly grown with

new markets opening up for African economies if high quality and standards can be met

(Lubinga et al., 2014). Plutella xylostella can have a negative influence on the

opportunities in these new markets (Lubinga et al., 2014).

1.7 Control of Plutella xylostella

Despite the efforts to establish integrated pest management programs, control strategies

rely mainly on insecticide application (Grzywacz et al., 2010). Other control strategies

include cultural and biological control (Shelton and Nault, 2004). The high pest status of

P. xylostella can mainly be attributed to insecticide resistance and the absence of natural

enemies in the areas where host crops are cultivated (Talker and Shelton, 1993).

1.7.1 Cultural control

Cultural control is important in establishing a long–term control strategy for P. xylostella

(Diez et al., 2012). Many studies focused on using plants or certain plant characteristics

to control insect pests (Boiça Júnior et al., 2013; Girão Filho et al., 2014). Plants have

specific modes of action in terms of their defence against various insect pests, which

include repellence, toxicity, growth regulation, oviposition inhibition and feeding

deterrence (Maia and Moore, 2011; Liang et al., 2012; Amoabeng et al., 2014).

Resistance development to insecticides by P. xylostella (Tabashnik et al., 1990; Shelton

et al., 1993; Zhao et al., 2002) in combination with food safety have motivated the

development of alternative management strategies using trap crops (Hooks and Johnson,

2003; Shelton and Badenes–Perez, 2006). Trap crops are plants cultivated to attract

insects and other organisms to protect the main crop against pest attack (Heikki and

Hokkanen, 1991). Trap crops are less economically important plants that are highly

preferred by P. xylostella which are planted between the Brassicas of choice in a

8

commercial field (Shelton and Nault, 2004). Protection is attained by either converging

the pest to a certain area or inhibiting the pest from reaching the main crop (Heikki and

Hokkanen, 1991). All pests prefer particular plants, which include different cultivars,

species and stages (Heikki and Hokkanen, 1991). For a trap crop to be effective in

reducing P. xylostella populations, the trap crop has to be in the vegetative stage and

healthy (Shelton and Nault, 2004) and should be available the entire growing season of

a commercial crop (Heikki and Hokkanen, 1991). The use of trap crops to control P.

xylostella has been attempted more than for any other insect pest with varying results

(Shelton and Badenes–Perez, 2006; Nethononda et al., 2016). Only a few of these were

applied to small scale testing or field experiments with no full–scale implementation of a

trap crop (Shelton and Badenes–Perez, 2006).

Additional reasons to implement trap crops are the environmental and economic benefits

related to this control strategy (Heikki and Hokkanen, 1991). Trap crop implementation

can reduce the costs of chemical control to a level compensating for the implementation

cost of the trap crops (Swezey and Daxl, 1988). Yields are also higher in trap crop

systems than in conventional fields (Hokkanen et al., 1986). Since the main crop does

not need excessive insecticide application in a trap cropping system, natural enemies are

left unharmed. Trap cropping can also attract and increase the incidence of natural

enemies and consequently biological control of the target species in the cultivated area

(Heikki and Hokkanen, 1991). This method is used successfully in the Soviet Union to

protect cabbage (Adashkevich, 1974; Vorotintseva, 1974; AI–Hitty and Sharif, 1987).

Trap crop implementation is negatively affected by farmers not willing to cultivate trap

crops with no market value (Khan et al., 2007). Other pests of the main crop such as

aphids can take refuge in the trap crop, when considering P. xylostella as the main pest

(Nofemela, 2008). Inconsistent results of trap crops are also problematic (Shelton and

Badenes–Perez, 2006).

The most proposed trap crops for P. xylostella is yellow rocket (Barbarea vulgaris)

(Brassicaceae) and Indian mustard (Brassica juncea juncea) (Brassicaceae) (Charleston

and Kfir, 2000; Badenes–Perez et al., 2004; Lu et al., 2004; Shelton and Nault, 2004;

9

Badenes–Perez et al., 2005). Yellow rocket is a biannual invasive weed that occurs

worldwide in temperate regions (Uva et al., 1997). Plutella xylostella moths prefer yellow

rocket as an oviposition substrate but it is not preferred by the larvae for feeding and larval

survival on yellow rocket is low (Sarfraz et al., 2006). Yellow rocket could therefore be

regarded as a dead–end trap crop. Dead–end trap crops are specific plant species, which

is highly preferred for oviposition with low larval fitness which can serve as a replacement

for conventional insecticides (Sarfraz et al., 2006).

Barbarea vulgaris deters feeding by P. xylostella larvae by means of triterpenoid saponins

3–0–b–cellobiosyloleanolic acid (saponin 2) and 3–0–b–cellobiosylhederagenin (saponin

1) (Badenes–Perez et al., 2010). Saponins are also cyto–toxic to P. xylostella cells

through the cell membrane, which is being permeated to induce apoptosis (De Geyter et

al., 2012). There are two morphological forms of B. vulgaris var. arcuatea, pubescent (P)

containing hairs and glabrous (G) being hairless. The two forms differ strongly in saponin

and glucosinolates content and in terms of genetics (Augustin et al., 2012). Plutella

xylostella larvae do not survive on G–type B. vulgaris var. arcuata due to saponins 1 and

2 being present (Badenes–Perez et al., 2010). P–type B. vulgaris var. arcuata contains

saponins, but P. xylostella larvae survive due to saponins 1 and 2 being absent (Kuzina

et al., 2011). Barbarea vulgaris var. variegate, resistant to P. xylostella larvae, similarly

contains saponins 1 and 2 (Badenes–Perez et al., 2011).

There are contradictions in reports from different parts of the world on the use of Indian

mustard as a trap crop. In South Africa, this species showed potential as a trap crop

although it has no specific advantages to cabbage production in particular (Charleston

and Kfir, 2000). This is in contrast to results from Indonesia where no effective control of

P. xylostella was achieved with Indian mustard (Omoy et al., 1995).

1.7.2 Biological control

Successful biological control programs depend mainly on the correct identification of

natural control agents (Collier and Steenwyk, 2004). Correct timing is essential since

biocontrol agents that colonize P. xylostella infested crops late in the season may not

provide effective control (Collier and Steenwyk, 2004). The controlled release of a high

10

number of natural enemies may provide effective biological control of this pest (Collier

and Steenwyk, 2004). However, the release of low numbers of biocontrol agents early in

the season allows for the establishment of a successful biocontrol agent population.

These agents can then provide pest control later in the season. This approach can be

effective in an area where P. xylostella overwinters and cause predictable damage later

in the season (Collier and Steenwyk, 2004).

Various natural enemies can suppress P. xylostella. It is reported that there are various

parasitoid species that attack the different stages of P. xylostella globally (Delvare, 2004).

These parasitoids can be grouped into 38 larval, six egg and 13 pupal parasitoids (Lim,

1986; Talekar and Shelton, 1993). Trichogramma and Trichogrammatoidea

(Hymenoptera: Trichogrammatidae) parasitoids are not well established bio–control

agents since egg parasitoids are not always host–specific and parasitize non–target

species (Talekar and Shelton, 1993). The best control by parasitoids is therefore from

larval parasitoids (Goulet and Huber, 1993). The genera Cotesia (Braconidae), Microplitis

(Braconidae) and Diadegma (Ichneumonidae) are the most effective parasitoids for P.

xylostella control (Hardy, 1938, Liu et al., 2000).

Biologically based efforts to control P. xylostella in Africa and Asia have focused strongly

on parasitoid introductions. However, the identification and deployment of promising

parasitoids in many regions, have had limited impact, due to insecticide applications

(Grzywacz et al., 2010). Studies done in Kenya, Uganda, Tanzania, Mozambique,

Zambia, Malawi and Zimbabwe reported low levels of parasitism by Oomyzus sokolowskii

(Kurdjimov) and Diadegma spp. (Löhr and Gathu, 2002). Diadegma is the most

widespread and effective parasitoid genus parasitising P. xylostella in Uganda, Kenya,

Tanzania and South Africa (Kfir, 1997; Seif and Löhr 1998). Diadegma collaris was

identified as a potential parasitoid for control of P. xylostella in East Africa since it is

commonly found in southern Africa (Kfir, 1997; Löhr and Gathu, 2002).

1.7.3 Chemical control

Information on chemical control of P. xylostella by farmers in South Africa is limited (Table

1.1) (Furlong et al., 2013), although these farmers rely completely on insecticides for

control of P. xylostella (Canico et al., 2013). Compared to global standards, maximum

11

application rates and frequencies are not adhered to by southern African farmers

(Williamson et al., 2008). In Botswana, most pesticides are applied at a rate of between

1 to10 fold greater than the recommended dosage rates (Machekano et al., 2019).

Organophosphates are applied at 1 to 3 fold and pyrethroids at 5 to10 fold the

recommended rate (Machekano et al., 2019). Frequency of application can vary from

once a month to more than three times a week (Obopile et al., 2008; Harvey, 2015). In

Botswana, 41 active ingredients sold under 56 trade names from 17 chemical groups, are

used to control P. xylostella (Machekano et al., 2019). Wandaat and Kugbe (2015)

reported that farmers use at least two to six alternating insecticides in southern Africa.

However, Ngowi et al. (2007), reported that up to five insecticides are mixed in a single

application without any recommendation of agro–chemical companies. It can result in

undesirable chemical reactions, phytotoxicity and longer withholding periods for the

respective insecticides (Baliga and Repetto, 1996). Uncontrolled use of insecticides

contributes to the increased cases of resistance and destruction of biological control

populations (Safraz et al., 2005; Grzywacz et al., 2010; Furlong et al., 2013).

Table 1.1: Insecticides and rates used for Plutella xylostella control on cabbage at six

sites in South Africa (Sereda et al., 1997).

Area Application

frequency

Insecticides Recommended

dosage (RD)

Brits Weekly Chlorpyrifos

Cypermethrin

Parathion

4xRD

RD

RD

Bloemfontein 1 Weekly Deltamethrin

Methamidophos

Parathion

Mevinphos

Betacyfluthrin

Bacillus thuringiensis var. kursaki

RD

RD

RD

RD

RD

RD

12

1.8 Insecticide resistance

Whalon and McGaughey (1998) defined resistance as an evolutionary process where

genetic adaptation is caused by biocide selection causing unsuccessful control. Plutella

xylostella is controlled mainly with insecticides in South Africa as in many other countries

(Mosiane et al., 2003). There are three reasons for the intensive insecticide application

on Brassicaceae crops (Sarfraz and Keddie, 2005). Firstly, insecticides are applied to

control P. xylostella, but it also kills the natural enemies and subsequent continued use

of selected insecticides results in resistance evolution and unsuccessful control (Sayyed

et al., 2002). Secondly, the highly migratory moths may migrate to areas with no natural

Bloemfontein 2 Twice a week Deltamethrin

Parathion

Betacyfluthrin

Bacillus thuringiensis var. kursaki

Methamidophos

RD

RD

RD

RD

RD

Ventersdorp Every 3–4 days Parathion

Methamidophos

Chlorpyrifos

Lambda–cyhalothrin

RD

RD

RD

RD

Skeerpoort Every 7–10 days Chlorpyrifos

Cypermethrin

Dimethoate

Pyrol

11xRD

16xRD

8xRD

RD

Roodeplaat 3 applications a

season

Mevinphos

Parathion

Carbofuran

RD

RD

RD

13

biological control agents resulting in insecticide applications (Chapman et al., 2002;

Sarfraz and Keddie, 2005). Thirdly, markets prescribe produce to meet certain standards,

which have an influence on control strategies, with farmers relying on insecticides for

effective control (Sarfraz and Keddie, 2005). Market pressure in Asia results in cabbage

farmers applying insecticides at least every 3–5 days with 12–16 applications in one

season (Shamsudin et al., 2004). The result of overusing insecticides is evident in the last

100 years where more than 500 insect species have developed resistance to selected

insecticides (Mota–Sanchez et al., 2002).

Plutella xylostella was the first crop pest to become resistant to DDT in the mid 1900’s

(Ankersmit, 1953) and also the first insect pest with resistance to Bacillus thuringiensis

(Tabashnik et al., 1990). By 2011, P. xylostella showed resistance to most insecticides

applied under field conditions in certain cropping areas (Ridland and Endersby, 2011)

and also to new insecticide groups, including diamides, spinosyns and phenylpyrazoles

(Table 1.2) (Gong et al., 2014). Worldwide P. xylostella is resistant to 91 active ingredients

from various modes of actions including 12 strains of B. thuringiensis (Bt) (Xia et al., 2014;

Legwaila et al., 2014). When comparing insecticide resistance for P. xylostella in southern

Africa to statistics from the rest of the world, southern Africa has a relatively low number

of reports (IRAC, 2015). Farmers who keep using the same active ingredient and apply

insecticides more frequently with an increase in dosage rates or as a mixture of various

insecticides promote resistant populations of P. xylostella (Ngowi et al., 2007; Canico et

al., 2013).

Table 1.2: Insecticide resistance of P. xylostella reported in Brassica producing areas in

2005 (Sarfraz and Keddie, 2005).

Area Insecticide Group Resistance

ratio (RR)

Reference

South

Carolina

Cry1C Bt 63 100 Zhao et al. (2000).

Malaysia Spinosad Spinosyns 20 600 Sayyed et al. (2004)

14

Abamectin Avermectins 16 300 Sayyed et al. (2004)

Fipronil Phenylpyrazoles 70 Sayyed et al. (2004)

Cry1 Ac Bt 1680 Sayyed et al. (2004)

Japan Acetamiprid Neonicotinoids 110 Ninsin (2004)

Hawaii Spinosad Spinosyns 1340 Mau and Gusukuma–

Minto (2004)

Dipel (Btk) Microbial

disruptors

3300 Tabashink et al. (1993)

India Fipronil Phenylpyrazoles 505 Mohan and Gujar (2003)

Cartap Nereistoxin

analogue

24 Perez et al. (2000)

Nicaragua Deltamethrin Pyrethroid 49 800 Perez et al. (2000)

Methamidopho

s

Organophosphate 468 Perez et al. (2000)

Cypermethrin Pyrethroid 232 Perez et al. (2000)

Chlorfluazuron Benzoylureas 200 Perez et al. (2000)

California Permethrin Pyrethroid 206 Shelton et al. (2000)

Spinosad Spinosyns 14.8 Shelton et al. (2000)

Australia Chlorpyrifos Organophosphate 199 Baker and Kovaliski

(1999)

Permethrin Pyrethroid 442 Baker and Kovaliski

(1999)

Florida Fenvalerate Pyrethroid 82 400 Yu and Nguyen (1992)

15

Cyhalothrin Pyrethroid 10 699 Yu and Nguyen (1992)

Carbofuran Carbamate 504 Yu and Nguyen (1992)

Diazinon Organophosphate 73 Yu and Nguyen (1992)

Endosulfan Acyclodiene

organochlorines

25 Yu and Nguyen (1992)

Insecticide resistance in field populations of P. xylostella cause control failures, but

farmers can use insecticides with alternative modes of action to which resistance should

not be present (Verkerk and Wright, 1997). Recurring infestations of P. xylostella and

control failure are also due to natural enemy destruction through the overuse of broad–

spectrum insecticides (Furlong et al., 2004). Early–season spraying of broad–spectrum

insecticides are lethal to parasitoids and thus exacerbate P. xylostella outbreaks

(Grzywacz et al., 2010). Recurring infestations and destruction of natural enemies are of

equal importance in control failure of P. xylostella (Verkerk and Wright, 1997; Talker,

2004). Newer insecticides have been introduced with more selective modes of action,

which are less harmful to natural enemies of P. xylostella (Sayyed and Wright, 2004).

Amongst the insecticides applied in Botswana for control of P. xylostella, primicarb

(carbamate) and Bt insecticides are the least harmful and therefore best for the

conservation of the natural enemies of P. xylostella (Machekano et al., 2017). Spinosad

and pymetrozine are less harmful to biological control agents and are regarded as soft

insecticides. Methomyl (carbamate), organophosphates and pyrethroids are highly toxic

to biological control agents especially Cotesia spp. which are regarded as the most

effective parasitoids in southern Africa (Machekano et al., 2017). Organophosphates,

pyrethroids and carbamates are, however, the most frequently used insecticides for

control of insect pests on vegetables (Obopile et al., 2008; Manyangarirwa et al., 2009;

Nyirenda et al., 2011; Obopile et al., 2013).

The downside is that these insecticides (spinosad and pirimicarb) were introduced in

combination with the older less effective insecticides in spraying programmes resulting in

16

rapid resistance development to newer insecticides (Mazlan and Mumford, 2005; Sayyed

and Wright, 2006). In only two to three years, resistance levels to selected insecticides

can reach major levels for newly introduced insecticides (Zhao et al., 2002; Sayyed and

Wright, 2006; Waters et al., 2009).

No data are currently available on the status of resistance of P. xylostella to registered

commercially available insecticides in southern Africa. The management tactics and

recommendations applied in southern Africa are based on reports from other countries

(Pakistan, Brazil, Australia, China, USA and India) (IRAC, 2015). Insecticide resistance

is based on insect strains with a history of chemical exposure and geographical range of

occurrence (IRAC, 2015). The recommendations from other countries could therefore not

necessarily be applicable to southern Africa. Consequently, the southern African farmers

do not have access to information on P. xylostella resistance for their cropping area. This

lack of information makes control and planning more complicated, and results in farmers

making uninformed decisions influenced by agro–dealers, chemical companies and

advertisements (Nyirenda et al., 2011). The lack of policies on pesticide usage with no

area–wide regulations also complicates integrated resistance management (Ngowi et al.,

2007; Wandaat and Kugbe, 2015). The development of an insecticide resistance

management (IRM) program is imperative to prolong the usage of insecticides and their

efficacy. An IRM program has been implemented in Canada and the USA, which is based

on the IRAC–MoA classification. A similar program was also implemented in Hawaii to

regulate the use of spinosad as constant exposure to insecticides lead to resistance

development (Mau and Gusukuma­Minuto, 2004). There is, however, no official IRM

program currently available for P. xylostella in South Africa. Examples of resistance to a

number of insecticides in different groups and countries are presented in table 1.3.

Table 1.3: Insecticide class, and country with resistant populations and the mode of action

of the respective classes (Furlong et al., 2013).

Insecticide class Country Mode of action (IRAC,

2012)

Anthranilic

diamides

China (Wang and Wu, 2012) Ryanodine receptor

modulators

17

Avermectins Pakistan (Attique et al., 2006), Brazil (Santos et al.,

2011), Taiwan (Sayyed et al., 2004), China (Zhou et al.,

2011), Malaysia (Iqbal et al., 1996)

Cl– channel activator

Azadirachtin Taiwan (Kao and Cheng, 2001) unknown

Benzoylureas Malaysia (Iqbal et al., 1997), Japan (Morishita, 1998),

Nicaragua (Perez et al., 2000), China (Zhao and Wang),

2006), Brazil (Santos et al., 2011)

Chitin synthesis

inhibitor

Bt (aizawai) USA (Liu et al., 1996), Thailand (Imai and Mori, 1999),

Malaysia (Sayyed et al., 2004), Taiwan (Kao and Cheng,

2001)

Midgut membrane

Bt (Kurstaki) China (Gong et al., 2010), India (Pereira et al., 2006),

USA (Tang et al., 1997), Thailand (Imai and Mori, 1999),

Taiwan (Kao and Cheng, 2001), Malaysia (Sayyed et al.,

2004)

Midgut membranes

Carbamates Taiwan (Kao and Cheng, 2001), South Africa (Sereda et

al., 1997), India (Sannaveerappanavar and Virktamath,

2006), China (Zhou et al., 2011), South Korea (Kim et al.,

2011)

AChe inhibitor

Chlorfenapyr Taiwan (Kao and Cheng, 2001), China (Zhou et al., 2011) Oxidative

phosphorylation

uncoupler

Cyclodiene

organochlorines

India (Lal and Kumar, 2004) ץ–aminobutyric acid

(GABA) Cl– channel

antagonist

Diacylhydrazines China (Zhou et al., 2011) Ecdysone agonist

Indoxacarb Pakistan (Attique et al., 2006), Malaysia (Sayyed et al.,

2006), Australia (Eziah et al., 2009), USA (Zhao et al.,

2006), Brazil (Santos et al., 2011)

Na+ channel blocker

Neonicotinoids Malaysia (Sayyed and Crickmore, 2007)

nAChR antagonists

Nereistoxin analogs India (Mohan and Gujar, 2003), Nicaragua (Perez et al.,

2000), China (Zhou et al., 2011), Taiwan (Kao and

Cheng, 2001)

nAChR blocker

18

Organophosphates Pakistan (Attique et al., 2006), India

(Sannaveerappanavar and Virktamath, 2006), China

(Zhou et al., 2011), Philippines (Sarmiento and Ocampo,

2010), Australia (Eziah et al., 2009), South Africa (Sereda

et al., 1997)

Acetylcholinesterase

(AChe) inhibitor

Phenylpyrazoles Malaysia (Sayyed et al., 2004), India (Mohan and Gujar,

2003), China (Zhou et al., 2011), Taiwan (Kao and

Cheng, 2001).

GABA Cl– channel

antagonist

Pyrethroids Australia (Endersby et al., 2011), China (Endersby et al.,

2011), Brazil (De Oliveira et al., 2011),India (Dhumale et

al., 2009), Japan (Morishita, 1998), Malaysia (Sayyed

and Crickmore, 2007), New Zealand (Cameron et al.,

1997), Philippines (Sarmiento and Ocampo, 2010), South

Korea (Kwon et al., 2009), South Africa (Sereda et al.,

1997), Pakistan (Attique et al., 2006), USA (Shelton et al.,

2000), Nicaragua (Perez et al., 2000).

Na+ channel inhibitor

Spinosyns USA (Baxter et al., 2010), Taiwan (Kao and Cheng,

2001), Malaysia (Sayyed et al., 2004), Pakistan (Attique

et al., 2006).

Nicotinic acetylcholine

receptor (nAChR)

activator

1.9 Resistance mechanisms in Plutella xylostella

Individuals in a population with behavioural resistance developed uncommon behaviour

to survive and reproduce successfully (McGaughey, 1998). For example, P. xylostella

moths oviposit on stems near the soil surface to avoid insecticide applied on leaf surfaces

(Sarfraz et al., 2006). Resistance can also be biochemical (Sarfraz and Keddie, 2005).

Several biochemical mechanisms can be present at the same time and can cause more

than 1000–fold resistance in P. xylostella to insecticides (IRAC, 2018). According to IRAC

(2018), these include the following:

1. Enhanced metabolic detoxification mechanisms:

Microsomal monooxygenases with different forms of cytochrome P450 that play a

major role in P. xylostella resistance to pyrethroids, organophosphates, abamectin

and benzoylphenyl ureas.

Glutathione S–transferases are reported to confer organophosphate resistance.

19

Carboxylesterases are involved in resistance to organophosphates and other

chemical classes of insecticides.

2. Insensitive acetylcholinesterase causes P. xylostella resistance development to

organophosphates and carbamates.

3. Reduced Cry1C binding to the target site in the midgut membrane and reduced

conversion of Cry1C protoxin to toxin are factors in resistance development to the

B.thuringiensis protein Cry1C.

4. Knock–down resistance is caused by a mutation(s) in voltage–gated sodium channels

responsible for pyrethroid resistance.

5. Other mechanisms – include modified GABA–gated chloride channels and reduced

penetration.

1.10 Integrated Pest Management (IPM)

Furlong et al. (2013) published steps for developing and implementing an IPM programme

suitable for most cultivation areas aimed at P. xylostella control. These are:

1. Action thresholds should be calculated for a specific area – a general threshold

cannot be used since each area has its own unique conditions, which change the

dynamics of a P. xylostella population. Action thresholds can reduce the number

of insecticide applications per season.

2. Development of an IRM program using the IRAC–MoA classification scheme is

crucial to ensure long term usage of selected insecticides in spraying programs.

These types of programs can only be successful if growers on a national or

regional level agree and comply.

3. Areas known to be invaded by seasonal migratory populations of P. xylostella can

use warning systems to alert growers of possible infestations. The Canola Council

of Canada (2014) uses a modeling system based on wind trajectory, incorporating

pheromone traps to estimate P. xylostella infestations beforehand.

20

4. Insecticide resistance should be monitored and insecticide applications should be

adjusted when resistance is present. The constant monitoring of resistance is

crucial in preventing control failures with insecticides.

5. Scouts should be trained to effectively and correctly identify pests and to monitor

pest populations and natural enemies.

6. Evaluation of an IPM program is vital to ensure the correct implementation of the

respective methods, which form part of the program. Scouting costs and savings

by reduced insecticide applications should be calculated to ensure economic

viability and delay of resistance evolution.

1.11 Aims and objectives

1.11.1 Aims

The aims of this study were to determine which brassica crops in South Africa can be

used as a trap crop for control of P. xylostella, and to establish the susceptibility of P.

xylostella to selected insecticides in South Africa.

1.11.2 Objectives

The specific objectives of this study were to determine:

the feeding damage and percentage growth of P. xylostella on selected host plants

in no choice tests.

the feeding preference of P. xylostella larvae on selected host plants in two–choice

tests.

the development and oviposition of P. xylostella on selected host plants for

possible use in a Push–Pull strategy.

the susceptibility of P. xylostella to selected insecticides.

The results of this study are presented in the form of chapters with the following titles:

Chapter 2: Cultural control of Plutella xylostella

Chapter 3: Susceptibility of Plutella xylostella populations to selected insecticides

Chapter 4: Conclusion and recommendations

21

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42

Chapter 2: Cultural control of Plutella xylostella

2.1 Abstract

Vegetables are grown in South Arica in rural areas for sustenance. The increase in

cruciferous vegetable production leads to increased Plutella xylostella populations.

Several cases of P. xylostella resistance to insecticides have been reported and prompted

the development of alternative management strategies, with trap crops being the focus of

cultural control. The aim of this study was to determine if any of the five selected P.

xylostella host plant species could be used as trap crops at Pico–Gro. Pico–Gro is a

commercial farm in Glen Austin, Midrand, South Africa. The objectives of this study were

to determine the oviposition and larval feeding preference of P. xylostella in no–choice

and two–choice tests, as well as to determine the larval development time on these

selected host plants. Five known P. xylostella host plant species were selected, viz. pak–

choi (Brassica rapa var. chinensis), cabbage (Brassica oleracea), tatsoi (Brassica

narinosa), kale (Brassica oleracea) and mustard (Brassica juncea). Data were analysed

by means of one–way analyses of variance (ANOVA), meanswere compared by means

of Tukey post–hoc tests. The proportion of P. xylostella that made a choice in the two–

choice tests was calculated, and data were analysed by means of a binomial distribution

test. IBM SPSS ® (version 23, Armonk, NY: IBM Corp). In the two–choice tests,

conducted mustard was preferred for oviposition as well as feeding preference by P.

xylostella larvae. The fact that mustard was the preferred choice over the other crops for

oviposition and feeding preference makes it a viable candidate for a trap crop for P.

xylostella.

Key words: Diamondback moth, feeding preference, oviposition, Plutella xylostella, trap

crops

43

2.2 Introduction

Vegetables in rural areas of South Africa are grown for sustenance (Charleston, 1998).

Generally, small–scale farmers generate a higher income per hectare with vegetables

compared to cereals, root and tuber crops (AVRDC 2006; Mazhawidza and Mvumi,

2017). In the 20 years up to 2012, the global production of cruciferous vegetables

(Brassicaceae) increased by 39% (FAO, 2012). The Brassicaceae consists of 300 genera

and 2 500 to 3 500 species (Marzouka et al., 2010).

Plutella xylostella is a cosmopolitan pest and is globally regarded as the primary pest of

cruciferous vegetables (Furlong et al., 2013). The pest status of P. xylostella, therefore,

increased with the increase in production of cruciferous vegetables. Although P. xylostella

is considered a Brassicaceae specialist (Sarfraz et al., 2010), Furlong et al. (2013) argued

that, with so many species and the global distribution of Brassicaceae, P. xylostella can

be considered a generalist just as much as a specialist. Several cases of P. xylostella

resistance to insecticides have been reported (Talekar and Shelton, 1993; Nyambo and

Löhr, 2005; Ayalew, 2006, Odhiambo et al., 2010, Santos et al., 2011, Zhou et al., 2011;

Agboyi et al., 2013 and Legwaila et al., 2014). This prompted the development of

alternative management techniques, with trap crops the focus of cultural control (Hooks

and Johnson, 2003, Shelton and Badenes–Perez, 2006).

2.2.1 Trap cropping

Trap cropping involves providing a pest with a more attractive host for oviposition and

nutrition than the cultivated crop (Holden et al., 2012). The success of trap crops depends

on the ability of the selected crop to not only attract but also retain pests in numbers

sufficient for the cash crop to experience reduced infestation (Holden et al., 2012).

Insects, in general, depend on host olfactory cues for locating their mates and mating, as

well as finding host plants and oviposition selection (Couty et al., 2006, Silva and Furlong,

2012). Generally, gravid females of P. xylostella are more sensitive to host–derived

volatiles (Mechaber et al., 2002), although there are also exceptions (Martel et al., 2009).

The successful use of a host plant as a trap crop, therefore, depends on specific

behavioural and physiological adaptations in the larval and adult stage. The adult females

44

should find and accept a plant for egg laying and larvae should accept it as well and be

able to develop on it (Bernays and Chapman, 1994). The ultimate trap crop is a dead–

end trap crop, which attracts female moths for oviposition but does not support larval

survival (Shelton and Nault, 2003). Insects have “evolutionary mistakes” when they are

attracted to dead–end trap crops. Insects are attracted to these plants due to their

olfactory restraints but these crops contain lethal underlying components (Badenes–

Perez et al., 2014). Trap cropping has the potential to reduce crop damage while

simultaneously reducing the use of conventional pesticides (Holden et al., 2012).

The choice made by moths to oviposit on certain plant species is essential in the success

of a trap crop since the immature stages of many phytophagous insects, especially

lepidopterans, which are not very mobile (Renwick, 1989). Oviposition by P. xylostella is

affected by stimuli such as leaf surface, abiotic factors and chemosensory stimuli

(Renwick et al., 2006). However, many insects use host plant chemicals in the relevant

concentrations to establish host suitability (Honda, 1995). Insects infesting Cruciferae use

the hydrolysis products from glucosinolates in decision making as stimulants for

oviposition (Hopkins et al., 2009) and larval feeding (Renwick et al., 2006). Variation in

the composition and concentration of glucosinolates can affect host plant suitability for P.

xylostella larvae. Natural selection will, therefore, be in favour of the females that are able

to differentiate between unsuitable and suitable host plants (Li et al., 2000).

Neonate larvae of P. xylostella are immobile and feed on plant material on which they

hatch (Sarfraz et al., 2010). P. xylostella is, therefore, an ideal candidate for trap

cropping. The established trap crops used for P. xylostella are unreliable and it was

reported that growers avoided this cultural practice (Banks and Ekbom, 1999, Shelton

and Badenes–Perez, 2006). Several researchers did, however, continue to study the

development and fitness of this pest on various Brassicaceae (George et al., 2009;

Badenes–Perez et al., 2010; Cunningham, 2012; Nethononda et al., 2016). White

mustard (Sinapis alba) was identified by George et al. (2009) as an effective trap crop for

P. xylostella. Wintercress (Barbarea vulgaris William Townsend Aiton) has also been

found as a preference by P. xylostella moths, but larvae do not survive well on these

plants, which qualifies it as a possible dead–end trap crop (Cunningham, 2012).

45

Although much research has been done on P. xylostella, questions about its host plant

preference in agroecosystems still remain (Newman et al., 2016). Trap cropping is a

viable cultural control method for P. xylostella in growth tunnels, since it has a strong

preference for some Brassicaceae species (Serizawa et al., 2001). Understanding

feeding and oviposition preference can help develop more accurate control measures for

various agroecosystems and assist in the improvement of viable trap crops for control of

P. xylostella in an integrated pest management system, especially in growth tunnels.

2.2.2 Pico–Gro test site

Plutella xylostella is one of the main pests of cruciferous crops cultivated at Pico–Gro in

Glen Austin, Midrand, South Africa (Personal observation). Vegetables and edible flowers

are produced commercially and supplied to both national and international markets. The

set–up consists of 12 greenhouses and an area covered with shade nets (Figure 2.1).

Over 190 plant products are grown at Pico–Gro (Personal observation).

Figure 2.1: Schematic representation of the farm section at Pico–Gro, Glen Austin

Midrand, South Africa.

46

Known host plants of P. xylostella, mustard (Brassica juncea), cabbage (Brassica

oleracea), tatsoi (Brassica narinosa), pak–choi (Brassica rapa var. chinensis) and kale

(Brassica oleracea) are intercropped with various other vegetables in five greenhouses

(Figure 2.3).

The average temperature in these greenhouses was kept at 30 °C and relative humidity

(RH) at 65%. An example of one of these greenhouses is provided in Figure 2.2.

Figure 2.2: Greenhouses used for the production of vegetables and other crops at Pico–

Gro.

Figure 2.3: One of the greenhouses at Pico–Gro where Brassicas are planted. These

greenhouses are divided into bays, each containing plants of a different plant family.

47

The different bays in one of these plant tunnels is depicted in Figure 2.4. It represents a

tunnel were plant species from different plant families are planted together. The plants

are nasturtiums, kale, shiso and bronze fennel.

Figure 2.4: A bay in a tunnel with plants from different families planted.

The arrangement of the various plants to be cultivated per bay in each greenhouse or

shaded area is carefully planned. An example of the layout in three of these greenhouses

is provided in Table 2.1.

48

Table 2.1: Greenhouses and shade net bays with the respective crops planted per bay

in these areas.

Bays Greenhouse 1 Greenhouse 2 Shade

Net

1 Flowers (viola and alyssums) Kale Radish

2 Red vain sorrel and fennel Peas

3 Nasturtiums, kale, shiso and bronze

fennel

Turnip

4 Pak–choi Cabbage

5 Tatsoi Rocket, pak–choi and

tatsoi

6 Rocket Tatsoi and pak–choi

7 Wild rocket

8 Pak–choi and tatsoi

9 Normal and wild rocket

10 Wild rocket

11 Samphire

Pico–Gro supplies national and international markets with various products. Pesticide and

quality requirements are according to the European Union maximum residue levels (EU

MRL) database, which is the standard for worldwide export to be Global Gap registered.

These requirements stipulate that the export product should be insect and residue free.

To ensure adherence, regular tests are done at the airport by the Perishable Product

Export Control Board and Department of Agriculture, Forestry and Fisheries (DAFF) as

well as maximum residue levels testing by the clients (Owner of Pico–Gro).

The implementation of an Integrated Pest Management (IPM) approach consisting of

chemical and cultural control (trap crops), will lower production costs because it can

reduce the number of insecticide applications for control of diamondback moth. It will also

lower the risk of insecticide residue concentrations and the associated health risks.

49

The aim of this study was to determine if any of the five–selected P. xylostella host plant

species, viz. mustard, cabbage, tatsoi pak–choi and kale could be used as trap crops at

Pico–Gro. The objectives of this study were to determine larval feeding and oviposition

preference of P. xylostella in no–choice and two–choice tests, as well as to determine the

larval development time on these selected host plants.

2.3 Material and methods

2.3.1 Plutella xylostella stock colony

Plutella xylostella larvae were collected from cabbage at a commercial farm in Glen

Austin, Midrand. The infested plant material was kept in plastic containers (40 x 20 x 15

cm) with aerated sides. Moths were transferred from these containers to wooden screen

cages (0.75 m (w) x 1 m (h)) with potted cabbage plants for oviposition. Eggs were

collected daily and kept in small plastic containers (52 mm high, 30 mm in diameter) with

a steel mesh–infused lid, in a rearing room at 26 ± 1 ºC, 65 ± 5% humidity and 14L:10D

photoperiod. Once emerged from the eggs, larvae were transferred into plastic containers

similar to the ones described above and reared on cabbage in a laboratory at 25 ±1 ºC.

2.3.2 Larval feeding preference for selected Brassicas

Mustard, cabbage, tatsoi, pak–choi and kale were planted in pots under shade nets at the

North–West University, Potchefstroom (Figure 2.5). For larval choice experiments, leaves

from five–weeks–old plants were used.

50

Figure 2.5: Brassica plants in pots that were used in no–choice and two–choice tests.

a: mustard, b: cabbage, c: tatsoi d: pak–choi and e: kale.

2.3.3 Larval feeding choice tests

2.3.3.1 No–choice tests

Leaf disks (3 cm diameter) were punched from mustard, cabbage, tatsoi, pak–choi and

kale. The bottoms of Petri dishes (100mm (d) x 15mm (h)) were covered with a 3 mm

layer of agar–agar (which contained no nutrients) to prevent desiccation of the plant

material. A single disc of plant material from a plant species was placed in the centre of

a Petri dish. The mass of 10 P. xylostella larvae (3rd instar) was determined before being

placed individually onto the agar next to the side of the Petri dish. The Petri dishes were

kept in a rearing room at 26 ±1 ºC, 65 ± 5% humidity and 14L: 10D photoperiod. Larvae

were weighed again after 48 hours to determine the mean mass gain of larvae per plant

species. This was done as an indication of the suitability of the respective host plant

species for larval growth. Each leaf disk was scanned after 48 hours with a Samsung S9+

cell phone, picture size (4:3 12MP) for foliar analysis application with BioLeaf

(http://bioleaftech.co.za/) (Machado et al., 2016), after which the percentage leaf surface

consumed was calculated (Figure 2.6). The no–choice test with each plant species

consisted of 10 replicates with five larvae used per replicate.

51

Figure 2.6: Calculation of the percentage damaged (defoliation) leaf area using the

BioLeaf application. The damaged area is shown in red and calculated as percentage

‘defoliation’.

2.3.3.2 Two–choice tests

To determine larval feeding preference for the respective plant species, two–choice tests

were done by placing a single leaf disc (each 3 cm in diameter) from two different host

plants, 4 cm apart and 5 cm away from the side per Petri dish. The same host plants used

in the no–choice tests were pair–wise presented. The bottoms of Petri dishes were

covered with agar–agar as described above (see 2.1). Five larvae were placed in the

centre of the Petri dish at an equal distance away from the respective leaf discs. Feeding

preference of the larvae was recorded and feeding damage was calculated after 48 hours

as described in 2.1. There were 10 replicates with five larvae for each combination.

52

2.3.4 Oviposition preference tests

Oviposition preference of P. xylostella moths was determined in two–choice tests with the

same plant combinations used in the two–choice larval preference tests (see 2.2). The

sex of 3rd and 4th instar larvae was determined according to the characteristics described

by Liu and Tabashnik (1997). The 5th abdominal segment of male larvae is lighter in colour

compared to that of female larvae where all segments are the same colour (Figure 2.7).

These larvae were kept individually in small plastic containers (20 cm x 15 cm x 7 cm)

until pupation.

Figure 2.7: Distinguishing characteristics of male and female Plutella xylostella larvae.

The male larvae have a lighter coloured 5th abdominal segment than female larvae.

On the day of exclusion, one male– and one female moth were released into containers

(40 cm x 20 cm x 15 cm) with ventilation openings on two opposite sides. From the

selected plant species, two were selected for each oviposition test with 20 replicates for

each two–choice oviposition test. The plants were placed approximately 20 cm apart and

watered daily. The plants were removed and carefully taken apart, 48 hours after moths

were released into the cages. Eggs on the selected plant pieces were counted under a

stereomicroscope (Nikon SMZ 1500).

2.3.5 Development time

Pupae were collected from the rearing colony and placed into containers (40 cm x 20 cm

x 15 cm) with ventilation openings on two opposite sides. Host plant species, viz. mustard,

cabbage, tatsoi, pak–choi and kale were presented in all possible combinations in these

containers (two plants per container). The plants were kept in each container for 24 hours

before being removed and transferred to similar containers, but without moths. Therefore,

Female

Male

53

the emerging moths were immediately exposed to the test plant combination, without any

previous experience of other host plants. The oviposition containers as well as the plants

removed from the oviposition containers that were transferred to insect–free containers,

were kept at 26 ±1 0C, 65 ± 5% humidity and 14L: 10D. Total development time (days)

from 10 eggs to moth emergence on each of the selected crops were recorded with 10

replications.

2.3.6 Data Analysis

Data were analysed for normality and homogeneity before further analyses. One–way

analysis of variance (ANOVA) was used and means were compared with a Tukey post–

hoc test if the data was normally divided and homogeneous. IBM SPSS ® (version 23,

Armonk, NY: IBM Corp) was used for statistical analyses and graphical representation.

The proportion of P. xylostella larvae that made a choice in the two–choice oviposition

and feeding tests was calculated, and data was analysed by means of a binomial

distribution test IBM SPSS ® (version 23, Armonk, NY: IBM Corp) was used for statistical

analyses and graphical representation. Significance between selected combination was

established by mean of t–tests and displayed as *P<0.05, **P<0.01 and ***P<0.001.

2.4 Results

Choice tests

2.4.1. No–choice tests

The mean mass increase by larvae that fed on the respective host plant species differed

significantly (F4, 20 = 11,48; P<0.001). Larvae that fed on mustard gained significantly less

mass than larvae that fed on pak–choi and tatsoi, but their mass gained did not differ

significantly from cabbage and kale–fed larvae. The increase in the mean larval mass of

P. xylostella larvae that fed on pak–choi was the highest, but it did not differ from larval

mass gained on tatsoi (Figure 2.8).

54

Figure 2.8: Increase in mean larval mass (g) after 48 hours of feeding on host plant

species, viz. cabbage, kale, mustard, pak–choi, tatsoi in no–choice tests. Bars capped

with the same letter are not significantly different (P < 0.05).

An example of feeding damage by P. xylostella larvae to a leaf disk of the respective host

plants evaluated in no–choice experiments is shown in figure 2.9.

F4, 20

= 11,48; P<0.05

ab

ab

a

c

bc

Ma

ss in

cre

ase

(m

g)

± S

E

Crops

55

Figure 2.9: Leaf disks showing Plutella xylostella feeding damage in the no–choice

experiments; a: mustard, b: cabbage, c: pak–choi, d: tatsoi and e: kale.

The mean percentage P. xylostella damage differed significantly between cabbage, kale,

mustard, pak–choi and tatsoi leaf disks (F4, 20 = 87,11; P<0.001). The mean percentage

damage inflicted by P. xylostella larvae was significantly higher on mustard compared to

cabbage, kale, tatsoi and pak–choi. However, significantly more pak–choi leaf material

was consumed by P. xylostella larvae compared to cabbage, kale and tatsoi. There was,

however, no significant difference in percentage damage caused by larval feeding on

cabbage, kale and tatsoi (Figure 2.10).

a c b

d e

56

Figure 2.10: Mean percentage damage after 48 hours of feeding on the respective host

plant species, viz. cabbage, kale, mustard, pak–choi, tatsoi in no–choice tests. Bars

capped with the same letter are not significantly different (P < 0.05).

2.4.2 Two–choice tests

The feeding preference of P. xylostella larvae in two–choice tests was significantly higher

on pak–choi, mustard and tatsoi compared to kale and cabbage (Figure 2.11).

F4, 20

= 87,11; P<0.05

c

b

a

a

a

57

Figure 2.11: Preference of Plutella xylostella larvae for different host plant species in

two–choice tests after 48 hours. Significance indicated by **P<0.01 and ***P<0.001. ‘NC’

indicates the number of larvae (out of 50 per combination) that did not make a choice.

2.4.2.1 Percentage damage

The differences in the quantity of leaf material removed by feeding larvae from leaf disks

from the respective host plant species in the two choice tests are shown in figure 2.12.

-1, -0,8 -0,6 -0,4 -0,2 0, 0,2 0,4 0,6 0,8 1,

Cabbage x Pak-choi

Cabbage x Tatsoi

Cabbage x Mustard

Cabbage x Kale

Tatsoi x Pak-choi

Kale x Pak-choi

Kale x Tatsoi

Mustard x Tatsoi

Mustard x Kale

Mustard x Pak-choi

Preference proportion

Ho

st-

pla

nt

co

mb

ina

tio

ns

Plant 1 Plant 2

NC 5

NC 14

***

***

**

***

***

***

***

***

**

***

58

Figure 2.12: Leaf disks provided in the two–choice test showing Plutella xylostella

feeding damage: a: cabbage vs pak–choi, b: cabbage vs mustard, c: kale vs mustard, d:

cabbage vs pak–choi, e: kale vs pak–choi, f: kale vs cabbage, g: mustard vs pak–choi,

h: mustard vs tatsoi, i: pak–choi vs tatsoi and j: kale vs tatsoi.

2.4.2.2 Mean percentage damage

The mean percentage damage of P. xylostella larvae that fed on mustard, pak–choi and

tatsoi was significantly higher when compared to the damage inflicted by larvae that fed

a

d

c

b

e

g

f

h

59

on cabbage and kale. There was, however, no significant difference in percentage

damage between pak–choi, tatsoi and mustard. The mean defoliation from larvae that fed

on cabbage and kale did also not differ significantly (Figure 2.13).

Figure 2.13: Mean percentage damage after 48 hours of feeding on the respective host

plant species, viz. cabbage, kale, mustard, pak–choi, tatsoi in no–choice tests. Bars

capped with the same letter are not significantly different (P < 0.05).

2.4.2.3 Oviposition preference

The mean number of eggs laid by P. xylostella in two–choice tests differed significantly.

Significantly higher numbers of eggs were laid on mustard, pak–choi and tatsoi when

compared to kale and cabbage (Figure 2.14).

a

bc

a

c

c

F8, 40

= 42; 12 P<0.05

60

Figure 2.14: The preference of Plutella xylostella moths for different host–plant species

in oviposition tests after 48 hours. Significance indicated by ***P<0.001.

2.5 Discussion

No–choice tests where P. xylostella are only exposed to one host plant are used to

determine specific development characteristics such as growth to compare between

various host plants (De Bortoli et al., 2013). Newman et al. (2016), however, used multiple

plant species within the Brassicaceae Brassicaceae plant family to establish larval

ingestion rates as well as contact preference. The tests were used to assess alternate

host plants for use as a trap crop. The study by Newman et al. (2016) showed that larval

growth was unfavourable on high–preferred oviposition species potentially having

negative effects on neonate larvae concluding the possible use of black mustard as a trap

crop. Niu et al. (2014) studied the reproductive potential, survival, and development of P.

xylostella on eight cruciferous species, i.e. Descurainia sophia (L.), Cardamine hirsuta

L., Capsella bursa–pastoris (L.), Thlaspi arvense L., Rorippa indica (L.), Orychophragmus

violaceus (L.), Cardamine leucantha (Tausch) and Cardamine macrophylla. The

development of P. xylostella differed greatly between plant species according to Niu et al.

-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6

MustardxPak-choi

Pak-choixKale

MustardxTatsoi

MustardxKale

CabbagexKale

TatsoixKale

TatsoixCabbage

MustardxCabbage

TatsoixPak-choi

Pak-choixCabbage

Oviposition proportion

Sele

cte

d c

om

bin

ation

Plant 1 Plant 2

***

***

***

***

***

***

***

***

***

***

61

(2014). The study by Niu et al. (2014) established that certain plant species such as C.

macrophylla had the longest development time, lighter weight per pupae and high

oviposition preference all good characteristics for a trap crop used for P. xylostella control.

No–choice tests gave valuable information regarding monoculture–cropping areas,

however, two–choice and development tests are essential to better understand host

preference of P. xylostella. Growing tunnels with various host plants or environments with

bordering wild plants in a nursery setting as seen at Pico–Gro is the ideal area to establish

known host plants as trap crops.

De–Bortoli et al. (2013) have performed multi–choice preference tests with cauliflower,

broccoli, cabbage and collard greens. Plutella xylostella larvae favoured collard greens,

however, cauliflower was preferred for oviposition by adult females (De Bortoli et al.,

2013). In the two–choice tests conducted mustard was preferred for oviposition as well

as feeding preference by P. xylostella larvae. The fact that mustard was the preferred

choice over the other crops for oviposition and feeding preference makes it a viable

candidate for a trap crop for P. xylostella. Charleston and Kfir (2000) reported the similar

results on a different cultivar with P. xylostella females to prefer Brassica juncea juncea

(Indian mustard) for oviposition compared to commonly grown crops such as cauliflower,

broccoli and Chinese cabbage. It therefore showed promise as a trap crop for P. xylostella

(Charleston and Kfir, 2000). The similar results are seen above for Brassica juncea var.

integrifolia (Giant red mustard) as it is preferred for oviposition compared to kale,

cabbage, tatsoi and pak–choi.

Plutella xylostella develops faster and survive better on Brassica napus and Brassica

compestris when compared to Brassica rapa, Brassica oleracea botrytis and Raphanus

sativus (Saeed et al., 2010). The similar results were found on different crop species

where P. xylostella developed faster on mustard than compared to cabbage, kale, pak–

choi and tatsoi. Oviposition was preferred on garden cress (Lepidium sativum) (L.) with

wintercress (Barbarea vulgaris) (L.) and black mustard (Brassica nigra) (L.) following

closely behind (Newman et al., 2016). The ingestion rates of the various plants with the

highest on aubretia (Aubretia deltoidea) (L.) and the lowest on black mustard (Newman

et al., 2016). In the study by Newman et al. (2016) black mustard was highly preferred for

62

oviposition, however, the ingestion rates were the lowest for the same crop. Mustard was

also highly preferred for oviposition when compared to pak–choi, tatsoi, kale and cabbage

but have contradicting results with the feeding preference (ingestion rate) concluding to

be the highest of the selected crop species.

Niu et al. (2014) established that development from egg to adult had significant variance

and was longer on R. indica (15.8 d) and C. macrophylla (20.8 d). The same results were

observed with mustard (10.6 d) and kale (16.9 d), however, the development time was

faster for the selected crops than the crops used in the study by Niu et al. (2014). Larval

development time of P. xylostella on cabbage was shorter than on kale and pak–choi.

When comparing this result with the two–choice and oviposition results it would make

sense that cabbage and kale had longer development times as they resulted in the lowest

feeding and oviposition preference and defoliation percentage. The presence of pak–choi

as one of the crops on which P. xylostella development is quicker is not surprising with

pak–choi present in all the highest subsets in feeding and oviposition preference and

defoliation percentage. Weight gained by P. xylostella larvae that fed on leaf tissue in no–

choice tests were the lowest for larvae feeding on mustard in comparison with the other

selected crops. Niu et al. (2014) had similar results on other crop species, however

instead of weighing larvae, pupae were weighed with T. arvense having the lightest pupae

(Niu et al., 2014). Oviposition on C. macrophylla, O. violaceus and Ca. bursa–pastoris

compared to R. indica indicated a fecundity of 305–351 eggs/female and 134 eggs/female

irrespectively (Niu et al., 2014). Oviposition on mustard and kale showed significant

variance with 16–27 eggs/female and 2–4 eggs/female irrespectively.

Choice attest and larval development evaluations conducted indicate the use of Brassica

juncea var. integrifolia (Giant red mustard) as a viable trap crop for the control of P.

xylostella. The integration of mustard as a trap crop can be used at sites like Pico–Gro to

help control P. xylostella populations. Planting the trap crop (mustard) in between

selected crops (tatsoi, pak–choi, kale and cabbage) can reduce the number of individuals

in the population by removing the trap crop after infestation occurs. The continued

planting of trap crops like mustard and removing infested trap crop plants may reduce the

population below the economic injury level.

63

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69

Chapter 3: Susceptibility of Plutella xylostella populations

to selected insecticides

3.1 Abstract

Plutella xylostella is the most important insect pest of Brassica crops worldwide and

occurs wherever its host plants are cultivated. Insecticide application is the main control

strategy and concerns are raised about the susceptibility of P. xylostella to insecticides in

South Africa. Plutella xylostella populations were sampled from two localities, Nelspruit

(Mpumalanga province) and Bapsfontein (Gauteng province). The toxicity of three

insecticides used for control of P. xylostella in South Africa, viz. lambda–cyhalothrin,

flubendiamide and spinosad has been estimated for both these populations using an

IRAC approved bioassay method. In the absence of a reference population, comparisons

with the recommended label rates were performed. When comparing the susceptibility of

the two populations (Nelspruit (N2018–01) and Bapsfontein (B2018–01)) for lambda–

cyhalothrin the LC80 values are higher (N2018–01: 9.35 and B2018–01: 24.87) than the

recommended dosage of 8ppm). These populations are highly susceptible to

flubendiamide with LC80 values of 2.70 (B2018–01) and 2.24 (N2018–01) 0% compared

to the recommended dosage of 48ppm and to spinosad with LC80 values of 24.87ppm

(B2018–01) and 9.35 (N2018–01) compared to the recommended dosage of 216ppm.

Plutella xylostella remains a persistent and extensive pest of Brassicaceae. Resistance

development to insecticides is a reality. It is costly to control the pest, but in South Africa

(Nelspruit and Bapsfontein), P. xylostella is still highly susceptible to two active

ingredients (flubendiamide and spinosad) which is important for effective control of P.

xylostella populations.

Keywords: Plutella xylostella, spinosad, lambda–cyhalothrin, flubendiamide, South

Africa, susceptibility

70

3.2 Introduction

Plutella xylostella is the most important insect pest of Brassica crops worldwide and it

occurs wherever its host plants are cultivated (Talekar and Shelton, 1993). For decades

P. xylostella was not an economically important pest in South Africa and insecticides

provided effective control on various crops (Dennill and Pretorius, 1995). Outbreaks of P.

xylostella in South Africa occurred in the early 1990s (Dennill and Pretorius, 1995). Since

then, insecticide application became the main control strategy and concerns are raised

about the susceptibility of P. xylostella to insecticides in South Africa (Sereda et al., 1997).

Various local populations of P. xylostella were found to be less susceptible to frequently

used insecticides (Sereda et al., 1997). These populations became so severe that farmers

sprayed their fields more frequently than recommended to overcome this problem (Dennill

and Pretorius, 1995; Sereda et al., 1997). Increased damage levels were reported on

crops in regions where insecticides were applied frequently (Waladde et al., 2001). High

levels of parasitism of P. xylostella were, however, reported in unsprayed crops in

Gauteng, North–West and Eastern Cape provinces of South Africa (Smith and Villet,

2002). The use, overuse and exploitation of broad–spectrum insecticides in some areas

could possibly have eradicated the natural enemies of P. xylostella in these areas (Sereda

et al., 1997). Dennill and Pretorius (1995) sampled only one parasitoid in a cabbage field

in Pretoria where insecticides were applied regularly. In the same year and area, Kfir

(1997) sampled and documented 21 species of parasitoids in areas where no insecticides

were applied.

It was mentioned that in Togo and Benin (Countries in West Africa) farmers use

insecticides in high dosages with frequent applications (Agboyi et al., 2016). However,

prior to the study done by Machekano et al. (2019), no comprehensive study was done

on practices, perceptions and knowledge in Botswana and southern Africa to better

understand the overuse of pesticides. The misuse of pesticides and the differences in

manufacturer recommendations and realities can all be linked to a lack of research

(Machekano et al., 2019).

71

3.2.1 Insecticide resistance

Yamada and Koshihara (1978) listed conditions contributing to the development of

resistance to insecticides by P. xylostella. These include long cultivation seasons,

monoculture cultivation of Brassicaceae and high temperatures. Plutella xylostella

infestations are severe in countries with warm climates since the pest completes multiple

generations per year (Liu et al., 1981).

Constant cultivation of host crops and the short life cycle of P. xylostella contributes to

the continuous development and population increase of the pest that resulted in 25

generations being sprayed with synthetic insecticides each year causing the development

of resistance to various insecticides in Malaysia (Sayyed et al., 2004). Farmers in

Zimbabwe and Kenya also depend on synthetic insecticides (pyrethroids,

organophosphates and carbamates) (Oruku and Ndun'gu, 2001; Sithole, 2005). Due to

their lack of knowledge on resistance and newly developed insecticide groups, poor

farmers in Africa and Asia (Rauf et al., 2004) aggravate resistance development to

insecticides. These farmers rely on broad–spectrum insecticides already overused and

more toxic to natural control agents (Badenes–Perez and Shelton, 2006). With no insect

resistance management (IRM) strategy in place, the pattern of introducing new

insecticides and failure of these insecticides to control P. xylostella populations is

continuing (Wright, 2004; Mau and Gusukuma–Minuto, 2004; Santos et al., 2011; Kang

et al., 2017).

The status of P. xylostella as one of the most damaging lepidopteran pests worldwide

could be ascribed to its ability to develop resistance to all conventional insecticide classes

in the field (Liu et al., 2015). With the introduction of newer insecticides (spinosad,

avermectin, fipronil and indoxacarb), different modes of action entered the market.

Resistance development is, however, enhanced with these insecticides not being rotated

(Amit et al., 2004). Resistance to spinosad was recorded in field populations of P.

xylostella in the United States (Zhao et al., 2002) and Malaysia (Sayyed et al., 2004),

while resistance to indoxacarb was reported from Pakistan (Attique et al., 2006).

Emamectin benzoate is a second generation semi–synthetic avermectin insecticide

72

(Syngenta, 2004), but cases of resistance to avermectins were reported in China (Liang

et al., 2003) and Malaysia (Sayyed et al., 2004).

The development of insecticide resistance is a global problem and it is critical for crop

protection and resistance management to clarify the mechanisms that establish

resistance (Xia et al., 2018). According to Liu et al. (2015) the first commercial insecticide

belonging to the diamide group is chlorantraniliprole. The diamide group of insecticides

became commercially available in 2006 with flubendiamide derived from

benzenedicarboxamide shortly followed by cyantraniliprole and chlorantraniliprole from

the anthranilic diamides (Nauen and Steinbach, 2016). This insecticide group is active on

the insect ryanodine receptors in the endoplasmic reticulum (Nauen and Staeinbach,

2016). In 2013, more than $1.2 billion of diamides were sold globally (Nauen and

Staeinbach, 2016). After only 52 generations of selection, P. xylostella showed resistance

to chlorantraniliprole in China (Liu et al., 2015).

The recurring selection pressure imposed onto P. xylostella by the application of

insecticides in a Brassica cultivation area in Brazil, caused multiple insecticide resistance

(Neto et al., 2016). Plutella xylostella has developed resistance to spinosad and

chlorfenapyr, but there is no shift in the resistance levels of chlorantraniliprole in field

populations (Neto et al., 2016). Cross–resistance between chlorfenapyr and spinosyns

was not found, however cross–resistance was found between spinetoram and spinosad

(Neto et al., 2016).

3.2.2 Research and case studies

Research on the management of P. xylostella in SA is limited with minimal data available

compared to other countries (Gryzwacz et al., 2010) (Figure 3.1). The efficacy of control

by a pyrethroid (cypermethrin) and Bt (B. thuringiensis var. kustaki) to P. xylostella was

evaluated in Botswana (Legwaila et al., 2014, Legwaila et al., 2015). There is, however,

no information available on the current susceptibility of P. xylostella to insecticides

registered for control in South Africa.

73

Figure 3.1: The number of P. xylostella insecticide resistance reports from countries in

the world (IRAC, 2018).

Insecticides for control of P. xylostella is registered in 14 groups with more than 160

active ingredients worldwide (Table 3.1). This multitude of registrations indicates the

difficulty of maintaining a high susceptibility of P. xylostella for insecticides.

110

13 10 10 8

24

14

1 0 0 0 0 0 2

0

20

40

60

80

100

120

NO

OF

RES

ISTA

NC

E R

EPO

RTS

(A

RTI

CLE

S)

COUNTRIES

74

Table 3.1: Insecticide groups registered for control of P. xylostella worldwide with examples of active ingredients (IRAC, 2018).

Insecticide group

Group number

Active ingredients

Diamides 28 Chlorantraniliprole, Cyantraniliprole, Flubendiamide

Carbamate 1A Alanycarb, Aldicarb, Bendiocarb, Benfuracarb, Butocarboxim, Butoxycarboxim, Carbaryl, Carbofuran, Carbosulfan, Ethiofencarb, Fenobucarb, Formetanate, Furathiocarb, Isoprocarb, Methiocarb, Methomyl, Metolcarb, Oxamyl, Pirimicarb, Propoxur, Thiodicarb, Thiofanox, Triazamate, Trimethacarb, XMC, Xylylcarb

Organophosphate 1B Acephate, Azamethiphos, Azinphos–ethyl, Azinphos–methyl, Cadusafos, Chlorethoxyfos, Chlorfenvinphos, Chlormephos, Chlorpyrifos, Chlorpyrifos–methyl, Coumaphos, Cyanophos, Demeton–S–methyl, Diazinon, Dichlorvos/ DDVP, Dicrotophos, Dimethoate, Dimethylvinphos, Disulfoton, EPN, Ethion, Ethoprophos, Famphur, Fenamiphos, Fenitrothion, Fenthion, Fosthiazate, Heptenophos, Isofenphos, Isoxathion, Malathion, Mecarbam, Methamidophos, Methidathion, Mevinphos, Monocrotophos, Naled, Omethoate, Oxydemeton–methyl, Parathion, Parathion–methyl, Phenthoate, Phosalone, Phorate, Phosmet, Phosphamidon, Phoxim, Profenofos, Propetamphos, Prothiofos, Pyraclofos, Pyridaphenthion, Quinalphos, Sulfotep, Tebupirimfos, Temephos, Terbufos, Tetrachlorvinphos, Thiometon, Triazophos, Trichlorfon, Vamidothion, Pirimiphos–methyl, Imicyafos, Isopropyl O–(methoxyaminothio–phosphoryl) salicylate

Cyclodiene organochlorines

2A Chlordane, Endosulfan

Fiproles 2B Ethiprole, Fipronil

Pyrethroids 3A Acrinathrin, Allethrin, d–cis–trans Allethrin, d–trans Allethrin, Bifenthrin, Bioallethrin, Bioallethrin S–cyclopentenyl, Bioresmethrin, Cycloprothrin, Cyfluthrin, beta–Cyfluthrin, Cyhalothrin, lambda–Cyhalothrin, gamma–Cyhalothrin, Cypermethrin, alpha–Cypermethrin, beta–Cypermethrin, theta–Cypermethrin, zeta–Cypermethrin, Cyphenothrin [(1R)–trans– isomers], Deltamethrin, Empenthrin [(EZ)– (1R)– isomers], Esfenvalerate, Etofenprox, Fenpropathrin, Fenvalerate, Flucythrinate, Flumethrin, tau–Fluvalinate, Kadathrin, Pyrethrins (pyrethrum), Halfenprox, Phenothrin [(1R)–trans– isomer], Prallethrin, Resmethrin, Silafluofen, Tefluthrin, Tetramethrin, Tetramethrin [(1R)– isomers], Tralomethrin, Transfluthrin, Permethrin

Spinosyns 5 Spinetoram, Spinosad

Avermectin 6 Abamectin, Emamectin benzoate, Lepimectin, Milbemectin

Benzylureas 15 Bistrifluron, Chlorfluazuron, Diflubenzuron, Flucycloxuron, Flufenoxuron, Hexaflumuron, Lufenuron, Novaluron, Noviflumuron, Teflubenzuron, Triflumuron

Dicylhyydrazine 18 Chromafenozide, Halofenozide, Methoxyfenozide, Tebufenozide

Oxadiazines 22A Indoxacarb

Semicarbazones 22B Metaflumizone

Pyridalyl UN

Chlorfenapyr 13 Chlopenapyr

Bt 11 Florbac, Dipel

75

The aim of this study was to evaluate the susceptibility of P. xylostella populations from

two areas in South Africa, namely Nelspruit (Mpumalanga province) and Bapsfontein

(Gauteng province) to selected insecticides with different modes of action.

3.3 Material and methods

3.3.1 Plutella xylostella sampling

In 2018, large numbers of P. xylostella larvae were collected on canola at Nelspruit

(25°26'14.2”S; 30°59'34.6”E) and on cabbage in Bapsfontein (26°94'34.2”S;

28°52'96.4”E) and used to establish rearing colonies. More than 500 larvae were

collected from each locality. An identification code was assigned to each collected

population and used for reference thereafter, viz. Bapsfontein (B2018–01) and Nelspruit

(N2018–01). The larvae were transported to the laboratory immediately after collection,

they were provided with additional food to limit possible temperature and humidity stress

and to ensure that they had enough food available at all times.

3.3.2 Experimental design

Commercial formulations of spinosad (spinosyn), lambda–cyhalothrin (pyrethroid) and

flubendiamide (diamide) were used in leaf–dip bioassays. Field collected larvae (F0) were

used for the initial range finder bioassays of spinosad, lambda–cyhalothrin and

flubendiamide for both P. xylostella populations prior to the toxicological tests. The

remainder of the F0 larvae was used to establish the respective rearing colonies. Plutella

xylostella larvae were kept in plastic containers (40 x 20 x 15 cm) with aerated sides.

Moths were transferred from these containers to wooden screen cages (0.75 m (w) x 1 m

(h)) with potted cabbage plants for oviposition. Eggs were collected daily and kept in small

plastic containers (52 mm high, 30 mm in diameter) with steel mesh–infused lids, in a

rearing room at 26 ± 1 ºC, 65 ± 5% humidity and 14L:10D photoperiod. Once emerged

from the eggs, larvae were transferred into plastic containers similar to these described

above and reared on cabbage in a laboratory at 25 ±1 ºC.

The susceptibility of the two P. xylostella populations to selected insecticides was

estimated using the IRAC Susceptibility Test Method 018.

76

Three stock solutions were prepared for each insecticide tested (replicates). Eight

accurate dilutions (concentrations) were prepared from each stock solution and 0.2 gL–1

Triton X added as a wetter (Figure 3.2). The control treatment consisted of water and

Triton X (0.2 gL–1).

Figure 3.2: Dilutions of the insecticides with a wetting agent added (Triton X).

Brassica oleracea (cabbage) leaves were sampled from plants planted in pots. These

leaves were cut into smaller pieces, (±5 x 2.5 cm). For the bioassays, 32 well trays

(Frontier: scientific services) were used. The leaf pieces (24 per concentration) were

individually dipped in the insecticide concentrations for 10 seconds and placed on a drying

rack (Figure 3.3).

Figure 3.3: Leaves left to dry off before inoculation.

77

After the leaf pieces were dry, they were transferred to bioassay trays labeled with the

corresponding concentration. The bottom of each well in these trays was covered with

a thin layer of agar–agar to prevent leaf pieces from desiccation. One 3rd instar P.

xylostella larva was placed on the leaf piece per well. The trays were sealed with

transparent, ventilated lids (Figure 3.4).

Figure 3.4: Containers with a single Plutella xylostella larva on one leaf piece per well

placed on agar–agar and sealed with transparent, ventilated lids.

All treatment trays were kept in an incubator at 25 ± 1°C and a 14:10 L:D photoperiod.

Mortality was assessed after 72 hours for lambda–cyhalothrin and spinosad. The

assessment period for flubendiamide was after 96 hours.

3.3.3 Data Analysis

Data on P. xylostella larval mortality was subjected to probit analysis using PoloSuite®

software (version 1.8) (LeOra Software).

The software provides the slope, lethal concentrations (LC’s) and 95% fiducial limits (FL)

of the lethal concentrations (LC’s) and tests the dose–mortality response for linearity.

Mortality was corrected for control mortality by means of Abbott’s formula (Abbott, 1925).

The estimated LC80 and LC95 values were compared with the maximum recommended

field rate. The assessment of potential insecticide control failure was carried out using the

78

same experimental unit as described for the concentration–response bioassay, but

comparing this mortality caused by the recommended label rate of each insecticide (mid–

concentration from the registered range) and the lower threshold at LC80 estimated for

each compound (Silva et al., 2011). The 80% mortality was used as a reference because

this is the minimum level of efficacy required for registration of an insecticidal compound.

This is the minimum level of efficacy without control failure due to insecticide resistance.

The insect mortality caused by the label rate was considered to be significantly lower than

80% when the recommended rate was lower than the lower threshold of 95% fiducial

interval of the LC80 of the insecticide for the tested insect population (Silva et al., 2011).

The registered and recommended label rates of the three insecticides in South Africa are:

8 ppm for lambda–cyhalothrin (mid–dosage), 48 ppm for flubendiamide and 144 ppm for

spinosad on cabbage for P. xylostella.

3.4 Results

The probit analyses results of spinosad (spinosyn), lambda–cyhalothrin (pyrethroid) and

flubendiamide (diamide) tested for both P. xylostella populations (B2018–01 and N2018–

01) are presented in table 3.2. The responses fitted the log (dose)/probit (mortality) model.

3.4.1 Spinosad

The slope of the response line to spinosad from N2018–01 was higher compared to that

of B2018–01, resulting in a lower LC50 value for N2018–01 compared to B2018–01 (Table

2.2). The recommended label rate (144 ppm) was higher than the LC95 and application at

the recommended rate will therefore result in 100% mortality based on the estimates of

the probit model (Table 3.2). The recommended rate is 24 times higher than the LC95.

3.4.2 Lambda–cyhalothrin

The slopes of the response lines of the two populations to lambda–cyhalothrin ranged

from 1.45 (B2018–01) to 3.68 (N2018–01) and the LC50 from 5.53 to 6.55 for N2018–01

and B2018–01, respectively. The LC50, LC80 and LC95 for both populations were higher

than the recommended dosage of 4 ppm (Table 3.2), indicating possible control failure

for both populations by this insecticide.

79

3.4.3 Flubendiamide

The slope of the response line to flubendiamide from N2018–01 was 1.86 and for B2018–

01, 0.82. The two populations (N2018–01 and B2018–01) susceptibility to flubendiamide

indicated that the LC50 values do not differ significantly (P > 0.05): N2018–01 (0.79) and

B2018–01 (0.25) (Table 3.2). The slope of N2018–01 was 1.86 (SE = 0.17) and (SE =

0.09). The recommended label rate (48 ppm) was almost eight times higher than the LC95

for both populations and would therefore result in 100% mortality based on the estimation

of the probit model (Table 3.2)

80

Table 3.2: Log dose probit mortality for P. xylostella populations from Nelspruit (N2018–01) and Bapsfontein (B2018–01).

Active ingredient

Recommended dosage (ppm)

Population n LC50 FL 95% LC80 FL 95% LC95 (ppm)

χ2 df H Slope SE

Spinosad 216 B2018–01 360 0.61 0.45–

0.88 1.95 1.23–5.07 5.93 2.8 3 0.9 1.66 0.22

N2018–01 288 0.47 0.31–

0.84 1.09 0.66–4.22 2.41 2.5 2 1.0 2.32 0.26

Lambda–

cyhalothrin 8

B2018–01 360 6.55 5.67–

7.79 24.87 18.98–35.02 88.92 0.2 3 0.08 1.45 0.21

N2018–01 360 5.52 3.86.89 9.35 7.51–13.04 15.46 5.8 4 1 3.68 0.37

Flubendiamide 48 B2018–01 432 0.25 0.16–

0.38 2.70 1.65–5.51 25.92 2.4 4 0.6 0.82 0.09

N2018–01 360 0.79

0.59–

1.04 2.24 1.64–3.47 6.03 2.3 3 0.8 1.86 0.17

n = number of larvae tested; FL = fiducial limits; LC50, LC80 and LC95 in ppm

3.5 Discussion

There is no baseline data on the susceptibility of P. xylostella to insecticides currently

available in South Africa. There are also no susceptible rearing colonies maintained which

can serve as reference colonies for toxicological studies. The results from this study,

therefore serve as baseline data for P. xylostella susceptibility to spinosad (spinosyn)

lambda–cyhalothrin (pyrethroid) and flubendiamide (diamide) in South Africa. Both P.

xylostella populations (B2018–01 and N2018–1) were highly susceptible to spinosad and

flubendiamide.

For flubendiamide P. xylostella exhibited high mortality levels > 80% at a low dosage (2.7

ppm) (Table 3.2), while the recommended dosage is 48 ppm. Resistance levels of P.

xylostella established by Liu et al. (2015) contradict the present study findings with 48.17–

fold resistance for chlorantraniliprole (diamide) with chlorantraniliprole also included in the

diamide group of insecticides.

Results from a study by Neto et al. (2016) indicated P. xylostella resistance to

chlorantraniliprole, spinosad and chlorfenapyr in Bezerros, Recife and Alegre. The LC50 (mg

a.i. /L) for spinosad in Recife was 0.017 compared to 3.64 in Bezerros with a resistance ratio

of 50 up to 200–fold (Neto et al., 2016). The LC50 and LC90 values for chlorpyrifos,

deltamethrin and spinosad were reported for P. xylostella populations from Kara and

Dapaong in Togo and Cotonou in Benin (Agboyi et al., 2016). These populations were

compared to a population from Kenya in Matuu as a control. Resistance to chlorpyrifos was

5 to 15–fold at LC50 and 9 to 885–fold at LC90 compared to the control population.

Deltamethrin had a 13 to 59–fold at LC50 and 149 to 1772–fold resistance at LC90 compared

to the control population (Agboyi et al., 2016). These P. xylostella populations were,

however, highly susceptible to spinosad (Agoyi et al., 2016). The estimates of the current

study for both populations tested, is in accordance with the findings of this study.

Kang et al. (2017) collected a P. xylostella population in Korea that exhibited 16.3–fold

resistance to chlorantraniliprole. After laboratory exposure to chlorantraniliprole for one year,

the resistance level increased to 2,157‐fold. This showed the ability of this pest to develop

resistance to insecticides (Kang et al., 2017). In an evaluation of various insecticide groups,

Kang et al. (2017) determined cross–resistance with only one diamide (flubendiamide)

showing 5,910 fold resistance (Kang et al., 2017). This resistant population did, however,

not show any cross–resistance to indoxacarb, spinetoram, metaflumizone and spinosad

(Kang et al., 2017). In this study, no–cross resistance was established between lambda–

cyhalothrin and any one of the other two insecticides evaluated, viz. spinosad, and

flubendiamide.

Sereda et al. (1997) reported a significant increase in P. xylostella populations on cabbage

in South Africa since 1995. In 1996, most populations sampled showed significant resistance

levels to deltamethrin (pyrethroid), methamidophos (organophosphate) and methomyl

(carbamate). Control failure for five out of six populations to deltamethrin was indicated at

the recommended dosage (RD) and double the RD rate by Sereda et al. (1997). Both the

populations tested in this study, N2018–01 and B2018–01, are also at risk of control failure

by lambda–cyhalothrin with the LC50, LC80 and LC95 for both populations found to be higher

than the recommended dosage. Sereda et al. (1997) stated that fewer larvae died after a

deltamethrin application compared to methamidophos and methomyl applied at RD and

double the RD. Resistance to deltamethrin at the RD was established in two populations,

Ventersdorp and Bloemfontein with 0% mortality (Sereda et al., 1997). Resistance by P.

xylostella to a pyrethroid (deltamethrin) was reported in South Africa in 1996. Sereda et al.

(1997) also reported less than 100% mortality of P. xylostella at the recommended dosages

for other deltamethrin, methamidophos and methomyl.

The long–term application of pyrethroids affects the effectiveness of this group, even after

application ceased. This, in turn, makes the introduction of new pyrethroids in a spraying

program, even based on rotation, obsolete (Sun et al., 1986). Plutella xylostella remains a

persistent and major pest of Brassicaceae with resistance development to insecticides a

reality. Control of this pest is also costly (Neto et al., 2016, Nauen and Staeinbach, 2016,

Kang et al., 2017). Populations from Nelspruit and Bapsfontein are still susceptible to

flubendiamide and spinosad. It is important to follow proper control programs where

insecticides with different modes of action are rotated to ensure effective control of P.

xylostella populations in South Africa.

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Chapter 4: Conclusion and recommendations

Brassicaceae (Cruciferae) comprises of a large number of frequently consumed vegetables.

The agricultural and economic importance of these vegetables is ascribed to their high levels

of minerals, polyphenols, antioxidants, vitamins and glucosinolates (Aires, 2015). Insects

are the most important crop pest with more than 10 000 species damaging crops worldwide.

However, only 700 inflict serious damage to crops in storage and in the field (FAO, 2019).

One of these is P. xylostella causing crop losses of up to 90% on cabbage (Verkerk and

Wright, 1996). Plutella xylostella is regarded as the most destructive pest of Brassicas

worldwide (Verkerk and Wright, 1996), with an estimated economic impact of US$4–5 billion,

if yield losses and control cost for all Brassicas are taken into account (Zalucki et al., 2012;

Bradshaw et al., 2016). Resistance development to insecticides by P. xylostella (Talekar

and Shelton, 1993; Nyambo and Löhr, 2005; Ayalew, 2006, Odhiambo et al., 2010, Santos

et al., 2011, Zhou et al., 2011; Agboyi et al., 2013 and Legwaila et al., 2014) in combination

with food safety have motivated the development of alternative management strategies

(Hooks and Johnson, 2003; Shelton and Badenes–Perez, 2006). The main focus of cultural

control is trap crops (Hooks and Johnson, 2003, Shelton and Badenes–Perez, 2006). The

most proposed trap crops for P. xylostella is yellow rocket (Barbarea vulgaris)

(Brassicaceae) and Indian mustard (Brassica juncea juncea) (Brassicaceae) (Charleston

and Kfir, 2000; Badenes–Perez et al., 2004; Lu et al., 2004; Shelton and Nault, 2004;

Badenes–Perez et al., 2005).

Choice and development tests conducted in this study indicated the use of Brassica juncea

var. integrifolia (Giant red mustard) very closely related to the already available trap crop,

Indian mustard (Brassica juncea juncea) as a viable trap crop for control of P. xylostella.

Mustard can be used as a trap crop for P. xylostella in cropping systems where tatsoi, pak–

choi, kale and cabbage are cultivated, since it was preferred to these crops for oviposition

and feeding by the pest in this study. Better control of P. xylostella can be achieved if

mustard as a trap crop can be integrated as part of an IPM system at Pico–Gro where these

crops are planted in greenhouses. Planting of mustard between other brassica crops such

as tatsoi, pak–choi, kale and cabbage, can reduce the number of individuals on the latter

crops if the trap crop is removed after infestation occurred. The continued planting of trap

crops like mustard and removing of infested trap crop plants as part of an IPM system may

reduce the population below the economic injury level.

Modern agricultural technology has allowed farmers to keep up with the need for food

security of a growing world population. Increased yields of major crops are achieved with

good cultivation practices and by applying pesticides (WHO, 2019). Although research on

P. xylostella has been done since the early 1900’s (Gunn, 1917), no comprehensive

integrated control strategy could be implemented for successful control of this pest. Despite

the efforts to establish integrated pest management, control strategies rely mainly on

insecticide application (Grzywacz et al., 2010). The pest status of P. xylostella can, however,

mainly be attributed to insecticide resistance and the absence of natural enemies in the

areas where host crops are cultivated (Taleker and Shelton, 1993). Arthropods cause 18–

20% of crop losses worldwide valued at US$470 billion (Sharma et al., 2017). To ensure

food safety on a global scale more than 1000 pesticides are currently used worldwide

against crop pests (FAO, 2019). It is possible to produce foods without the use of pesticides

in certain instances, however, these instances are rarely found. Compared to global

standards, maximum application rates of pesticides and frequencies of application are not

adhered to by southern African farmers (Williamson et al., 2008).

Plutella xylostella was the first crop pest to become resistant to DDT in the mid–1900’s

(Ankersmit, 1953) and also the first insect pest with resistance to Bacillus thuringiensis

(Tabashnik et al., 1990). Since 1995, Sereda et al., (1997), reported a significant increase

in P. xylostella populations on cabbage in South Africa. By 2011, P. xylostella showed

resistance to most insecticides applied under field conditions in certain cropping areas

(Ridland and Endersby, 2011) and also to new insecticide groups, including diamides,

spinosyns and phenylpyrazoles (Gong et al., 2014). This is, however, not the case in the

Nelspruit and Bapsfontein cropping areas with P. xylostella still being highly susceptible to

flubendiamide (diamide) and spinosad (spinosyns). In contrast, high levels of resistance in

P. xylostella populations sampled from these areas was found to lambda–cyhalothrin.

The possibility of resistance development to insecticides prompts more research of other

control methods (cultural, biological) for P. xylostella. The only viable solution is an IPM

program to ensure susceptibility to insecticides by P. xylostella as long as possible.

Therefore, continued research, especially in developing countries such as South Africa

remains important.

4.1 Recommendations

In this study, the potential to use mustard as a trap crop in integrated pest management of

P. xylostella was confirmed. It can thus be recommended to farmers to be integrated into

their IPM Systems for control of this pest. This aspect of the study can be expanded to

include more potential trap crops. It can then be applied in an area–wide IPM system for

control in heavily infested areas. Evaluation of the susceptibility of P. xylostella populations

to all registered insecticide groups must be continuously done according to the

recommended application set out on the label of the selected insecticides. This information

of frequently used insecticides can then be used to identify shifts in susceptibility.

The integration of trap crops and application of insecticides with the high efficacy of control

of P. xylostella can contribute to reducing the number of insecticide applications per season.

It can also prolong the effective use of these insecticides for control of P. xylostella in South

Africa.

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