Euphytica 86: 95-102, 1995. 95 © 1995 KluwerAcademic Publishers. Printed in the Netherlands.

Banana weevil resistance and corm hardness in Musa germplasm

R o d o m i r o Ort iz , D i rk Vuyls teke , Bon i f ace D u m p e & R.S.B. Ferr is Plantain & Banana Improvement Program, International Institute of Tropical Agriculture (IITA ), High Rainfall Station, Onne, PMBO08 Nchia-Eleme Rivers State, Nigeria

Received 6 December 1994; accepted 22 May 1995

Key words: banana, breeding, corm characteristics, Cosmopolites sordidus, Musa, plantain, stemborer, weevil

Summary

Banana weevil (Cosmopolites sordidus Germar) is a major limiting factor in the cultivation of plantains and bananas (Musa spp. L.) in sub-Saharan Africa. The larvae damage the crop by tunneling in the corm. Chemical control is feasible but not sustainable, whereas host plant resistance is safe and has long term benefits. Banana weevil damage and infestation levels and corm hardness were assessed in Musa germplasm in order to determine the genetic control and potential mechanisms of resistance to this insect pest. Susceptibility/resistance to weevil was measured by the percentage coefficient of infestation (PCI) and damage in cross sections (CS) of the corm. All plantains were equally susceptible to the banana weevil, while a wild banana accession and some cooking and dessert banana cultivars showed high levels of resistance. Differential genotypic responses were observed in euploid plantain-banana hybrids. Segregation results suggest that host plant response to weevil in Musa is controlled by gene(s) exhibiting partial dominance towards the resistant parent and modifier genes with additive and dosage effects for susceptibility in the plantain parent. In natural banana germplasm, resistant clones showed increased corm hardness, as measured by a penetrometer in longitudinal and cross sections of outer and central corm tissues. This might suggest a non-preference mechanism for weevil resistance. However, the lack of correlation between corm hardness with PCI and CS scores in the segregating progenies suggested that other mechanisms may be more important in conferring resistance to banana weevil.

Introduction

An important insect pest affecting the perennial pro- duction of plantain and banana (Musa spp.) in Africa is the banana weevil (Cosmopolites sordidus Germar), whose larvae make tunnels in the corm or rhizome. This weakens the plant and results in reduced bunch weight or toppling of old plants before harvest (Vuylsteke et al., 1993a). Moreover, the damage caused by the larvae may be increased by attack of other pests, this acceler- ating the destruction of the corm tissues (Mesquita et al., 1984). The insect is native of the Indo-Malaysian region and has worldwide distribution throughout the tropics (Cuill6, 1950). It only attacks Ensete and Musa (Simmonds, 1959). Though it is possible to control the pest with insecticides (Stover & Simmonds, 1987), these are hazardous and expensive for the smallholders who grow most of the crop in Africa.

Biological control of banana weevil has been suggested as a potential intervention (Neuenschwan- der, 1988), because the weevil has natural enemies and at least 15 potential predators are known (Ost- mark, 1974). Ants (Tetramorium guinieensis, Cam- ponotus spp.), entomopathogens (Beauveria bassiana, Metarhizium anisopliae), larval parasites such as the dipteran Sarcodexia innota, and larval and egg preda- tors (Hololepta spp. and Alegoria dilatata) have been recommended for the control of banana weevil in Colombia and Cuba (Belalcazar, 1991; Roche & Abreu, 1983). Recently Walangululu et al. (1993) reported that powdered leaves of Tephrosia spp. had a repellent effect on adult banana weevils. Hence, they considered the insecticidal effect of this plant as a potential method to prevent oviposition at the pseu- dostem base. Traps to catch weevils seem impractical

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as a control measure (Stover & Simmonds, 1987) due to the labor costs associated with such intervention.

Host plant resistance offers a safe and long term control strategy for the banana weevil (Seshu-Reddy & Lubega, 1993) within the framework of integrated pest management (Sikora et al., 1989). Other useful attributes may be added to the desired cultivar through proper genetic manipulations.

Susceptibility to weevil varies within Musa (Mesquita et al., 1984), plantains (Musa spp., AAB group) being among the most susceptible culti- vars. Ityeipe (1976) mentioned that weevil infesta- tion ranged from very high in plantain and medi- um for 'Cavendish' banana to very low for diploid AA cultivars. Seshu-Reddy & Lubega (1993) showed that weevil development and survival was significantly different among East African bananas, with cooking and beer cultivars being on average more suscepti- ble than dessert types. M. balbisiana, donor of the B genome, seems to be resistant to banana weevil (Simmonds, 1959); however, Haddad et al. (1979) reported that AAB and ABB groups of cultivars had higher coefficient of infestation than AAA bananas. Significant differences between Musa cultivars in host response to weevil were established in field screening trials in Cameroon (CRBP, 1992), with AAB plan- tains being generally more susceptible than ABB and AAA bananas. A negative correlation between corm hardness and infestation rate led to the hypothesis of mechanical resistance to oviposition or larval develop- ment (Pavis & Minost, 1993).

This paper reports on field screening of diverse Musa germplasm under natural infestation of banana weevil and on the assessment of corm hardness as a potential resistance mechanism to this pest. Breed- ing schemes and selection methods for population improvement and cultivar development are proposed based on a genetic model which ensued from the anal- ysis of phenotypic segregation in euploid plantain- banana F1 hybrids.

Materials and methods

Five AAB plantain cultivars, namely 'Agbagba' (medi- um False Horn), 'Big Ebanga' (giant False Horn), 'Ubok Iba' (medium Horn), 'Bobby Tannap' (medi- um French), and 'Obino l'Ewai' (medium French), were assessed for banana weevil damage and corm hardness in a 6 year old field at the High Rainfall Sta- tion of IITA, Onne, Rivers State, southeastern Nigeria.

Similarly, plants of the triploid AAA dessert banana cultivars 'Yangambi Km. 5' and 'Valery', the diploid AA cultivars 'Pisang lilin' and 'SH-3362', the wild seeded non-edible AA banana 'Calcutta 4', and the triploid ABB cooking bananas 'Bluggoe' and 'Foug- amou', were screened for banana weevil resistance in ratoon crops in field of different ages. Additional- ly, 97 euploid hybrids derived from artificial crosses between the susceptible plantains 'Bobby Tannap' and 'Obino l'Ewai' with the highly resistant 'Calcutta 4' banana (Vuylsteke et al., 1993b) were evaluated for banana weevil attack and corm hardness in a 3 year old field. The euploid hybrids were either, in order of decreasing numerical importance, diploid, tetraploid, or triploid, the latter being produced very infrequently. The euploid hybrids were randomly planted in single row plots of 4 to 5 plants. All plots were surrounded by plants of the susceptible plantain cv. 'Agbagba', whose corms were evaluated to establish the distribu- tion of weevil infestation in the field and to compare its response to weevil with that of the euploid hybrids.

At harvest, tissue injury in the corm due to weevil damage was recorded using the percentage coefficient of infestation or PCI (Mitchell, 1978), which is a mod- ification of the coefficient of infestation (CI) technique developed by Vilardebo (1973). The PCI is basically a template conversion of external tunneling points into a percentage index representing the degree of corm infestation by the pest (Ogenga-Latigo & Bakyalire, 1993). This coefficient determines the extent of dam- age seen by superficial paring of the corms of harvest- ed plants. Data were taken with PCI templates which have 10 sections of 18 °. The template was placed in the center of the exposed area (5 cm below the base of the pseudostem) and presence (1) or absence (0) of weevil damage was recorded in both top (upper) and bottom (lower) parts of each section of 18 °. Hence, PCI provides a measure of pest infestation and may give information about oviposition preference.

Internal assessment of weevil damage in cross sec- tions (CS) was done in the same corms. Cross sections were cut at the base and 5 cm below the base of the pseudostem. Each cross section was divided into 3 con- centric circles (outer, middle and center) and each cir- cle scored as follows: 0 (no damage), 1 (slight damage or < 10% tunneled), 2 (moderate damage or 11-30%) and 3 (heavy damage or > 31%). CS scores might be regarded as a measure of insect feeding preference.

Fragments of the same corms were taken from the outermost parts and fibrous central parts and kept for measuring hardness. Measurements were taken within

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Table 1. Weevil infestation and corm hardness of plantains (Musa spp., AAB group) in a naturally infested 6 year old field, Onne, Nigeria

Cultivar Group Weevil score Corm hardness (N)

PCI 1 CS 2 CT 3 CL 4 OT 5 OL 6

'Bobby Tannap' medium French 12.5 0.8 18.7 :t: 0.97 22.4 -4- 2.4 18.2 20.24- 1.0

'Obino l'Ewai' medium French 10.0 0.5 12.04- 1.6 10.1 4-4.1 11.5 18.34- 1.8

'Agbagba' medium False Horn 10.7 0.8 16.4 -4- 0.6 13.5 4- 1.7 17.7 14.84- 0.7

'Big Ebanga' giant False Horn 14.0 1.2 17.6 4- 1.1 15.3 4- 2.9 17.9 17.0 -I- 1.3

'Ubok Iba' medium Horn 13.6 0.5 15.1 4- 1.6 14.0 4- 4.1 17.2 13.1 4- 1.8

Differences among cultivars NS NS * * NS **

I PCI = percentage coefficient of infestation. 2 CS -- weevil damage in cross sections on a scale from 0 (no damage) to 3 (heavy damage). 3 CT = transversal measurement in central corm fragment. 4 CL = longitudinal measurement in central corm fragment. 50T = transversal measurement in outer corm fragment. 6 OL = longitudinal measurement in outer corm fragment. 7 Mean -t- standard error. NS, *, ** indicate non-significant or significant differences between cultivars at c~ = 0.05 or 0.01, respectively.

1 hour after collection in the field. Quadrants arcs of approximately 2.5 cm thickness and 3.5 cm circumfer- ence were excised using a blade to obtain a fiat uni- form surface from transverse sections of corm tissue. Hardness was assessed using a digital force gauge or penetrometer (Lloyd Instruments DFG50, Hampshire, England), which measured corm hardness in Newtons (N = 105 dynes). Three transveral and three longitu- dinal random measurements were taken on each corm fragment.

One-way analyses of variance were used for sta- tistical analysis of the results for plantain cultivars and euploid hybrids. Phenotypic correlations (Beck- er, 1975) based on clonal means of the euploid hybrids were calculated to determine the extent of association between banana weevil damage and infestation levels and the degree of corm hardness as measured by the penetrometer. Chi-square tests for independence were carried out to determine whether frequency distribu- tions of banana weevil scores (PCI and CS) were equal between ploidy levels and among half-sibs of the same ploidy.

Results

Plantain cultivars showed a similar weevil infestation level (Table 1), while some banana cultivars, such as 'Yangambi Km. 5' and 'Pisang lilin', and the wild banana 'Calcutta 4' were highly resistant to this

pest (Table 2). The cooking bananas 'Fougamou' and 'Bluggoe' were rated as highly resistant to this pest whereas the dessert banana 'Valery' and 'SH-3362' , an improved diploid (from Fundacion Hondurena de Investigacion Agricola), were rated as partially resis- tant and susceptible, respectively (Table 2). There were significant differences in corm hardness, except in transversal measurements of outer sections, among equally susceptible plantain cultivars (Table 1). In the banana germplasm, resistant clones showed increased corm hardness (Table 2).

Significant differences were observed among euploid plantain-banana hybrid progenies for banana weevil damage and corm hardness (Table 3). Genotyp- ic variation was calculated as the percentage of total sum of squares in the analysis of variance. Most of the variation for host-plant response to the pest was accounted for by the genotypes (Table 3). There was a 75% incidence of weevil damage in 'Agbagba' but weevils were evenly distributed across the 3 year old field where this susceptible cultivar was grown with the euploid hybrids.

The means and standard errors of PCI and CS scores of 'Agbagba' in this field were used to develop a pest severity scale, based on confidence intervals at 5% (CI0.05), for evaluation of the euploid hybrids to banana weevil damage and infestation levels. Clones with severity ratings significantly greater or less than the mean of Agbagba at the 5% level were considered to be very susceptible (> CI0.05) or partially resistant

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Table 2. Weevil infestation and corm hardness of bananas in naturally infested fields of at least 3 years old, Onne, Nigeria

Cultivar Group I Weevil score Corm hardness (N) PCI 2 CS 3 CT 4 CL 5 OT 6 OL 7

'Calcutta 4' AAw 0.0 0.0 19.4 4- 1.0 19.2 4- 1.0 20.3 4- 0.8 19.2 4- 0.7 'Pisang lilin' AAc 0.0 0.0 21.6 4- 1.1 21.3 4- 1.0 25.9 4- 1.0 21.1 4- 0.8

'Yangambi Km. 5' AAA 0.0 0.0 21.0 4- 1.5 19.9 4- 1.0 21.1 4- 0.8 20.7 4- 0.7

'Bluggoe' ABB 0.0 0.0 21.6 4- 1.5 21.8 4- 1.5 21.6 4- 1.2 17.8 4- 1.1 'Fougamou' ABB 0.0 0.0 24.7 -I- 1.1 21.5 4- 1.0 21.6 4- 0.9 19.2 4- 0.8

'Valery' AAA 2.54- 1.08 0.14-0.1 16.24- 1.0 15.64- 1.0 17.64-0.8 15.94-0.7

'SH-3362' AAi 7.04- 1.0 0.64-0.1 14.84- 1.0 15.04- 1.0 16.44-0.8 14.9-t-0.7

LSD0.05 2.8 0.3 2.7 2.7 2.2 2.0

1 AAA = triploid dessert banana, AAc = cultivated diploid banana, AAw = non-edible wild banana, AAi = improved diploid banana of the Fundacion Hondurena de Investigacion Agricola, ABB = cooking banana. 2 PCI = percentage coefficient of infestation. 3 CS = weevil damage in cross sections on a scale from 0 (no damage) to 3 (heavy damage). 4 CT = transversal measurement in central corm fragment. 5 CL = longitudinal measurement in central corm fragment. 60T - - transversal measurement in outer corm fragment. 7 0 L = longitudinal measurement in outer corm fragment. 8 Mean 4- standard error.

Table 3. Weevil infestation and corm hardness of euploid plantain-banana hybrids in a naturally infested 3 year old field, Onne, Nigeria

Genotypes Genome Weevil score Corm hardness (N) PCI l CS 2 CT 3 CL 4 OT 5 OL 6

'Agbagba'

Euploid hybrids

Differences among euploid hybrids

Variation accounted

by genotypes (%)

AAB 6.5 4- 0.87 0.5 4- 0.09 16.3 4- 0.5 15.0 4- 0.5 17.7 4- 0.7 16.4 4- 0.6

AAB × AAw 1.9 4- 0.8 0.2 4- 0.02 18.0 4- 0.1 17.4 4- 0.1 20.6 4- 0.2 16.5 -4- 0.6

64 57 49 52 48 45

1 PCI = percentage coefficient of infestation. 2 CS = weevil damage in cross sections on a scale from 0 (no damage) to 3 (heavy damage). 3 CT = transversal measurement in central corm fragment. 4 CL = longitudinal measurement in central corm fragment. 50T = transversal measurement in outer corm fragment. 6 0 L = longitudinal measurement in outer corm fragment. 7 Mean 4- standard error. ** indicates significant differences between euploid hybrids at a = 0.001.

(< CI0.o5) (Tab le 4). C l o n e s w i t h b a n a n a weevi l sever-

i ty r a t i ngs s im i l a r to t hose o f the pa ren t ' C a l c u t t a 4 '

we re ra ted as h i g h l y res i s tan t .

T h e n u m b e r o f d i p l o i d and p o l y p l o i d c lones eval-

ua ted pe r f a m i l y and the i r r eac t ion to b a n a n a wee-

vi l are l i s t ed in Table 5. C h i - s q u a r e tests for inde-

p e n d e n c e i n d i c a t e d tha t g e n o t y p i c r e s p o n s e to b a n a n a

weev i l was d i f f e ren t b e t w e e n p lo id i e s bu t no t b e t w e e n

ha l f - s ib s at the s a m e p l o i d y level (i.e., c o m p a r i n g

' O b i n o l ' E w a i ' x ' C a l c u t t a 4 ' vs ' B o b b y T a n n a p ' x

' C a l c u t t a 4 ' hybr ids ) , excep t for c ross sec t ion d a m -

age scores a m o n g d ip lo ids . O n ave rage p o l y p l o i d s

(e i the r t e t rap lo ids or t r ip lo ids ) were m o r e su scep t i b l e

to b a n a n a weevi l t han the i r d ip lo id half - or fu l l -s ibs .

B o t h b a n a n a weev i l scores , P C I and CS, p r o v i d e d

very s imi la r i n f o r m a t i o n r ega rd ing g e n o t y p i c r e s p o n s e

to b a n a n a weev i l as s h o w n by the h i g h l y s ign i f i can t

p h e n o t y p i c co r re l a t ion (Table 6). Th i s sugges t s tha t

Table 4. Scale for grouping euploid plantain-bananahybrids based on host-responses to banana weevil infestation of the susceptible plantain cultivar 'Agbagba' and highly resistant bananas in 3 year old fields at Onne, Nigeria

Host-plant response PCI l CS 2 Statistics 3

Very susceptible > 8.02 > 0.68 > CIo.o5

Susceptible 4.92-8.02 0.32-0.68 CIo.o5 of Agbagba

Partially resistant 0.01-4.91 0.01-0.31 < CIo.o5 Highly resistant 0.00 0.00 Scores of 'Calcutta 4'

or 'Yangambi Km. 5'

Agbagba mean -4- s.e. 6.47 5:0.79 0.50 5:0.09

i PCI = percentage coefficient of infestation. 2 CS = weevil damage in cross sections on a scale from 0 (no damage) to 3 (heavy damage). 3 Based on confidence intervals (CI) at c~ = 0.05 (C10.05).

Table 5. Banana weevil borer damage and infestation values in euploid plantain-banana hybrids derived from crosses between the plantain cultivars 'Obino l'Ewai' (OL) and 'Bobby Tannap' (BT) with the wild banana 'Calcutta 4' (C4)

Ploidy/Cross Highly Partially Susceptible Very x2(P) resistant resistant susceptible

Percentage of coefficient of infestation (PCI)

2x OL × C4 7 10 1 1 2x BT × C4 33 21 0 1

Among diploids

Poly-x OL × C4 1 6 6 5

Poly-x BT x C4 0 4 0 1

Among polyploids

Between 2x and polyploids

Weevil damage in cross sections (CS)

2x OL x C4 9 4 4 2

2x BT × C4 38 15 1 1

Among diploids

Poly-x OL × C4 2 3 7 6

Poly-x BT x C4 0 3 1 1

Among polyploids

Between 2x and polyploids

5.62 (0.13)

4.00 (0.26)

36.78 (< 0.001)

1 1 . 6 4 (< 0.01)

4.00 ( 0 . 2 6 )

32.57 (< 0.001)

99

on ly o n e m e a s u r e m e n t m a y suffice to assess b a n a n a

weev i l d a m a g e in seg rega t ing popu la t i ons . Moreover ,

s o m e h y b r i d s w i t h low PC I va lues had h i g h CS scores

w h i l e the r eve r se d id no t occu r (da ta no t shown) .

Hence , i t m a y b e m o r e app rop r i a t e to assess weevi l

a t tack o n l y in c o r m cross sec t ions becaus e it assesses

in t e rna l damage .

P e n e t r o m e t e r m e a s u r e m e n t s in l ong i t ud i na l and

t r ansve r sa l sec t ions o f ou te r and cent ra l c o r m t issues

were h i g h l y cor re la ted (Table 6). There fore , several

m e a s u r e m e n t s in d i f fe ren t par t s o f the c o r m m a y no t

be r equ i red to assess c o r m hardness . T h e coef f ic ien ts

o f va r ia t ion o f m e a s u r e m e n t s f r o m the cen t ra l c o r m

were lower than those o f the ou te r c o r m t i ssues (da ta

no t s h o w n ) sugges t i ng tha t the f o r m e r s h o u l d b e used

to assess c o r m h a r d n e s s in M u s a g e r m p l a s m . However ,

p h e n o t y p i c cor re la t ions were no t s ign i f ican t b e t w e e n

co rm ha rdnes s m e a s u r e m e n t s and b a n a n a weev i l scores

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Table 6. Phenotypic correlations (p) in plantain-banana euploid hybrids between measures of banana weevil infestation1 and corm hardness 2

CS CT CL OT OL

PCI 0.714 - 0.015 - 0.105 0.107 0.029 ***3 ns ns ns ns

CS 0.043 0.019 0.118 0.097 ns ns ns us

CT 0.933 0.798 0.767

CL 0.766 0.767

OT 0.851

i Measured by the percentage coefficient of infestation (PCI) and weevil damage in cross sections (CS), and corm hardness. z Measured by a penetrometerin transversal (CT) or longi- tudinal (CL) central corm fragments, and transversal (OT) and longitudinal (OL) outer corm fragments. 3 ns, *** indicate that p was non-significantly or signifi- cantly different from zero at c~ = 0.001, respectively.

(PCI and CS) in neither plantains (Table 1) nor on plantain-banana euploid hybrids (Table 6). This means either that a clone resistant to attack of Cosmopolites sordidus may not have a hard corm or that a clone with a soft corm will not always be susceptible to banana weevil.

Discussion

Genetic analysis of host-plant resistance to banana weevil

Analysis of frequency distributions of genotypic response to weevil based on the PCI and CS scores observed in segregating euploid Fl ' s (Figs la and lb, respectively) suggested that gene(s) exhibiting partial dominance towards the highly resistant parent 'Cal- cutta 4' and additive effects of modifier genes for sus- ceptibility in the plantain parents, may control banana weevil resistance in Musa. The shift towards suscepti- ble responses in polyploid hybrids could be the result of dosage effects (Stern, 1929) of the genes for suscepti- bility in the triploids and tetraploids. Indeed, the devi- ation from expected phenotypic expression in Musa polyploids seems to be due to dosage compensation towards an effect proportional to the gene dose. This

has also been reported for host responses to sigatoka diseases in banana (Vakili, 1968) and plantain poly- ploid hybrids (Ortiz & Vuylsteke, 1994a). Alternative- ly, the faster ratooning of diploids as compared to poly- ploids (Ortiz & Vuylsteke, 1994b) could reduce plant stress to this pest because the followers (or ratoons) are subjected to weevil attack for a shorter period (Mesqui- ta et al., 1984). Such improved suckering behaviour should not be considered as true resistance to banana weevil but as an evasion mechanism of the host plant (Smith, 1989).

Breeding for host-plant resistance to banana weevil

Significant variation in PCI and CS scores among euploid hybrids (Table 3) suggests that genetic improvement for host-plant resistance against weevil is feasible. Phenotypic selection based on weevil damage scores in cross sections of the corm may suffice to select clones for further multiplication and testing in infest- ed fields across environments before cultivar release. However, any population improvement scheme aiming at an increase in the frequency of favourable alleles for banana weevil resistance requires progeny testing due to the partial dominance exhibited by this trait. Selfing may increase the frequency of alleles for resistance in a specific $1 genotype and allows the determination of the parental genotype. As such, the potential breeding value of a specific clone could be assessed. For exam- ple, the plantain-derived diploid hybrid ' 1206-1' was selfed and highly or partially banana weevil resistant $1 progeny were recovered (Ortiz et al., unp. results). Hence, this clone has a high breeding value and may be selected as parent for a population improvement scheme.

The breeding value of the susceptible plantain par- ents were also determined by progeny testing. The fre- quency distributions of CS scores were significantly different between 'Bobby Tannap' and 'Obino l 'Ewai ' derived diploids (Table 5), with the former being less affected by the pest. Similarly, 'Bobby Tannap' had rel- atively more resistant derived polyploid hybrids (60%) than 'Obino l 'Ewai ' (28%). However, the plantain cul- tivars themselves had similar reactions to the banana weevil (Table 1). This shows that susceptible plantains may have different breeding values for banana weevil resistance, which are not always associated with their phenotypic values. Hence, a population improvement scheme such as recurrent selection should be based on combining ability or $1 progeny tests.

No. of clones 40

30

'Calcutta 4'

I n Diploids [ ] Polyploids

101

20

10

AAB plantains

0 ~ High. Resistant Part. Resistant Susceptible Very Susceptible Genotypic response

(a)

No. of clones 50

40

30

20

10

0

'C~-cutta 4'

High. Resistant Part. Resistant

AAB plantains

S Susceptible Very Susceptible

I N Diploids I [ ] Polyploids

Genotypic response

(b) Fig. 1. A. Frequency distribution of genotypic response to attack of Cosmopolites sordidus, as measured by the percentage coefficient of infestation (PCI) in the corm, in euploid plantain-banana hybrids and their parents ('Calcutta 4' and AAB plantains). Mean values of parents are indicated in arrows; B. Frequency distribution of genotypic response to attack of Cosmopolites sordidus, as measured by weevil damage in corm cross sections (CS), in euploid plantain-banana hybrids and their parents ('Calcutta 4' and AAB plantains). Mean values of parents are indicated in arrows.

Mechanisms of host-plant resistance to banana weevil

Morphological, physiological or other traits of the host plant may affect the insect population by negative- ly affecting its biology or reducing the severity of attack (Smith, 1989). Two types of insect resistance are known: antixenosis (non-preference) and antibio- sis (Painter, 1958), and should be combined to develop

plantain and banana cultivars with potential durable resistance to banana weevil attack.

Eggs of Cosmopolites sordidus are to a large extent laid at ground level in the corm and the pseudostem base (Koppenh6fer, 1993). Hence, PCI scores may provide information about host plant resistance to ini- tial infestation because it measures surface tunneling. However, selection based on PCI scores masks the real extent of corm infestation and damage because eggs

102

are also laid below ground and rarely in exposed posi- tions (Froggat, 1925). Indeed, some clones with low PCI had high CS scores, indicating that a susceptible genotype could be scored as resistant based on PCI only. Also, selection based on CS scores is important because resistance to weevil in the corm surface does not limit the extent of internal damage.

Attraction of the pseudostem to adults was not used as a criterion of resistance to weevil because no cor- relation was found between attraction and infestation (Pavis & Minost, 1993). Similarly, the lack of phe- notypic correlation between corm hardness and host plant resistance in the plantain-banana hybrids (Table 6) ruled out this physical barrier as a non-preference type resistance mechanism to banana weevil. This also indicates that corm hardness cannot be used as a quick and reliable parameter for evaluation of banana weevil resistance. Further investigations of resistance mecha- nisms in the corm should consider either the absence of essential nutrients for the banana weevil or compounds which inhibit its development.

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