Effects of plant architecture on parasitism rates by Trichogramma

fasciatum on the eggs of the invasive Light Brown Apple Moth. Ervin Herrera, Julie Hopper* & Nicholas Mills*

University of California Berkeley, *Department of Environmental Science, Policy and Management.

Introduction

References

Methods Results/Discussion

• The Light Brown Apple Moth (LBAM), Epiphyas

postvittana, is an invasive pest species native to

Australia and was discovered in California in 2007

(Suckling & Brockerhoff, 2010).

• LBAM is considered a generalist in its diet resulting in

economic costs on a wide variety of crops (Venette,

2003)

• One type of pest control is using LBAM’s natural

enemies to control its population, also known as

biological control.

•One natural enemies is an egg parasitoid,

Trichogramma fasciatum. Female T. fasciatum oviposit

eggs into Lepidopteran eggs, such as LBAM .The

young utilize the host eggs as nutrition for development

into the adult stage.

•We observed the effects of plant architecture on T.

fasciatum’s ability to find and parasitize hosts.

•Plants we investigated; Apple, Grape, Hardenbergia,

Leptospermum, Manzanita, Penstemon, Plantain,

Raspberry, and Strawberry.

•Hypothesized that our data would be consistent with

previous studies that suggest higher plant complexity

leads to less parasitoid success.

Whole plants containing a single egg clutch were put into cages along with 20 female T.

fasciatum. Cameras on tripods were focused on the single egg clutch and set to record

for three hours.

A single leaf containing one egg clutch was

fastened in a small glass vial with water placed

inside of a plastic container. One female T.

fasciatum was put onto the stem of the leaf.

Containers were then sealed and observed. When

the female reached the egg, the time was recorded.

Acknowledgements Suckling, D.M. and E.G. Brockerhoff, “Invasion Biology, Ecology, and Management

of the Light Brown

Apple Moth (Tortricidae)”. Annual Review of Entomology, 2010. 55: p. 285‐30

Robert C. Venette, Erica E. Davis, Michelle DaCosta, Holly Heisler, & Margaret

Larson. “Mini Risk Assessment

Light brown apple moth, Epiphyas postvittana”

Department of Entomology, University of Minnesota, 2003

x20 females

At San Francisco State University we had six replicate

blocks, each containing nine different randomized plant

types that each had 10 LBAM egg clutches on their

leaves. These plants were picked up after one week and

observed for signs of parasitism.

Source: http://cisr.ucr.edu

Source: http://nature.berkeley.edu/millslab/lbam.html

Container Experiments

Cage Experiments

We left another set of plants. This time the plants

contained approximately 70 larvae each. We left the plants

at the field site for two weeks and observed the larvae for

signs of parasitism.

Field Experiments

Trichome Quantification

A 5 millimeter cross section of

each type of leaf was taken from

the midpoint. The number of

trichomes were then counted, on

both the lower and upper sides

divided by the surface area

(Image J). Figure 1

The trichome density per mm2 cross-section of the

leaves significantly varied among plant type

(Kruskal-Wallis, H = 68.70, df = 8, P < 0.0001).

Figure 2 Figure 3

Figure 4

Trichomes on the lower

side of Raspberry Leaf

Lack of trichomes on top of

Penstemon Leaf

Discussion

Comparison of parasitism rates among differing

plant types in our field experiment among six

replicate blocks in August 2010.

Increased trichome density increased the average time

it took T. fasciatum to reach the egg on the leaf, but

was not significant (ANCOVA, F= 0.002, df = 1, P =

0.967).

The success rate of reaching the host egg by T. fasciatum

significantly decreased with increasing number of leaves used

(leaves were variable so that surface area was held constant

among plant types) (Generalized linear model (binomial), F =

8.381, 71, P = 0.004). When modeled with plant type and total

trichome density, the number of leaves used and the plant type

did not significantly effect success rate, and increasing

trichome density decreased the success rate of T. fasciatum

reaching the egg clutch on the leaf (GLM (binomial), F = 3.13 1,

63, P = 0.08).

I would like to thank Julie Hopper for being the best mentor I could ever have asked

for this summer. I also want to thank everyone in the Mills lab who would answer all

of the questions that I had on my project and their projects. I want to thank Chris

Lever for being a great program director and Lynn Huntsinger because without her

none of this would have been possible. A big thanks to Nina Nim, my lab partner, for

sticking this through all the tediousness with me.

Comparison of time it took T. fasciatum to

reach egg clutches on certain plant types.

D.A. Andow and D.R. Prokrym, “Plant structural complexity and host-

finding by a parasitoid” Oecologia, 1990. 82: 162-165

D.M. Olson and D.A. Andow, “Walking pattern of Trichogramma nubilale

Ertle & Davis(Hymenoptera; Trichogrammatidae) on various surfaces”

Biological Control, 2006. 39: 329–335

Figure 5

• This research focused on how plant architecture effects host-finding by natural enemies. We utilized the natural setting of a

field as well as the laboratory with whole plants and leaves. Our findings support previous studies on both natural and

artificial materials that have determined increased plant complexity decreases the success rate and increases the search

time of an egg parasitoid for its host (Andow& Prokrym, 1990)

• Raspberry plants had the highest trichome densities while Penstemon plants had zero trichomes (Figure 1). Trichome

density could play an important part in how a single T. fasciatum because of their relative size. It is also noted that trichomes

can affect the walking patterns of Trichogramma in search of a host (Olson & Andow, 2006). There is a trend that an

increased trichome density increases the time it takes T. fasciatum to find their hosts (Figure 3).

• Parasitism rates were highest in our field apple plants (50%) followed by our grape and penstemon plants (both 33%). All

other plants showed no signs of parasitism (Figure 2). Future work should investigate why these plants had zero parasitism.

• The average time for T. fasciatum to reach the egg clutch in caged plants varied depending on the type of plant (Figure 4). I

would like to investigate what volatile differences in these plants cause Trichogramma to not find a host as quickly.

• It has been observed that higher plant structure complexity leads to a lower success rate for Trichogramma (Andow&

Prokrym, 1990). Another good proxy for plant complexity is the number of leaves on the plant. An increased number of

leaves lead to a trend of lower success of T. fasciatum of finding the eggs (Figure 5).

• The findings of this study suggests that T. fasciatum has higher success in finding hosts on simpler crops when being used

in biological control.