Title

The oviposition response of the imported willow leaf beetle (Plagiodera versicolora): Does mother really know best?

Date of Award

5-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Advisor(s)

James S. Coleman

Second Advisor

Larry L. Wolf

Keywords

Plagiodera versicolora, Oviposition, Willow leaf beetle

Subject Categories

Ecology and Evolutionary Biology | Entomology

Abstract

This dissertation addresses whether the oviposition responses of the female imported willow leaf beetles ( Plagiodera versicolora ) maximizes individual lifetime fitness. Rather than addressing this question on the scale of across habitats, species, or individuals, I chose to investigate the female's oviposition response on a plant as a function of different leaf stages differing in suitability as oviposition and larval growth sites.

The first series of observations examined the distribution of eggs in the field in relation to availability of leaf developmental stages. Females place a disproportionate number of eggs on the mature leaves compared to the younger ones. In the laboratory, larvae fed younger leaves grow faster and bigger than those fed mature leaves. This presents an interesting anomaly.

Experiments in the field examined the success of eggs placed on different leaf developmental stages. Traditionally, some measure of time, size, and/or survival is utilized to approximate the offspring's ultimate success and the mother's fitness. Using all these variables, I conclude that offspring success is most influenced by oviposition site in the egg stage. Eggs placed on mature and older leaves have a higher probability of hatching than those placed on other leaf stages. This higher hatching success more than compensates for the below average growth rate and survival experienced by early instars in comparison to their growth rate on younger leaf stages.

The final series of experiments examined factors that influence the probability of an egg hatching and attempted to quantify the advantage gained by eggs on mature leaves over other stages. Egg predators, Podisus spp., concentrate searches on the upper leaves, therefore mature leaves are less likely to be searched. Upper leaves experience lower humidities than those at the base of the plant. Eggs have a higher probability of hatching at higher humidities (>67%).

The results of this study question the assumptions upon which most oviposition studies are based. Questions include the validity of some estimates of fitness such as adult or larval size, testing of just the larval stage as an indicator of offspring success, and the focus on nutritional value of the host with the exclusion of other possible selective factors. These assumptions are shown not to hold true, bringing into question some of the conclusions from the existing body of literature.

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