Genetic and Phenotypic Differentiation as a Consequence of Host Plant Use by Lepidopteran Herbivores

Abstract

In this dissertation, I focused on the role of plant hosts as a driving force leading to phenotypic and genotypic changes in insect herbivores. There are three main questions addressed: (1) Do generalist species' populations have broad diet breadth or do they represent a mosaic of sub-populations, each having narrow diet breadths? (2) How do host plants affect the immune response of polyphagous herbivores? and (3) Do host plants or host plant such as allelochemicals, alter the interaction between herbivore defense and parasitoid counter-defense?

Do generalist species' populations have broad diet breadth or do they represent a mosaic of sub-populations each having narrow diet breadths?

In Chapter 1, I determined, using amplified fragment length polymorphisms (AFLPs), whether host plant-associated genetic differentiation (HAD) was exhibited by a suite of polyphagous tree feeding macrolepidoptera. I determined this by using polyphagous species that exhibit traits expected to be important in the formation of genetically divergent sub-populations.

How does host plant affect the immune response of polyphagous species?

In Chapter 2, the objective was to examine the effect of host plant species on the immune defenses of polyphagous lepidopteran herbivores, specifically the intensity of encapsulation measured as percent melanization, of three common forest Lepidoptera species.

In Chapter 3, I discuss and assessed the potential role of immune responses in insect outbreaks. I present a brief background on immune responses, discuss the methods used to experimentally measure the components associated with immune response and how immune response varies. Lastly, I draw on the studies available and present several potential hypotheses to stimulate further research.

Does host plant, or some aspect of host plant such as allelochemicals, alter the interaction between herbivore and parasitoid?

In the final chapter, I explored the ecological consequences of viral-plant allelochemical interactions. The objective of this study was to use a model system, Manduca sexta and Cotesia congregata, to directly test the effect of the allelochemical nicotine and the presence or absence of polydnavirus (PDV) on larval immune responses. PDV allows the parasitoid egg to escape encapsulation (an herbivore defense against parasitism).