Are Caterpillars Evolving Through Feeding Behavior in Response to Parasitoids?

Abstract

Cabbage loopers (Trichoplusia ni) are herbivores that are commonly preyed upon by many different species of parasitoid wasps. These wasps reproduce by injecting its gestated eggs into 2-3rd instar caterpillars. After injection of wasp eggs, studies have shown some caterpillars will shift behavior in reaction to the injection. The result of the hatching eggs is termination of the caterpillar. I specifically look at this feeding behavior in the caterpillar after injection, using different phytochemically active plant species to show that there is a conscious attempt at self-medication from the caterpillars. This is extremely important as the trophic effect that herbivores have on the local plant competition, especially between native and non-native plant species, which create a domino effect on the entire ecosystem and its survival. We use two different phytochemically active plant species: Nasturtium officinale, a phytochemically rich plant, and Mimulus guttatus, a phytochemically benign plant. We then measured the percent consumed on a given leaf, before and after injection of a silica bead into 4-5th instar cabbage loopers, using different injection type groups totaling 65 caterpillars each assigned to one specific plant individual that it will feed on for the duration of the experiment. For the percent of leaf consumed, injection type was not significantly different. Caterpillars that were stabbed but not injected (SN) were significantly different than those that were not stabbed. Plant type and plant type with an interaction term of injection type were not significantly different in percent of leaf tissue consumed. Suggesting no preference in diet, or respective feeding behavior change.