Document: JAM-3-43-15

Gregarious feeding by a specialist insect herbivore increases host plant susceptibility.

FORDYCE, J.*

University of California, Davis, CA 95620 ¹

Abstract:
Local adaptation to host plants by insect herbivores can result in physiological mechanisms to overcome plant defenses or behavioral modifications to circumvent plant defenses. The California pipevine swallowtail, Battus philenor, lays eggs in larger clutches than observed in other populations. In California, females lay 12 to 14 eggs per clutch and larvae feed gregariously in early instars. In other populations studied, the average clutch size ranges from two to six. Here I present evidence that a modification in this life history trait is a response to local hostplant characteristics in California. Increased clutch size and gregarious feeding by California pipevine swallowtails on its only available hostplant, the Dutchman's pipe (Aristolochia californica), facilitates increased larval growth. The observed increase in larval growth rate reduces the time larvae spend in the first instar, when they are most vulnerable to insect predators. The increase in larval growth rate is mediated by plant responses to increased herbivore loads and not the result of group feeding stimuli. These data indicate that plant responses to herbivory are important influences on the life history evolution of insect herbivores. The plant response to herbivory induced by large group feeding gregariously also deters herbivory by later emerging clutches on the same plant. This response is observed within 24 hours of initial herbivore damage. Thus, plant responses to herbivory may mediate competitive interactions among conspecifics.

Keywords: Battus philenor, Aristolochia californica,herbivory, gregarious feeding, induction, plant defenses, life history evolution

This abstract is being presented at: 11:45 AM in session:
Oral Session #26: Invertebrate Herbivore - Plant Interactions.