PARENT SESSION
Oral Session #66: Plant-Animal Interactions: Webs and herbivory.
Presiding: J.N. Holland
Wednesday, August 7. 1:00 PM to 4:45 PM. Apache Meeting Room, TCC.

Plant-plant-herbivore interactions: phenotypic plasticity in response to plant competition affects herbivores of common milkweed.

AGRAWAL, ANURAG^*,1, ¹ University of Toronto, Toronto, Ontario, Canada

ABSTRACT- Phenotypically plastic responses to one environmental cue may have profound consequences for other members in the community (i.e., trait-mediated indirect effects). I studied the effect of grass-competition on the interaction between native milkweed plants and their herbivores. Early season competition for light from grasses resulted in 30% taller, 50% thinner milkweed plants that received 60% less herbivory by a specialist stem-inhabiting weevil. As observational and experimental data show that weevils prefer to oviposit on thicker stems, the reduction in herbivory on plants subject to competition was likely mediated by competition-induced thinner stems. Late season plant competition for underground resources reduced milkweed height by 15% and more than doubled herbivory by adult cerambicid beetles. As this monophagous (on milkweed) beetle requires grass stems for oviposition, the enhanced herbivory on milkweed plants subject to competition was likely a result of available resources (grass stems) for oviposition. Thus, competition with grasses resulted in altered milkweed phenotypes and properties of the neighborhood that affected herbivory. Given that most plants experience and respond to both competition and herbivory, I predict that diverse ecological consequences of binary interactions are a general outcome in natural communities.

KEY WORDS: plant-herbivore interactions, phenotypic plasticity, stem elongation response, milkweed herbivores