PARENT SESSION
Oral Session # 53: Herbivory IV: Communities, Populations, and Genetics.
Presiding: JA Rudgers
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 103.
Community-level impacts of induced plant responses to herbivory.
Van Zandt, Peter1, 1 University of Toronto, Toronto, ON, Canada
ABSTRACT- Numerous forces affect herbivorous insect communities, but the role that induced plant responses play in altering community structure has been little studied. To better understand the potential for early season herbivory to affect subsequent herbivore species richness, I conducted field experiments over two years involving the community of milkweed herbivores. Plants were either undamaged (controls) or damaged by one of three herbivores: monarchs (Danaus plexippus), milkweed leaf beetles (Labidomera clivicollis), or weevils (Rhyssomatus lineaticollis). Following treatment, naturally colonizing herbivores were counted on all plants for two months to assess host plant usage and herbivore species richness. In 2001, monarch damaged plants had 26% greater species richness, while weevil damaged plants had 17% lower richness than undamaged plants. In 2002, herbivory by monarchs reduced subsequent insect species richness on host plants by 33%, but weevil damaged plants did not differ from controls. In 2001, monarch treated plants exhibited induced susceptibility, with over 6 times more damage compared to controls, while in 2002, weevil damaged plants received nearly 4 times more damage. In general, colonizing herbivores were less likely to be found on weevil treated plants in 2001, whereas in 2002 they were attracted to weevil treated plants. The net effects of initial weevil treatment were unaffected by subsequent plant damage, while the effects of monarchs dissipated in strength as natural herbivore damage accumulated. This suggests that induced responses to initial weevil feeding persisted through the season while monarchs were more likely to affect colonization via alterations in the trajectory of community assemblage. Therefore, induced plant responses to herbivory can substantially alter herbivore community structure, but the nature of the effects are likely to depend on the identity of the initial herbivore.
Key words: bioassay, diversity, Asclepias, priority