PARENT SESSION
Oral Session # 65: Herbivory V: Deer, Geese, and Chemistry.
Presiding: K Bjorndal
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 103.
Successionally driven changes in leaf nutrients and spatial patterning of herbivory on lupines at Mount St. Helens.
Apple, Jennifer*,1, Bishop, John1, Fagan, William2, 1 Washington State University-Vancouver, Vancouver, WA2 University of Maryland, College Park, MD
ABSTRACT- Previous work demonstrated that several lepidopteran herbivores of lupines (Lupinus lepidus) in primary successional habitat on Mount St. Helens exhibit similar distributions with respect to host plant density, with damage concentrated at the sparse margins of high-density (core) patches or in lower-density (edge) patches. We focus on one of these herbivores, the cutworm Euxoa sp. (Noctuidae), to determine whether host plant tissue quality is responsible for the observed negative density-dependent pattern of herbivory. Larvae hatched from eggs laid by field-caught females were reared on leaves collected from either core or edge lupine patches. Caterpillars fed leaves from core plants were significantly delayed in their development: after 8 weeks, they were about 33% smaller than larvae fed edge tissue. Eventually, larvae on core diets achieved comparable maximum weights as larvae on edge diets. Euxoa larvae also exhibited four-fold higher mortality on diets of core leaves during the first 2 weeks of development. Comparisons of C, N, and P content of the core vs. edge leaves comprising the experimental caterpillars' diets revealed that edge plants provide a higher quality food source for larvae, with elevated %P and lower C:P and N:P elemental ratios. In addition, we found that the nutrient content of both core and edge leaves declined significantly over the course of the summer. These results suggest that the nutritional content of plants available to early instar Euxoa larvae is critical in determining their growth trajectory and survivorship, especially since late instar larvae will be subject to deteriorating food quality. Thus, successionally driven bottom-up processes that produce density-dependent variation in host plant quality are likely responsible for the spatial distribution of Euxoa at Mount St. Helens.
Key words: herbivory, nutrients, primary succession, lupine