Meeting Abstract
49.9 Jan. 6 Ecological stoichiometry of Manduca sexta herbivory on Datura wrightii. BARKER, JENNIFER S.**; HUXMAN, T.; BRONSTEIN, J.; DAVIDOWITZ, G.; University of Arizona; University of Arizona; University of Arizona; University of Arizona jbhepcat@email.arizona.edu
Host plants can differ in quality among different habitats, among plants in the same microhabitats, as well as among leaves within an individual plant. The herbivorous insect Manduca sexta (tobacco hornworm: Sphingidae) obtains all its nutrients from its host plant Datura wrightii (jimson weed). It has been proposed that phosphorus (P) content in diet determines organismal growth rate (Growth Rate Hypothesis). We examined how C:N:P in the host plant D. wrightii translated into C:N:P in the herbivorous caterpillar M. sexta in a semi-natural environment that emphasized differences in the nutritional quality of the host plant. D. wrightii were subjected to one of the four combinations of high/low water and high/low nutrient treatments in either soil or sand substrate. The sand substrate stressed the plants, reduced their photosynthetic rates, and amplified treatment differences more than soil substrate. M. sexta hatchlings fed within a single treatment until pupation. Pupae were tested for C:N:P by mass spectrometry and colorimetric assay. Results show that the C:P and N:P ratios in the leaves varied widely among treatments but were confined to a narrow range in the insects. No association could be found between pupal mass or development time with P levels. A strong correlation between insect growth rate and P levels of plants grown in sand existed, but the correlation was not apparent for caterpillars that fed on soil-grown plants. This indicates that the growth rate hypothesis is most evident for animals under stressful conditions. C:N:P levels in the caterpillars� diets did not translate directly into caterpillar C:N:P. This lack of correspondence between leaf and caterpillar C:N:P is partly due to selective foraging behavior by the caterpillar.
