Oviposition Patterns and Hostplant Suitability: Parsnip Webworms and Wild Parsnip

Abstract

-The parsnip webworm (Depressaria pastinacella) is an oligophagous herbivore with limited powers of dispersal in its larval stages. Because there is substantial variation between individual wild parsnip hostplants (Pastinaca sativa) in the ability to support larval growth, we investigated patterns of egg deposition within a wild parsnip population in order to determine if ovipositing females are capable of recognizing phenotypes that confer high larval survivorship. The numbers of eggs laid on 38 plants in a two-M2 plot during a 16day period were tallied, as were the numbers of larvae surviving to the ultimate instar on the same plants. As indices of plant size, basal stem circumference and final plant biomass were measured. Partial regression analysis revealed that plant mass was a better predictor of larval success than was the number of eggs laid on a plant. Ovipositing females did, however, lay more on larger plants, a behavior that improves the chances of offspring survival. That plant size is the principal determinant of egg deposition patterns was suggested by a laboratory experiment demonstrating that females lay eggs in direct proportion to the area of oviposition substrate. Thus, the density of eggs per unit area of substrate was constant. While leaf area was not measured in the field, a similar relationship between egg numbers and plant size was observed; the number of eggs per gram of plant tissue was not significantly different between plants with above-average mass and plants with below-average mass.