Size-mediated performance of a generalist herbivore feeding on mixed diets

摘要

Mixed diets can allow generalist insect herbivores to obtain nutritionally balanced resources or dilute toxins from specific foods, but also present the generalist with greater challenges in decision-making and require a greater ability to detoxify a wide range of plant defensive compounds. Young and small generalist larvae can have different nutritional requirements, ability to detoxify compounds, and mobility compared to older and larger larvae. In this field study, I asked how larval size affected performance of the woolly bear (Platyprepia virginalis), a generalist herbivore, on a uniform diet of bush lupine (Lupinus arboreus) compared to a mixed diet including bush lupine. Large larvae bad greater survival on the mixed diet treatment compared to the lupine-only diet, but survival of small larvae did not vary with diet. Larval size also influenced growth on each diet, but this effect varied with year. In 1997, large larvae had higher growth on a lupine-only diet compared to a mixed diet, whereas small larvae had equivalent growth on both diets. In 1998, larvae of each size did not differ in their response to diet treatments. In the field, large larvae apparently eat a more diverse diet than small larvae, which contrasts with the growth result for 1997. This suggests that factors other than growth, Such as parasitism or predation, might influence choice of host plants.