Specificity of Adaptation to a Novel Host Plant be a Seed Beetle

Adaptation to one novel host plant may simultaneously improve an insect\u27s performance on other unfamiliar hosts, as a kind of cross-adaptation. In selection experiments using an Asian population of the seed beetle Callosobruchus maculatus (Fabricius) (Coleoptera: Chrysomelidae: Bruchinae), larval survival in lentil seeds increased from about 1 to \u3e90% in fewer than 30 generations, and acceptance of lentil by egg-laying females increased two- to three-fold. We examined whether rapid adaptation to lentil altered beetle performance on other grain legumes, including hosts more closely related to lentil than to the ancestral host, mung bean. Three replicate, lentil-adapted lines were compared to the mung-bean (control) line with respect to both host acceptance (oviposition under no-choice conditions) and larval performance (survival to adult emergence, development time, and adult mass at emergence). In most experiments, females from the lentil lines laid more eggs on unfamiliar hosts than did females from the mung-bean line. Greater oviposition on novel hosts could not be explained by variation in potential fecundity, and did not appear to depend on a host\u27s relatedness to lentil. Although survival in lentil remained extremely divergent between the mung-bean and lentil lines (0 vs. \u3e90%), the lines did not differ in larval performance on two other novel hosts (pea and fava bean) that are much more closely related to lentil than to mung bean. Because larval performance is most likely the limiting factor in the potential colonization of a novel host by C. maculatus, our experiments did not provide strong evidence for cross-adaptation. The results suggest that adaptation to even a highly marginal host need not lead to a general expansion of an insect\u27s host range