A swallowtail butterfly (Papilio glaucus) is an exceptional insect herbivore because of the breadth of its diet (polyphagy) and extent of geographic distribution. Evidence suggests that this is allowed by genetic variability within and among P. glaucus populations. Parallel investigations of host quality have also indicated important intraspecific variability. The array of host plants encountered by P. glaucus at 3 sites (southern Michigan, northern Michigan, and interior Alaska) will be compared. Variablility between sites, between host species within sites, and between trees within host species will be compared using bioassays. Variability of full-sibling families across 7 sympatric host species. Reciprocal transplants of P. glaucus populations among the 3 sites will allow simultaneous tests for geographic differentiation between insect populations and host populations. Collectively, these experiments test the constancy of insect-plant interactions. The hypothesis that average characteristics of the insect and plant species are poor predictors of interactions between individual plants and individual insects will be tested. As a counterpoint, abiotic environments encountered by P. glaucus at each site will be contrasted, and localized adaptation of the insect to different thermal regimes will be tested. Patterns of genetic variability in insect traits surveyed here will provide information about evolutionary lability of important physiological attributes, and may separate environmental factors that lead to local adaptation from those that constrain distributions.
