Variation in the Nutritional Physiology of Tree-feeding Swallowtail Caterpillars

A key problem in addressing patterns of interaction between forest insects and their host trees is determining the level at which important ecological and evolutionary interactions occur. We commonly view plant-herbivore relations as herbivore species interacting with plant species, tacitly assuming that variation among members of either species is small and one can represent their interaction by the average characteristics of the insect and the tree species (Ehrlich and Raven 1964, Feeny 1976, Rhoades and Cates 1976, Scriber and Feeny 1979). This simplistic view is violated if insects within a species differ markedly in their ability to grow on a particular host, or if trees within a species differ substantially in quality as perceived by the insect. If variation within both insect and host species is large and pervasive, then insect-plant interactions may be best studied at the level of genotypes and populations (Edmunds and Alstad 1978, Fox and Morrow 1981, Thompson 1988a, Ng 1988, Karban 1989). Clearly intraspecific variation exists. For example, potato beetles, checkerspot butterflies, tortoise beetles, autumnal moths, and tiger swallowtails are each comprised of populations that differ in their ability to use key hosts (Hsiao 1978, Rausher 1982, 1984, Haukioja and Hanhimaki 1985, Scriber 1986a, Hare and Kennedy 1986). Likewise mountain birch, cottonwood, honeylocust, and a variety of other trees are comprised of populations that differ in their suitability for specific herbivores (Hanover 1980, Haukioja and Hanhimaki 1985, Pakash and Heather 1986, H e m et al. 1987). Even within populations, individual trees may be highly variable (Edmunds and Alstad 1978, Whitham and Slobodchikoff 1981, Smiley et al. 1985, Ayres et al. 1987, Whitham 1989). The emerging problem is to quantify variation within and between populations relative to variation at other levels.