POPULATION DIFFERENTIATION IN EUPHYDRYAS EDITHA BUTTERFLIES: LARVAL ADAPTATION TO DIFFERENT HOSTS.

Abstract

Adaptation of phytophagous insects to novel larval host plants can involve changes in at least three different suites of characters: those associated with female oviposition behavior, with larval feeding behavior, and with larval digestive physiology (Dethier, 1954, 1970; Bush, 1974; Feeny, 1975). Differences between closely related insect species and between conspecific populations indicate that evolutionary change in oviposition behavior is common (Singer, 1971; Smiley, 1978; Jones and Ives, 1979). Moreover, similar differences in larval growth rates and survivorship when different species or populations are reared on the same host plant indicate that larval adaptation often accompanies changes in oviposition behavior (Smiley, 1978; Futuyma et al., in press). However, few investigations have explicity attempted to separate the contributions of differences in larval feeding behavior and digestive physiology to observed differences in growth performance. Because both the number and type of characters involved in larval adaptation will presumably influence the probability that a newlyfounded insect population will successfully adapt to a novel host before becoming extinct, it seems desirable to characterize the types of changes that occur during adaptation of a population to its host plants. This study was conducted to provide such a characterization for the checkerspot butterfly, Euphydryas editha. In western North America, E. editha exists as a series of discrete populations (White and Singer, 1971; Ehrlich et al., 1975). In most populations, females oviposit on only one or perhaps two different plant species in the family Scrophulariaceae or the genus Plantago (Plantaginaceae), even though other species used by other populations may be available. Although populations differ both in acceptance responses of females to a standard array of host species (Singer, 1971, and pers. comm.) and in electrophoretically detectable patterns of enzyme variation (McKechnie et al., 1975), little is known about variation among populations in characters associated with larval adaptation. Although populations are normally highly localized and dispersal between them is low (Ehrlich, 1961; Ehrlich et al., 1975), population extinction and refounding by occasional migrants is probably fairly common (Ehrlich et al., 1980). Moreover, because populations specializing on one host species interdigitate geographically with populations specializing on others (White and Singer, 1974), it is likely that the founding of a new population often involves colonization of a novel host. A comparison of populations using different host species may elucidate the types of evolutionary changes that accompany such colonization. In this study I compare the growth performances of larvae from two E. editha populations that use different hosts. Specifically, I ask (1) Do larvae from different populations exhibit differential adaptation to their own host plants, as measured by growth rates and survivorship? and (2) If so, are observed differences due to differences in digestive physiology, feeding behavior, or both?