VARIATION IN PHOTOPERIODIC RESPONSE WITHIN AND AMONG SPECIES OF MILKWEED BUGS (ONCOPELTUS)

Diapause is an important component of many insect life histories. First, it allows escape in time and hence the avoidance of unfavorable conditions; and second, it interacts with developmental rates to synchronize life cycles, influence patterns of voltinism, and regulate seasonal phenologies (Tauber and Tauber, 1976). Recent studies have shown that the trait displays considerable environmental and genetic variance (cf. papers in Dingle, 1978a). Environmentally diapause is frequently a function of a temperature-photoperiod interactions, and genetically it often displays high proportions of additive genetic variance (Dingle et al., 1977), although Mendelian segregation of diapause from nondiapause with dominant gene action has been reported from several species (Tauber and Tauber, 1977; Lumme, 1978; Vepsiiliiinen, 1978). In many species photoperiodic diapause displays considerable ecogeographic variation which contains both environmental and genetic components (Danilevskii, 1965; Dingle et al., 1977; Dingle, 1978a). Typically there is clinal variation in the critical photoperiod necessary to induce dormancy. Usually this occurs along a north-south gradient, but instances are known of longitudinal and altitudinal variation as well (Bradshaw and Lounibos, 1977), reflecting adaptation to local climatic conditions. Other aspects of diapause such as intensity and rate of emergence may also show geographic variation and local adaptation (Dingle, 1978a). Varying selection pressures along clines apparently maintain high proportions of additive genetic variance for diapause related traits (reviewed in Dingle et al., 1977; Tauber and Tauber, 1978). Determination of these selective forces constitutes an important problem in evolutionary biology, not least because diapause is so intimately related to life history strategies and therefore to fitness (Dingle, 1968, 1974a; Istock et al., 1976; Dingle et al., 1977; Istock, 1978; Solbreck, 1978). Prompted by these considerations, we present here a study of variation in photoperiodic influences on age at first reproduction within and among species of milkweed bugs (Oncopeltus). This is one of a series of papers on life history variation in these insects; flight and body size are discussed in Dingle et al. (1980), size dependent timing of metamorphosis in Blakley and Goodner (1978), and other papers are in preparation. The genus Oncopeltus is of additional interest because adult reproductive diapause is concurrent with migration in the North Temperate migrant 0. fasciatus, demonstrating interaction with a further escape mechanism available to insects (Kennedy, 1961; Southwood, 1977; Dingle, 1978b; Solbreck, 1978). The wide distribution of the genus Oncopeltus in the New World (O'Rourke, 1977) allows comparison of a number of species and populations with respect to photoperiodism and environmental variables influencing seasonal phenologies. It is also possible to examine historical factors because some species have evidently evolved on Caribbean islands, while others have only recently been introduced as 1 Present address: Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado 80309.