The intensity of diapause has a distinct seasonal pattern in the Colorado potato beetle, Leptinotarsa decemlineata (Say). During August, temperature and photoperiod only slightly affected the very intense diapause in field populations. By summer's end, diapause intensity diminishes, but photoperiod and temperature maintain diapause. Subsequently, the beetles lose their responsiveness to photoperiod at high temperatures (approximately 200c), but at low temperatures (18-15°C) the beetles remain photoperiodically sensitive after emerg ing from the soil and initiating oviposition in the spring. Mated females that overwinter can lay fertile eggs without mating in the spring. Populations from climatically different areas in New York state (Riverhead, Long Island and Freeville, Upstate New York) have similar thermal thresholds (approximately 12°C) for reproductive development after dormancy. However, the Riverhead population has a less intense diapause and lower thermal require ments for initiating oviposition (K = 135 HDD12 (heat-degree daysl) than the Freeville population (K = 213 HDD,.). This diversity results in large differences in the timing of vernal emergence and oviposition at the two sites. The results suggest that natural selection acts on both the timing of emergence and oviposition after dormancy and on the variation in the occurrence of the events. EVOLUTIONARY ANALYSES of life histories generally are undertaken from demographic or genetic view points; seldom do they include physiological con siderations. However, physiological processes are the vital connection between the ecological expres sion of life history traits that determine fitness, and the components of population genetics that con stitute heritability and genetic covariances (Stearns 1983, Tauber & Tauber 1986). When physiological aspects are neglected, life history studies offer an incomplete and static view of the dynamic inter actions between populations, organisms, and the environment. Seasonality is a primary environmental variable that influences the expression of life history traits. The environmental cues that regulate reproduc tion, development, migration, and dormancy, and the physiological responses to these cues change with seasons. Thus, comparative studies of the eco physiological interactions between organisms and their seasonal environments give a vital dimension to the understanding of evolutionary processes. The Colorado potato beetle, Leptinotarsa de cemlineata (Say), is an excellent subject for eco physiological investigations of how life histories