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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 emerging 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 requirements for initiating oviposition (K = 135 HDD12 [heat-degree days]) than the Freeville population (K = 213 HDD12). 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.