Cohorts of mating pairs of newly-emerged first-generationLeptinotarsa decemlineata(Say) adults were established each week on potato foliage to examine seasonal changes in reproductive and diapause activity and to determine when diapause induction occurred in the field in western Massachusetts. The percentage of newly emerged females that oviposited before entering the soil was much higher in the 25 July cohort (81.8%) than in the 1 August cohort (8.3%), indicating that diapause induction occurred by early August in this region. Population data supported this finding. Differences in within-season population dynamics over a 6-yr period (1981–1986) were evaluated with respect to the role of timing of arrival of colonizing adults and diapause induction. Because oviposition consistently dropped off by mid-August due to diapause induction, the extent of second-generation populations was most influenced by the timing of oviposition by first-generation females. Substantial differences in the abundance of second-generation populations relative to that of first-generation resulted from differences in time of colonization by overwintered adults and subsequent emergence of first-generation adults. A temperature-dependent phenological model corroborated the importance of diapause and time of colonization in the phenology and population dynamics of this insect. Implications for pest management are discussed.