Many insect species show intraspecific variation in their life-history patterns. Such variability may affect a wide range of traits including size, social behavior, sex ratios, mating tactics, and competition for resources; it may also affect population dynamics and interactions with parasites and prey. Trypoxylon politum has a seasonally variable, partially bivoltine life history (i.e., some wasps produce offspring that emerge before the end of June whereas others produce overwintering offspring) in the southern part of its range and a univoltine life history (one generation per year) north of central Virginia. Wasps from three partially bivoltine populations differ in the pattern of adult emergence, suggesting a latitudinal gradient in their responsiveness to photoperiod and other environmental cues affecting diapause. Wasp size, sex, the date on which the egg was laid, and rearing conditions also affect diapause and the timing of adult emergence. Few behavioral differences were found between wasps from Florida partially bivoltine populations and wasps from New Jersey univoltine populations, but their life histories and patterns of adult emergence differ markedly. When wasps from partially bivoltine populations are reared in the north (NJ), they emerge at the same time as local wasps. When wasps from univoltine populations are subjected to warm winter conditions (FL) they show a bimodal pattern of adult emergence: some emerge slightly later than wasps from partially bivoltine populations reared under the same conditions whereas others emerge at the same time as those developing in the north. This suggests variability in the effect of environmental cues on diapause and adaptation to local environmental conditions. The advantages of different emergence patterns are discussed in terms of their effect in avoiding unfavorable conditions such as freezing, and in taking advantage of favorable conditions such as synchronizing emergence with the occurrence of appropriate prey resources.
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Vol. 77 • No. 4