The Effect of the Timing of Post-Diapause Egg Development on Survival, Growth, and Body Size in Gryllus pennsylvanicus

Abstract

A trade-off between early mortality and reproduction may influence the evolution of the timing of post-diapause development in insects that exploit seasonal environments. To test this prediction, we manipulated hatching date experimentally in three successive years within a population of Gryllus pennsylvanicus, a univoltine cricket that overwinters in the egg stage. Each year, cohorts of nymphs derived from eggs maintained under identical conditions in the laboratory were simultaneously raised in the laboratory and in the field from where the population was collected. In the field, nymphal mortality was high early in the season, but was independent of hatching date in the laboratory under favorable rearing conditions. This suggests that the low temperatures encountered by the nymphs early in the season had a detrimental effect on survival. Growth rate and adult body size decreased with a delay in hatching date in the laboratory, which suggests that delayed hatching had a direct effect on the timing and rate of subsequent life history processes. The nymphs in the laboratory developed under a long and constant photophase indicating the onset of summer, whereas nymphs in the field experienced a declining photophase. Such a difference in photoperiod apparently induced a plastic response in growth rate, with nymphs growing faster under the declining than the constant photophase. It also appears that the declining photophase reduced the degree-days required for metamorphosis in the cohorts, and induced a decline in adult size with development time within each cohort. In either case, such an accelerating effect on metamorphosis implies a decrease in fecundity with a delay in hatching date. Therefore, these findings provide evidence for the hypothesis that a trade-off between early mortality and reproduction influences the evolution of the timing of post-diapause development at this locality.