Seasonal variation of life history traits in the nymph-overwintering grasshoppers, Arphia sulphurea and Chortophaga viridifasciata.

Abstract

To understand the selective pressures that shape the phenology of a species, it is essential to know how variation in the timing of life cycle events affects individual fitness. In this research, I first describe how oviposition date affects offspring fitness in Arphia sulphurea and Chortophaga viridifasciata, two species of nymph-overwintering grasshoppers. I then use the observed seasonal changes in offspring fitness to test life history models concerning optimal offspring size and the size and age at maturity. Offspring that are produced late in the breeding season have a lower fitness, compared to early nymphs. This is primarily because they experience higher mortality during the winter. Overwinter survival is size-dependent, with larger nymphs having higher survival. Late nymphs have less time to grow before the onset of winter, and therefore do not survive as well. In addition, late nymphs produce fewer egg pods, because they have a shorter adult longevity. Current theory on optimal offspring size would predict that females of these two species should produce larger offspring later in the season, due to the seasonal declines in offspring fitness. For both A. sulphurea and C. viridifasciata, individual females produce larger offspring in successive egg clutches, an unusual result for insects. There are also increases in offspring size between early and late C. viridifasciata females. The seasonal declines in offspring fitness result in a strong selection for early reproduction. Under these conditions, slow growing individuals should mature at a smaller body size, in order to prevent delayed maturation. Experiments where the growth rates of juvenile A. sulphurea were varied by manipulating the thermal environment showed, however, that date of emergence is much more responsive to the effects of growth rate than is adult body size. Females show no differences in body size among the growth rate treatments, while males do respond to low growth rates with smaller body sizes. This may be due to different determinants of reproductive success for the two sexes: large size increasing fecundity for females and early emergence increasing access to females for males.