Maternal Effects in Gypsy Moth: Only Sex Ratio Varies with Population Density

A number of species of forest caterpillars fluctuate in density with a periodicity of 8–11 yr. One explanation for these ‘‘cycles’’ is that changes in food quality or quantity and crowding influence the growth rate and final size of moths at high density. Carryover of these influences to the next generation through maternal effects could modify the dynamics of the population. To determine whether development, survival, pupal size, fecundity, or sex ratio varied among offspring of gypsy moths from highand low-density populations, we collected eggs from three sites where moths had been at low density and three sites where moth density had been high for several years. We reared caterpillars hatching from these eggs in the laboratory under controlled conditions. Sex ratio was the only characteristic to vary in a consistent way with the density of the maternal population. Egg masses from low-density sites produced significantly more females than those from high-density sites, for which the numbers of males and females were equal or slightly in favor of males. The female bias of the sex ratio of low-density populations increases the potential rate of increase of the population and may arise from local mate competition when populations are sparse. The reduced rate of increase in high-density populations associated with the higher production of males could delay the recovery of populations following decline. Whether the sex-ratio deviation observed in low-density populations persists should be studied in the future.