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 influ
ences to the next generation through maternal effects could modify the
dynamics of the population. To determine whether development, surviva
l, pupal size, fecundity, or sex ratio varied among offspring of gypsy
moths from high- and 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 h
atching from these eggs in the laboratory under controlled conditions.
Sex ratio was the only characteristic to vary in a consistent way wit
h 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 sligh
tly in favor of males. The female bias of the sex ratio of low-density
populations increases the potential rate of increase of the populatio
n and may arise from local mate competition when populations are spars
e. The reduced rate of increase in high-density populations associated
with the higher production of males could delay the recovery of popul
ations following decline. Whether the sex-ratio deviation observed in
low density populations persists should be studied in the future.