Some Factors that Distort the Sex Ratio of the Gypsy Moth Porthetria dispar (L.) (Lepidoptera: Lymantriidae)

Abstract

AbstractDuring a study on the population dynamics of the gypsy moth, Porthetria dispar (L.), conducted in the Town of Glenville, New York, some factors were found to affect the sexes differentially. The importance of this differential mortality is indicated by the fact that 78 per cent of the variation in the logarithm of an index of population trend (the ratio of population density from year to year) was associated with the logarithm of adult sex ratio.Disease and desiccation during instars IV-VI and among pre-pupae were strongly selective against the female insects. This differential mortality caused a change in the pupal sex ratio from about 70 per cent females where no disease occurred to less than 25 per cent female pupae following an epizoötic. Ichneumonids, on the other hand, usually killed more male pupae than females, except when host size was reduced by excessive larval density and competition. The net result from this series of factors that distort the sex ratio has been to produce adult sex ratios varying from more than 80 per cent female moths to only 2 per cent females.In this host species, as in most other animals, it seems that the population consequences of a mortality factor that kills the host sexes in different proportions should be evaluated in terms of the more critical (female) sex destroyed.