Environmental Causes of Variation in the Sex Ratio of an Arrhenotokous Insect,Dahlbominus fuliginosus(Nees) (Hymenoptera: Eulophidae)

Abstract

Unlike most other arrhenotokous species,D.fuliginosusfemales cannot control fertilization of their eggs and in an optimal environment the sex ratio of any one strain is constant. Variations in the sex ratio occurred when unfavourable environmental conditions prevailed before, during and immediately following mating. Beyond half their life span, males inseminated less than half the females they mated and the inseminated females produced fewer female progeny. Temperatures above 27 °C. during post-embryonic development sterilized otherwise functional males but had much less effect on females. Structural defects of either sex were rare. Mating was successful only within the normal range for optimal adult activity. Although females were inseminated by more than one male, the sex ratio of their progeny was not affected. Larval mortality from superparasitism above 65 larvae per host differentially favoured survival of males, heightening the incidence of pairing in nature at low host population densities. The percentage females was significantly reduced when oviposition was interrupted, or delayed by low temperature, providing a possible explanation for the numerous reported differences in the sex ratio between laboratory and different geographically located populations.