Sex Ratio Adjustment by a Koinobiotic Parasitoid Lysiphlebus delhiensis (Subba Rao & Sharma) (Hymenoptera: Aphidiidae) in Response to Host Size

Koinobiotic parasitoids are said to be unresponsive to host resources as measured by external host dimensions and, therefore, are unable to allocate sexes of progeny differentially into smaller versus larger hosts. The present paper examines the effect of host size on the sex allocation by a koinobiotic parasitoid Lysiphlebus delhiensis (an aphid parasitoid). It tests the following hypotheses, assumed in host size model: (1) a greater proportion of females will emerge from larger hosts, (2) survival of the developing parasitoid stages differ between the sexes in relation to host size and (3) fitness of female offspring is better in large hosts. The results demonstrate that L. delhiensis deposits fertilised (diploid) and unfertilised (haploid) eggs differentially in response to host size. The female laid more haploid eggs in unsuitable host stages (I, II and IV instar nymphs, and adult apterous), than in suitable host size (III instar nymphs). However, perception of host size by females was not absolute and was dependent on the amount of temporal variation in the host size distribution and on the previously experienced host size. The measurement of certain performance criteria of the progeny of L. delhiensis emerging from small versus large hosts (developmental period, survival, mating ability, fecundity and OSR) indicates that the host size related characters have a greater effect on the fitness of the female than male progeny. No evidence of differential mortality of the sexes of L. delhiensis in small versus large hosts was observed. This implies, therefore, that while ovipositing in growing stages of the host, L. delhiensis exhibits maternal adjustment of OSR as a function of host size. By adjusting the sex ratio of the progeny in response to host size the female maximises her fitness without knowing about the future host quality.