Host-size-dependent sex ratios among parasitoid wasps: does host growth matter?

Abstract

Waage's (1982) hypothesis that host-size-dependent sex ratios will occur in parasitoids of nongrowing hosts and not in parasitoids of growing hosts is examined using published data on parasitoid wasps. Waage's hypothesis is supported as a general, but not absolute, rule: among solitary parasitoid wasps, a significantly greater proportion of parasitoids of nongrowing than of growing hosts show some evidence of host-size-dependent sex ratios (85% versus 42%, G=6.54, P< 0.05). The premise of Waage's hypothesis-that for parasitoids which develop in a growing stage, host size at oviposition is not a good predictor of the amount of resources available to the developing parasitoid-is also examined. It is suggested that across host species Waage's premise will hold for some, but not all, parasitoids of growing hosts. Likely exceptions to Waage's premise, and thus his prediction, are discussed. Parasitoids of growing hosts which are expected to have evolved hostsize-dependent sex ratios include parasitoids which utilize a narrow size range of host species, parasitoids which can distinguish among host species by some criterion other than size, and parasitoids which utilize host species whose susceptible instars do not overlap in size.

Using optimality models to improve the efficacy of parasitoids in biological control programmes

“…The host-quality model (Charnov et al, 1981) predicts that, if the response of sons and daughters to host quality differs, females should allocate to low-quality hosts the sex that suffers less. The fitness of females is generally more affected by host quality and therefore males are allocated to lowquality hosts as has been observed in several species (King, 1987;Cleary & van Ginkel, 2004) although exceptions have been reported (King, 1989;West & Sheldon, 2002). An important prediction of the host-quality model is that sex allocation in relation to host quality is not absolute but rather relative (West, 2009).…”

Sex ratios

“…In some species (idiobionts) females kill the host at oviposition (egg laying), and in such cases host size will be a reliable cue as to the resources that offspring will have available for development. However, in other species (koinobionts) the host is not killed by the female, and so can continue to grow, in which case host size at oviposition is a less reliable predictor of the resources that their offspring will have available for development (King, 1989). Consistent with the possible importance of environmental predictability, species in which the host was killed at oviposition (idiobionts) were more likely to show facultative sex ratio variation than species in which the host was not killed (koinobionts) (King, 1989).…”

Effects of host pupal age on host preference and host suitability in Brachymeria lasus(Walker)(Hymenoptera: Chalcididae).

“…Also, host age can have a greater effect on sex allocation by idiobiont than by koinobiont parasitoids, because host age is more important to the fitness of offspring in idiobionts (King, 1987(King, , 1989.…”