Opportunistic predation and offspring sex ratios of cicada-killer wasps (Sphecius speciosus Drury)

1. The way in which hunting of prey affects the sex ratio of the predator's offspring is not well understood. Female cicada-killer wasps are convenient for study because they specialise in capturing cicadas to provision their offspring. Cicada prey are nearly twice as heavy as the wasps that carry them, hence some degree of prey selectivity by the wasps is to be expected. It has been suggested that wasps bias their offspring towards females by foraging selectively for female prey, whereas there is some evidence that sex ratios are actually male-biased. This study was designed to establish the connection between foraging and offspring sex ratio. 2. Three, non-exclusive, hypotheses of selective predation were tested. The frequency of predation on different classes of prey in conjunction with their availability was estimated by intercepting the wasps on their way to their nests and by sampling cicadas in the environment. The hypothesis of selective predation was not supported; predation appears to be opportunistic and non-selective. Cicada prey weight was not a simple linear function of wasp weight, although the smallest wasps were constrained to carry small prey. 3. Wasp offspring (larvae) were excavated from subterranean nests and found to be male-biased (3 : 1 or 4 : 1) in 2 years. The observed ratios are close to expectation from Fisher's equal-investment model, taking account of sexual size dimorphism, and are evidently unrelated to the sex of the prey. A simple binomial probability rule of sex allocation provides a behavioural mechanism for producing the observed sex ratio of offspring.