Does brood size in field populations of Microctonus aethiopoides Loan vary with host availability?

The release of a gregarious parasitoid as a biological control agent in a relatively simple grassland ecosystem provides a rare opportunity to observe the outcomes of parasitoid density dependent oviposition behaviour in the field. A retrospective analysis was undertaken of monthly host population data collected over the three years following the release of a parthenogenetic and facultative gregarious Irish genotype of the endoparasitoid Microctonus aethiopoides Loan in New Zealand in 2006 for control of Sitona obsoletus Gmelin in pasture. Brood size ranged from one to seven parasitoids/host, with a mean of 1.63 ± 0.04, and was not influenced by locality, host gender or host weight. It was positively correlated with percent parasitism and to a lesser degree with host abundance. Scramble competition varied with brood size, but not host weight, and resulted in 33% larval mortality at prepupae emergence. Size and fecundity of adults from broods increased with increasing host weight up to a threshold of 4 mg host resources/parasitoid larva. Above this level, size and fecundity stabilised. A simple fitness model suggested that optimum brood size varies with host availability from 1, when the number of available hosts/ adult parasitoid is 65 or greater, to 3 when hosts/parasitoid is 29 or fewer. The ability to vary brood size with host abundance to maximise reproductive efficacy contributes to the observed efficacy of Irish M. aethiopoides as a biological control agent in New Zealand.