Does brood size in field populations of <i>Microctonus aethiopoides</i> Loan vary with host availability?

<p dir="ltr">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 <i>Microctonus aethiopoides</i> Loan in New Zealand in 2006 for control of <i>Sitona obsoletus</i> 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 <i>M. aethiopoides</i> as a biological control agent in New Zealand.</p><p dir="ltr"><br></p>