Host preference and sex allocation of three hymenopteran parasitoid species (Ichneumonidae and Braconidae) of a longicorn pest, Oemona hirta (Fabr.) (Col., Cerambycidae)

The host preference and sex allocation of two ichneumond parasitoids, Xanth ocryptus novozealandicus (Dalla Torre) and Campoplex sp., and one braconid parasitoid, Apsicolpus hudsoni Turner of the longicorn borer, Oemona hirta, were studied in New Zealand. The size of the borer larvae attacked by the parasitoids can be determined by measuring the gallery width of the borers. These parasitoids are idiobionts and their secondary sex ratio is female-b iased. Females of these species are significantly larger than males, and th e females allocate the sex of offspring depending on the size of host larva e, by laying female-producing eggs on significantly larger host larvae. Xan thocryptus novozealandicus is larger in body size and shorter in ovipositor length than Campoplex sp. and A. hudsoni, and consumes late instar larvae or prepupac in shallow galleries whereas the latter two utilize early to mi ddle instar larvae in deep galleries. Campoplex sp. and A. hudsoni preferre d host larvae in significantly larger tree twigs than X. novozealandicus.