none Gregariously reared Mythimna separata Walker larvae consume 12% less maize foliage, live longer, and convert food less efficiently than larvae reared in isolation. Parasitism by Cotesia ruficrus Haliday reduces larval food consumption substantially, and estimates of reductions in population food consumption, based upon known levels of field parasitism and age at death, range from 54 to 69%. Both food consumption and age at death (measured by head capsule width) of parasitised larvae are positively related to the number of parasites emerging from the host. It is suggested that this feeding reduction may be an important factor in the successful biological control of M. separata in New Zealand, and that this aspect of the parasite-host interaction has received insufficient recognition in biological control theory and practice. Significant reductions in host larval feeding resulting from parasitism will reduced pest damage even in the absence of population suppression. However, parasites which increase food consumption may exacerbate pest problems.