The influence of food supply on the survival and oviposition ability of Cotesia plutellae, the larval parasitoid of the diamondback moth, Plutella xylostella, was examined in the laboratory. The longevity of the parasitoid decreased drastically under a lack of sugar availability. The oviposition ability, evaluated by functional response to host density, was highly suppressed by a lack of food supply. Thus, sugar availability is a very important condition for the effective utilization of C. plutellae as a biological control agent of P. xylostella in greenhouses.