BEHAVIOR OF THE PARASITOIDPLEOLOPHUS BASIZONUS(HYMENOPTERA: ICHNEUMONIDAE) IN RESPONSE TO CHANGES IN HOST AND PARASITOID DENSITY

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Abstract
A synthesis of observations on density-dependent interactions in parasitoid populations is attempted, working from the particular aspects of discrimination against parasitized hosts and detailed observations on parasitoid behavior, to more general aspects of parasitoid ecology, dispersal, distribution, and response to host populations. Females ofPleolophus basizonus(Gravenhorst) are able to discriminate against parasitized hosts ofNeodiprion swaineiMiddleton between narrow limits (mean densities of 0.4 to 1.4 eggs per cocoon) set by the probability of a female finding an unparasitized host. They show mutual interference in egg laying at high parasitoid:host ratios, and a density-dependent escape reaction to adult parasitoid density. These behaviors all result from the recognition of repellent trail odors left by searching females. Response of individual females to high parasitoid densities may therefore lead to dispersal, the occupation by the population of more, but not so favorable habitats, and an apparent density-independent response to host populations.