Inversely Density-Dependent Parasitism: The Role of Plant Refuges for Hosts

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Abstract
(1) The response of a parasitoid species to host density was investigated for a host species concealed by plant tissues. The parasitoid was Pteromalus sp. and the host was a stem-galling sawfly. Euura lasiolepis on the willow, Salix lasiolepis. (2) For three consecutive years in natural populations the parasitoid responded in an inversely density-dependent manner to host density, as density varied among patches of hosts on willow clones. (3) There was a systematic positive relationship between mean gall diameter per willow clone and host density, such that hosts in high population patches were better concealed from parasitoids than hosts in low population patches. The size of the refuge from parasitoid attack increased as host density increased. (4) The negative relationship between mean gall diameter per willow clone and percentage parasitism was significant in 1984, and when the 3 years of data from this study were combined, and when all data for seven generations of sawfly were grouped (1979-185). (5) Plant traits and other concealing factors are important in defining the response of parasitoids to host density and may play a role in mechanistic and functional explanations of such responses.