SPATIAL DENSITY DEPENDENCE - EFFECTS OF SCALE, HOST SPATIAL PATTERN AND PARASITOID REPRODUCTIVE STRATEGY

The parasitoid assemblage of the goldenrod gall moth, Gnorimoschema gallaesolidaginis was studied to determine the effects of scale and host spatial pattern on patterns of parasitism. In the absence of discrete host patches, the relationship between densities of hosts and parasitized hosts was examined on scales of 0.25, 1, 4, and 9-m2 within and between twenty contiguously sampled 144-m2 sites. For scales of 1, 4 and 9-m2, sample size was held constant by random subsampling of quadrats with replacement. Positive density dependence was detected for total host mortality, host mortality caused by larval and total parasitoid assemblages, and host mortality caused by a solitary parasitoid Eurytoma bolteri, and a polyembryonic parasitoid Copidosoma gelechiae. Positive density dependence was predominant on the 1-m2 scale and was attributed to behavioural aggregation of parasitoids in areas of high host density. The effect of scale was most pronounced for parasitoid assemblages and least for Copidosoma. Density dependence was correlated with high mean host crowding for Copidosoma, and low mean host crowding for Eurytoma. Differential effects of host spatial pattern and scale on these two species is attributed to the high reproductive capacity of Copidosoma and the tendency for subpopulations of Copidosoma to respond numerically through time to localized host outbreaks within a site.