"Searching time aggregation" and density dependent parasitism in a laboratory host-parasitoid interaction.

Abstract

Assuming random search by parasitoids within host-containing patches, and a constant search rate, current host-parasitoid models suggest that "searching time aggregation" by parasitoids in patches of high host density should tend to produce spatially density dependent parasitism at the patch level. It is not clear, however, that statistically significant "searching time aggregation" necessarily implies that significant density dependent parasitism will occur. In actual host-parasitoid systems the amounts of searching time allocated to patches of equal host density may vary a great deal from patch to patch. Such behavioral variability may be capable of obscuring an underlying density dependent trend, producing density independent parasitism at the patch level despite significant searching time aggregation in patches of high host density. This possibility is tested using data from an earlier laboratory study (Morrison and Lewis; Ent. exp. et appl. 30:31-39 [1981]) of the foraging behavior of Trichogramma pretiosum Riley, a generalist parasitoid of lepidopteran eggs. Searching time allocation is found to be highly variable among patches of equal host density, and significant density dependent parasitism does not occur despite significant searching time aggregation in patches of high host density. This suggests that in cases in which density independent or "density vague" patterns of parasitism are observed in field samples, direct field measurements of searching time allocation in patches of different host density may be necessary to demonstrate the presence or absence of significant searching time aggregation by foraging parasitoids.