The effects of spatial differences in host density on parasitism of Heliothis zea (Boddie) eggs by Trichogramma spp. were investigated in cultivated soybean plots. H. zea egg density per leaf and the distance separating egg-bearing leaves within lots were experimentally manipulated. A kairomone-containing extract of H. zea scales was used to mimic natural oviposition sites. Spatial relationships between the probability of parasitism and egg density per leaf were the result of 2 opposing trends. The probability of at least one egg on an eggbearing leaf being encountered and parasitized by Trichogramma (here termed a “discovery” of that leaf by parasitoids) increased as the egg density per leaf increased; the conditional probability of parasitism for other eggs present, given an attack on one egg, decreased with increasing egg density. The net result of these opposing trends varied with separation distance and, statistically, percent parasitism varied from inversely density-dependent to density-independent as separation distance increased from 25 to 150 cm.
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Morrison et al. (1980) studied this question.