none. Fourth and fifth instar larvae, and also pupae of the apple ermine moth, Yponomeuta malinellus Zeller, are attacked by the ichneumonid Herpestomus brunnicornis Gravenhorst in central Europe. The parasitoid was studied as a potential biological control agent of the apple ermine moth in British Columbia. Investigations of the biology of H. brunnicornis, its distribution of attack within the tree canopy and the rate of parasitism in relation to spatial variation in the number of hosts per tent were studied over a 4 year period in Europe. The following biological parameters of H. brunnicornis were studied: adult emergence, sex ratio, copulation, potential fecundity per day, host feeding, oviposition behaviour, handling time, and larval and apple ermine moth varied from 0.5 to 1.3 per 30-leaf cluster and pupal pupal development. In the present study, the number of tents of the parasitism by H. brunnicornis ranged from 3.9% to 8.5% in 1993-1995. Host tents were randomly distributed within and between apple trees. rants within the tree canopy in 1993-1995. The index of dispersion indicated that the distribution of emerging parasitoids per host tent was contagious. Percentage parasitism between tents by H. brunnicornis was inversely related to the number of host larvae/pupae per tent. Herpestomus brunnicornis is a synovigenic parasitoid whose females have a small maximum egg load. In addition, handling time on host pupae is high. Both these characteristics appear to provide an adequate explanation for the inverse density-dependence in parasitism on a spatial scale.