HYPERPARASITOID AGGREGATION IN RESPONSE TO VARIATION IN APHIDIUS-ERVIHOST DENSITY AT 3 SPATIAL SCALES

1. This article investigates the pattern of hyperparasitism of the hos t Aphidius ervi Haliday (Hymenoptera, Aphidiidae), a primary parasitoi d of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae ) at three spatial scales, 2. In the laboratory, the hyperparasitoid A saphes lucens (Provancher) (Hymenoptera: Pteromalidae) was introduced into cages containing sixteen alfalfa plants with varying numbers of A . ervi mummies (the stage susceptible to hyperparasitism). The pattern of hyperparasitism at the end of the 48-h trials showed no density-de pendent hyperparasitoid aggregation, although there was strong density -independent hyperparasitoid aggregation. 3. In the field, the density of A, ervi mummies was manipulated in twelve 2 x 2-m plots containing 1309-1654 alfalfa stems. Variation in hyperparasitism among plots sho wed no density-dependent aggregation, although there was strong densit y-independent aggregation, 4. Finally, at the largest scale of the stu dy, the distribution of hyperparasitism was sampled among twelve alfal fa fields within a 5 x 3-km area. At this scale there was both density -dependent and density-independent hyperparasitoid aggregation, 5, The natural variation in A, ervi mummy density is greatest at the larger scales of study. Therefore, density-dependent hyperparasitism occurs o nly when there is high natural variation in mummy density.