Gregarious nesting of a digger wasp as a “selfish herd” response to a parasitic fly (Hymenoptera: Sphecidae; Diptera: Sacrophagidae)

Abstract

Females of a digger wasp Crabro cribrellifer nested in a 234 nest aggregation covering 2x65 m in area. Variability in 3 edaphic factors (soil temperature, hardness, and per cent moisture) had no significant influence on the non-random spatial distribution of nests within the aggregation. A significant positive correlation between wasp nest density and the mean number of parasitic flies, Metopia campestris , present was shown (Fig. 1). Yet, based on 20 excavated nests, the probability of cell parasitism decreased as nest density increased (Fig. 2). This may give females nesting within a dense aggregation a selective advantage, and supports the “selfish herd” model developed by Hamilton (1971).