Mating at High Population Density in a Colonial Territorial Wasp, Sphecius speciosus Drury (Hymenoptera: Crabronidae)

Abstract

Male Sphecius speciosus Drury (Hymenoptera: Crabronidae) emerge before females and compete for mating territories in nest colonies. In a large, dense aggregation of S. speciosus at a chemical plant in Channahon, IL, some males appeared to exhibit territorial behavior, but most flew through the nest area in apparent search of females, which are sexually receptive upon emergence. As females emerged, they were quickly surrounded by clusters of several males, all attempting to copulate. We wanted to determine whether the high male density influenced which males would be successful. Previous studies had shown that large male Sphecius have an advantage, because of superior ability to secure and defend territories. However, as territorial males were unable to repel the nearly constant intrusions by other males, we predicted large males would have no advantage. We captured a sample of mating clusters following initiation of copulation, and measured body mass, wing length, and mesoscutal width of successful and unsuccessful males. Though successful males were larger than a haphazard sample of the general male population, they were not significantly larger than unsuccessful males in the clusters. There was also no difference in symmetry, based on relative lengths of wings, between the successful and unsuccessful males. Apparently, small males either avoid or are excluded from mating clusters. However, within clusters the ability of males to compete and/or the possible choice of mates by females, are not influenced by male size and wing symmetry.