Alternative male mating tactics in Bembecinus quinquespinosus (Hymenoptera: Sphecidae): correlations with size and color variation

Summary

The digger wasp Bembecinus quinquespinosus exhibits characteristics not typical of other species in the family Sphecidae: there is no sexual size dimorphism (Fig. 1) and there is a size-linked color polymorphism among males (Fig. 2). We studied a population of this species in eastern Colorado, USA to determine if the above attributes were related to the male mating strategy.

Data on seasonal changes in sex ratio suggest that this species displays protandry (Fig. 4). Each morning for about a week after they first appear, large numbers of males congregate in the emergence area and search for pre-emergent females. Upon locating females just below the surface of the ground, males attempt to dig them out. If more than one is present when the female emerges, a cluster of struggling males forms around her. One male eventually succeeds in grasping the female and carrying her away from the emergence area. Most males in the emergence area are the large yellow color form. Within this group, larger individuals are also more likely to mate with emerging females. 92% of males participating in clusters or “mating balls” around females were 2.8 mm or larger in head width, although only 24% of the males in the population were this large (Fig. 1).

Because of the advantage enjoyed by larger males in the emergence area, smaller dark colored males cannot compete successfully. Therefore, they have adopted the alternative, probably less profitable, tactic of searching for females in areas adjacent to the emergence area. Some females leave the emergence area without mating and probably mate with males in this group.

Although there is a strong relationship between color and the mating tactic adopted by males, we do not know the function of the color polymorphism. It is suggested that sexual size dimorphism has disappeared in this species because males have evolved larger size, rather than because selection pressures on female size have been relaxed. Intrasexual selection related to success in aggressive interactions and the ability of large males to protect the female from competitors after coupling by covering her and carrying her away from the emergence area has been the most likely cause of the evolution of increased male size.