Sublethal effect of brood parasitism on the grass-carrying wasp Isodontia mexicana

Abstract.  1. The nests of solitary, nest-provisioning wasps (Sphecidae and Crabronidae) are commonly attacked by brood parasites, including flies of the families Phoridae and Sarcophagidae. Larvae of the flies commonly kill the wasp offspring directly or starve it by consuming prey provided by the adult female wasps.
2. To determine whether brood parasitic flies can have a sublethal effect (i.e. reduced body size) on wasp offspring, nests of the wasp Isodontia mexicana were collected at two field sites in upstate New York, U.S.A. Nest diameter had no effect on the probability that a wasp cell would be inhabited by a brood parasite. Most offspring that developed in cells also containing phorids or sarcophagids managed to complete development and emerge as adults. Nevertheless, they had significantly smaller body size than conspecifics emerging from unparasitised cells in which the developing wasp did not have to compete for food (which in this species consists of tree crickets and katydids). Apparently, this is the first time that a sublethal effect of brood parasitism on offspring body size has been quantified for a solitary wasp species. Known effects of body size on the reproductive success of adult wasps suggest that sublethal consequences of the presence of brood parasites may have a significant effect on the fitness of adult I. mexicana .