The Evolution of Clutch Size in Parasitic Wasps

Parasitic Hymenoptera can be divided into solitary species, in which only one individual develops in a host, and gregarious species, in which more than one develops. In the majority of solitary species, larvae have large fighting mandibles, and if more than one egg is placed in a host, the larvae fight among themselves until only one remains. Gregarious species lack these mandibles. Analyses of one-locus genetic models suggest that a gene for fighting invades a population of nonfighting gregarious individuals when the parent produces a clutch size of two or three. However, it is much harder for a gene for nonfighting to invade a population of fighters, and the solitary fighting condition has the properties of a locally absorbing state. Over a wide range of parameters a parent wasp will be prevented from realizing its optimal clutch size by this evolutionary constraint. It is suggested that the limited behavioral options available to parasitic wasp larvae in a host and the frequency of superparasitism are responsible for the prevalence of larval fighting in parasitic wasps.