The metabolic enzyme phosphoglucose isomerase (Pgi) affects the outcome of intra‐specific competition in a polyembryonic wasp

1. Polyembryonic parasitoid wasps in the family Encyrtidae (Hymenoptera) have evolved a caste system consisting of morphologically and functionally distinct larvae called soldiers and reproductives. 2. Two selective pressures are thought to underlie the evolution of the soldier caste: defence against competitors and resolution of the sex ratio conflict. Previous studies also indicate that soldier development time strongly affects the outcome of intra‐specific competition in the polyembryonic encyrtid Copidosoma floridanum Ashmead. This study builds on prior findings by showing that alleles of the metabolic enzyme phosphoglucose isomerase (Pgi) differentially affect soldier development time and the outcome of competition. 3. Soldier larvae with the Pgi alleles 100 or 120 emerged on average 65 h post‐parasitism, whereas soldier larvae with a third allele, 54, emerged at 67 h. In turn, C. floridanum broods homozygous for the 100 and 120 alleles outcompete broods homozygous for the 54 allele. 4. Pgi allelic diversity may be maintained through a life‐history trade‐off affecting female brood sizes with homozygous broods bearing the developmentally disadvantageous 54 allele producing more adult females than broods bearing alternate common alleles.