As larvae, solitary species of parasitoid wasps eliminate rivals, including sibs, through contest competition. In gregarious species, larvae tolerate each other and several individuals can develop from a single host. We report experiments on two congeneric braconid species aimed at understanding how gregarious development evolved in one genus, and the consequences of larval interactions for parental behavior. In the transition from solitary to gregarious development, one possible intermediate stage is if solitary species frequently lay more than one egg per host. If the risk of conspecific superparasitism (another female of the same species oviposits into a single host) is high, optimal clutch size theory predicts the laying of multiple-egg clutches, because a female will increase the probability of the surviving larva being her own. Under the same conditions, theory predicts that gregarious species should reduce their clutch size. In our experiments, the solitary Aphaereta genevensis increased their clutch size under the risk of intraspecific competition for hosts whilst the gregarious A. pallipes reduced their clutch size, as predicted. In A. genevensis, the response to the risk of competition was dependent upon body size, being greatest amongst small females. In A. pallipes, the response did not depend on female body size, but clutch size increased with body size in all treatments, unlike in A. genevensis. Under the risk of competition from the other species, the response differed relative to the intraspecific treatments and differed between species. Thus, our data suggest that: (1) competition for hosts may have contributed to the evolution of gregarious development in this genus; (2) female wasps can combine information from both external and internal cues in complex ways during clutch size adjustment; and (3) the social environment may alter the adaptive response to both external and internal cues in adult females. Drosophila virilis (Drosophilidae) : Host Braconidae (Ichneumonoidea) Aphaereta genevensis (Braconidae) : Parasite Aphaereta pallipes (Braconidae) : Parasite Reproduction; Reproductive productivity; Life cycle and development; Development; Evolution; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Competition Aphaereta genevensis; Aphaereta pallipes; Egg number; Clutch size on single dipteran host; Larval development; Solitary/gregarious development in dipteran host; evolutionary considerations; Dipteran hosts; Drosophila virilis; Intraspecific competition Braconidae; Evolution; Gregarious larval development within host Drosophila virilis; Hymenopteran parasites; Solitary/gregarious development of parasitoids; evolutionary considerations; Aphaereta genevensis & Aphaereta pallipes none