Examining potential benefits of group living in a sawfly larva, Perga affinis

Although defense may serve as a primary selective force behind the evolution and maintenance of social behavior, numerous other benefits play a role as well. Larvae of the Australian sawfly, Perga affinis, live in colonies and retain their gregarious lifestyle through pupation. To evaluate selective pressures maintaining social behavior in P. affinis, I investigated potential benefits of group living. These included predation protection, immune function, thermoregulation, foraging facilitation, and pupation success where I compared treatments between individuals versus groups and between groups of different sizes. I found no evidence of predation on larvae; however, the mortality risk was significantly higher for single versus grouped larvae, suggesting that other grouping benefits are important. Immune function was not a benefit as hemolymph from single versus grouped larvae did not differ in its ability to limit bacterial growth. Investigating thermoregulation revealed that grouped larvae attained significantly higher temperatures than single individuals and that large groups reached higher temperatures than small groups. Larvae grew significantly faster at higher temperatures, indicating that the increased heat absorption and retention capacity of large groups may speed up development. Group size did not affect foraging facilitation (measured via weight gain) except during the last time period, where individuals in large groups gained significantly more weight. Weight gain in the last instar may be critical for pupation as high larval weight significantly increased the odds of pupation success. Thus, the group size effect on weight gain during the last instar provides an adaptive explanation for the observed ontogenetic increase in colony size and coalescence. Copyright 2009, Oxford University Press.