Effects of colony size and stage of development on defense response by the swarm-founding wasp<Emphasis Type="Italic"> Polybia occidentalis</Emphasis>

In social insects the level of defensive effort generally increases with colony size, but the effects of the size of the defending adult population and the size of the brood population being defended have not been separated. We used pre-emergence-stage colonies of the swarm-founding wasp, Polybia occidentalis, to distinguish the effects of these two parameters on the intensity of colony defense. We elicited defensive behavior by using both mechanical (taps) and chemical (alarm-pheromone-containing venom) stimuli, and scored the responses as the number of attacks on a standard target. The intensity of defense increased linearly throughout the colony's first 25 days of development, despite the decrease in absolute numbers of adults during the same period. That is, the per capita attack rate increased strongly during this period, leading us to conclude that the intensity of defense is a function of the colony's investment in brood and not of the number of defending adults. We provide evidence that the increase is attributable neither to sensitization due to frequent disturbance, nor to the aging of the adult wasps. We further show that defensive effort on a per capita basis decreased with increasing size across colonies, whether colony size was measured as number of adults or as investment in brood. This pattern suggests that the cost-benefit relationship of defending the colony changes with colony size.Communicated by R.F.A. Moritz