Social structure and the defensive role of soldiers in a eusocial bamboo aphid, Pseudoregma bambucicola (Homoptera: Aphididae): A test of the defence‐optimization hypothesis

A complete understanding of the evolution of sociality in aphids requires a detailed knowledge of the patterns of soldier investment in their ecology. The eusocial bamboo aphid Pseudoregma bambucicola has a morphologically specialized first‐instar soldier caste. The proportion of soldiers was positively correlated with colony size. Within a colony, soldiers were evenly distributed among subcolonies; within each subcolony, however, their distribution was biased toward peripheries which were exposed to many predators. Field experiments introducing natural enemies such as Eupeodes confrater (Diptera: Syrphidae) and Synonycha grandis (Coleoptera: Coccinellidae) revealed that the survival rate of these predators was negatively correlated with the density of soldiers, suggesting that soldiers can more or less defend their colonies by killing or removing a range of natural enemies. Observations suggest that large mature colonies attract more predators than newly established small colonies and that, within a colony, the predators attack each subcolony regardless of its position on bamboo shoots. This implies the presence of a positive correlation between colony size and predation risk. Thus, the investment in soldiers seems to reflect the attacking pattern of predators within a colony. These results agree with the defence‐optimization hypothesis in soldier investment of P. bambucicola colonies.