Effect of group size and caste ratio on individual survivorship and social immunity in a subterranean termite

Individuals living within social groups may benefit from the efficiencies of division of labour, but on the other hand render themselves vulnerable to socially transmitted disease. This cost to social living should promote cooperative barriers to disease transmission, especially in eusocial taxa where spatial and genetic proximity to nestmates are characteristically pronounced. Termites are eusocial yet little is known about how their sociality is deployed to resist contagion. In this study, we manipulate two variables that are expected to affect the number and nature of social interactions and measure the ability of individuals within groups to resist fungal infection. From laboratory experiments on field-collected colonies, we report that both group size and caste composition directly affect the survivorship of individuals within groups, but only caste composition moderates survivorship upon immune challenge. Our study therefore provides no statistical evidence that individual Eastern subterranean termites (Reticulitermes flavipes) have increased resistance to disease in crowded groups-that is, there is no evidence for a density-dependent social immune response. Our results do suggest, however, that the caste-specific nature of interactions may be important for controlling disease in a social context.