Survival and productivity benefits of sociality vary seasonally in the tropical, facultatively eusocial bee Megalopta genalis

Tropical habitats are characterized by strong wet and dry seasons, but the effects of seasonality on the costs and benefit of sociality are largely unknown for tropical insects. This is an important gap in our understanding of sociobiology because many social bees and wasps are in the tropics. We found evidence of seasonal effects on the costs and benefits of social and solitary behavior in the tropical sweat bee Megalopta genalis. Productivity, whether measured as brood cell production per nest, or brood cell production per female, was greater in the dry season than the wet, likely reflecting floral resource availability. Per nest productivity was greater in social nests than solitary, but this difference was only significant in the dry season. Conversely, per capita productivity was greater in solitary than social nests, but again only in the dry season. Nest failure rates were also higher in the wet season, although roofs protecting nests from rain did not increase survival, suggesting that increased foraging effort in the face of declining resources rather than wetness per se led to nest failure. Newly initiated nests had higher failure rates than established nests, but these were not affected by season. Social nests collected late in the wet season after reproduction has largely ceased show that M. genalis can live in social groups without reproduction; these bees are likely waiting together until provisioning resumes in the subsequent dry season. Our results suggest that the productivity benefits of social nesting are greatest in the dry season, but that insurance-based benefits to social nesting may be greater in the wet season. This reveals that the costs and benefits underpinning sociality are dynamic across seasons, even in tropical systems.