Eating Versus Heating: A Study Of The Allocation Of Workers Between Foraging And Nest Incubation In Bumble Bees

1. Bumble bees are important eusocial pollinators whose worker caste, fuelled by nectar sugar, accelerate colony growth through brood incubation, but allocating workers to nectar foraging could compromise nest thermoregulation in daytime and produce 'incubator limitation'.2. We hypothesise that colonies in nectar-poor habitats experience stronger incubator limitation by diverting workers from incubation to foraging, but this potential effect of habitat quality on colony fitness via task allocation is not fully understood.3. We therefore modelled levels of nectar foraging in relation to forage richness and theoretically estimated the impact of incubator-limitation on colony fitness. Additionally, we investigated whether live colonies of bumble bees (Bombus terrestris) responded to manipulation of their sugar supply as if incubator-limited.4. Model solutions for environmentally realistic scenarios showed that colonies allocate up to one third of workers to nectar foraging in order to avoid sugar starvation during sustained incubation. The incubator-limitation that results can strongly reduce colony reproduction, but its impact depends on the temperature regime of the nesting habitat.5. Live colonies allocated between one sixth and one third of daytime worker effort to foraging and responded to sugar supplementation as if incubator-limited.6. These findings suggest that forage richness and nest temperature relations could interact to affect the fitness of wild bumble bee colonies through incubator limitation. Our testable theory promises an integrative understanding of how forage quality and climate could govern the distribution and abundance of bumble bees and other eusocial insects that utilise brood incubation by their worker caste.