Harbouring Blochmannia incurs costs: a trade-off between the necessity of the obligate primary endosymbiont for brood development and its costs for adult carpenter ants (Hymenoptera: Formicidae)

All ants of the species-rich genus Camponotus ("carpenter ants") possess the obligate intracellular bacterial mutualist Blochmannia. We tested the relevance of the endosymbiont Blochmannia for offspring rearing using cross-fostering experiments between Camponotus sp. colonies, which were either treated with antibiotics to remove Blochmannia or untreated. Our antibiotic treatment reduced the level of Blochmannia endosymbionts in eggs, larvae, and workers significantly. Corroborating previous results, we found that eggs from treated colonies had a significantly reduced probability to develop into larvae and almost zero probability to become adults. Surprisingly, workers treated with antibiotics (symbiont-free workers) had a significantly higher success in raising their own and foreign eggs both from treated and untreated colonies than untreated workers. This indicates that the Blochmannia symbiosis entails substantial costs for the host in terms of brood rearing, that is, antibiotic-treated workers are more successful in brood rearing than symbiont-harbouring workers. If confirmed, this would be a case where the costs of a symbiosis can be empirically measured and quantified. Alternatively, the antibiotic treatment increased, as a side effect, the brood rearing effort of workers leading to the observed increase in brood rearing success of treated workers. But even if that would be the case, it still indicates that workers that have either lost or have a significantly reduced number of endosymbionts can still raise brood from antibiotic-treated and untreated colonies better than untreated workers. Thus, Blochmannia, although crucial for brood development in general, may reduce the amount of brood a colony can raise due to negative effects on ant workers.