Blochmannia endosymbionts reduce brood rearing success in a carpenter ant (Camponotus sp.)

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 and subcolonies (worker groups), 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, subcolonies treated with antibiotics had a significantly higher success in raising their own and foreign eggs from treated and untreated colonies than untreated subcolonies. This might indicate that the Blochmannia symbiosis entails significant costs for the host in terms of brood rearing, i.e., symbiont-free workers are more successful in brood rearing than untreated workers. If confirmed, this would be a rare 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 differences 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, may reduce the amount of brood a colony can raise.