Waste management by ants: the enhancing role of larvae

Abstract

Insect societies are challenged by harmful pathogens that originate from waste, such as faeces, food leftovers or corpses. The discarding of waste to outside the nest reduces these sanitary risks and contributes to the social immunity of ant colonies. In this study, we tested whether the nest-cleaning behaviour in Myrmica rubra colonies differed depending on the pathogenicity of waste or the presence of brood as well as over successive exposure to waste. We introduced waste items covered with entomopathogenic Metarhizium brunneum conidia and items without conidia in either broodright or broodless colonies. The greater the pathogenicity of waste, the faster it was discarded by the ants, although this did not prevent higher mortality rates in workers and larvae. When exposed a second time to waste items, ant colonies improved the discarding of nonpathogenic waste but became less efficient at removing infected waste, probably due to morbidity in the workers’ population. Most surprisingly, we found that the presence of brood enhanced the hygienic responses of workers, with ant colonies doubling their probability of discarding waste items to outside the nest. Overall, we show that ants can detect entomopathogenic conidia on waste and take steps accordingly to hasten the removal of infected items. Furthermore, we demonstrate the upregulating role of larvae which results in enhanced performance of hygienic tasks and reinforces sanitary control inside the colony.