Caste composition contributes to improve the social immunity in a nasute termite species exposed to entomopathogenic fungus

The evolution of a reproductively altruistic caste contributed to the success of eusociality through the division of labour. In termites, the proportion of soldiers in the colonies increased throughout the group's evolution. In Nasutitermitinae, soldiers have a crucial role in defence, resource selection and foraging. However, the role of soldiers and workers in colony immunity is still poorly understood.Here, we evaluated the role of mixed caste groups in the social immunity of Nasutitermes corniger (Termitidae: Nasutitermitinae) exposed to the entomopathogenic fungi Metarhizium anisopliae (Metsch.). We tested the hypothesis that the presence of workers and soldiers optimizes defence against pathogens, and this defence is increased when the soldier ratio is closer to what naturally occurs in colonies.Our results showed that mixed caste groups of N. corniger exposed to M. anisopliae survive longer compared to groups with only one caste. In addition, termite groups that died from fungi were less likely to show fungal infection in mixed caste groups.The behavioural observations suggest that in the natural proportion of soldiers, allogrooming and trophallaxis play a crucial role in the control of disease and its death hazard in termites. Soldiers may play an important role in colony immunity, being able to start the alarm signal indicating the threat of pathogens.Our results show that the presence of the two castes promotes possible mechanisms for socially mediated immunity. This study may help in the understanding of the function of the soldiers and workers and the significance of pathogens in termite eusocial evolution.