Seasonal changes in host ant aggressiveness towards social parasites of different types

Social parasites are social insects that exploit the labour and resources of other social insects. In ants, several types of social parasites are known, including temporary parasites, in which host workers are present only during colony founding, and slave-maker ants, which continually supplement their workforce with host workers throughout the colony's lifetime. Because defence against parasites is costly, host species are expected to modulate their responses according to parasite-imposed risks. In this study, we investigated the seasonal patterns of aggressiveness in a common host, the silky ant, Formica fusca , towards its temporary social parasite (European wood ant, Formica polyctena ) and a slave-maker (blood-red ant, Formica sanguinea ). Behavioural assays were conducted in spring, summer and autumn using host workers collected from colonies located (1) near F. polyctena colonies, (2) near F. sanguinea colonies, and (3) in parasite-free (control) areas. Hosts from areas dominated by F. polyctena were more aggressive towards F. polyctena in spring and summer than in autumn, showed peak aggression towards F. sanguinea in summer, and were more aggressive towards F. sanguinea than F. polyctena in summer and autumn. Hosts from areas with F. sanguinea were most aggressive towards both parasite species in spring and summer, with similar responses to both parasites across all seasons. Control colonies showed an intermediate pattern, combining elements of both responses. Seasonal variation in host aggressiveness appears to track differences in the timing and severity of threats imposed by the two parasites: colony founding by F. polyctena in spring and early summer, and slave raids by F. sanguinea in summer. Such temporal adjustment of defensive behaviour may help minimize the costs of defence against social parasites by increasing aggression only when the threat is greatest. • Social parasites are social insects that exploit other social insects. • Defence costs against parasites vary, driving the evolution of defensive strategies. • The aggressiveness of the hosts depends on the type of social parasite. • The aggressiveness of the hosts is influenced by neighbouring social parasites. • Seasonal changes in host aggressiveness align with the activity of social parasites.