Ant fungus farmers nutritionally manage symbionts

1. When parasites exploit mutualisms involving food exchange, they can destabilise the partnership with costs to interacting partners. For instance, the ant Sericomyrmex amabilis farms fungal symbionts to produce food, but, in so doing, attracts parasitic Megalomyrmex symmetochus guest ants that infiltrate fungus‐farming ant societies and live with their hosts their entire lives.2. The present study examined whether host foraging in parasitised colonies shifts towards nutritional requirements of the parasitic guest ants as inferred from the parasite's elemental content (%C, %N, and C:N).3. Laboratory feeding experiments with nutritionally defined diets indicated that S. amabilis ants harvest protein‐biased substrate, and more total substrate when hosting M. symmetochus relative to when provisioning their fungus gardens and nestmates.4. Field surveys further showed that parasitised colonies incur reductions in fungus garden nutritional quality and quantity, brood mass, and host worker body condition. And yet these costs appear manageable across growing seasons, as parasitised fungal cultivars appear to provide sufficient nutrition for stable populations of host ants.5. The approach developed here shows how behavioural strategies for nutrient regulation can extend beyond the needs of the individual to entire fungus‐farming systems, and implies that S. amabilis dynamically adjusts collective foraging strategies when parasitised to enhance long‐term symbiotic stability