Leaf-cutting ants show species-specific hygienic behavior toward microfungi in their fungal gardens

Background/Question/Methods

Leaf cutting ants (Atta colombica) and their fungal garden rely on each other for nutrition in a tightly coevolved mutualism. In the course of cultivating their own mutualistic fungus, leaf-cutting ants encounter many microfungi that may occur as parasites, entomopathogens, or neutral/beneficial microsymbionts. The source of these microfungi may be the surrounding soil or the plant material brought to the nest by the ants for use as a compost. Whether the ants’ hygienic behavior toward these microfungi is generalized or specific to different microfungal species is unknown. We used sub-colonies to test the response of worker ants to 15 species of microfungi on agar plugs placed near their garden. Eight of these microfungal species were isolated from freshly cut leaves carried by leaf-cutting ants (i.e. they were fungal endophytes), while seven were isolated from other leaf-cutting ant fungal gardens (i.e. they were fungal garden contaminants or symbionts). The response intensity of the worker ants to the agar plug was measured as the mass lost from the plug as the worker ants cut away at the fungi and placed the pieces in their garbage pile. 

Results/Conclusions

We found that worker ants removed microfungi isolated from freshly cut leaf material faster than microfungi isolated from fungal gardens. Our data suggest that the worker ants moderate their behavior in a species-specific rather than generalized fashion when responding to different types of microfungi they encounter.