Credit: MATTHEW KWESKIN

Leaf-cutting ants of the tribe Attini do not eat the vegetation they gather. Rather they use it to grow fungi, and then feed off the fungi. The ants' vast nests, containing up to eight million individuals, have chambers devoted to fungus cultivation. But like all farmers, the ants face competition for the crop in the form of pathogens — in this case other fungi, which threaten to destroy the fruits of their labours.

A couple of years ago, Cameron Currie and colleagues showed how the ants use antibiotic-producing bacteria for pest control. From observations of the species Atta colombica (pictured), he and Alison Stuart now describe other strategies to that end (Proc. R. Soc. Lond. B 268, 1033–1039; 2001). One is simple cleanliness: the ants lick the surfaces of both the nest itself and new leaf material to keep them clear of contaminants.

The authors also sprayed fungal-cultivation chambers with spores of two pest fungi, Trichoderma viride and Escovopsis, to see the reaction. When rogue spores were found by the inhabitants, large numbers of workers congregated at the infected site to gather up spores in their mouthparts and cart them away. If the spores reached the stage of germination, the ants turned to weeding by removing chunks of leaf along with crop and pest fungi.

The ants seem to be able to tell the difference between the two invading fungi. The specialist pathogen Escovopsis triggered more ants to take up cleaning duties, and their efforts went on much longer, than those directed against Trichoderma. But the ant farmers don't have everything their own way. Although Trichoderma was eliminated rapidly, Escovopsis hung on in the nests. It may be that this pathogen grows too quickly for the ants to keep up, or that the stickiness of the spores makes it impossible for the ants to remove 100% of them from a nest.