Leaf-cutter ants (Acromyrmex and Atta spp.) exhibit ancient and complex interactions with the symbiotic fungus Leucoagaricus (Basidiomycetes: Agaricales) from which they feed, and with the virulent and specific fungus Escovopsis weberi J.J. Muchovej & Delia Lucia (Ascomycetes: Hypocreales) that attacks the ants' fungal gardens. This system offers a unique opportunity to study possible avenues for replacing polluting pesticides with a biological control agent against the ants. We isolated both Leucoagaricus sp. and E. weberi from the gardens of Acromyrmex lundii Guérin-Méneville colonies and confronted them with each other by growing Leucoagaricus on petri dishes and placing inoculated pieces of agar inoculated with E. weberi at the edges. Here we present growth curves of Leucoagaricus sp. and E. weberi in the absence of each other, as a baseline to which we compare the effect of the fungi on each other. As expected, we found a negative effect of E. weberi on the ant cultivar from different colonies of A. lundii. E. weberi increased its growth rate, as well as the levels of conidiation, in the presence of the ant cultivar. We determined that a soluble and diffusible compound, released by the cultivar, triggered, was responsible for, or did both for the increased levels of conidiation in E. weberi, and that this response was reversible. We discuss why our results are encouraging from a biological control perspective.