The interplay of symbiotic beings in the Pampa: the encounter between an alien grass and leaf‐cutting ants

Symbiotic relationships across trophic levels influence invasions. Although Epichloë endophytes are often linked to forage grasses spread through alkaloid-mediated herbivore resistance, this link oversimplifies a complex, multifactorial process. We investigate whether this fungal endophyte interacts with the fungus garden of the native leaf-cutting ant Acromyrmex ambiguus by inducing the release of a widespread green leaf volatile, thereby altering its foraging behavior. We conducted 5-d choice experiments to assess ant preferences for leaves and seeds of Lolium multiflorum plants, either infected (E+) or uninfected (E-) with Epichloë occultans, along with a Petri dish assay to evaluate the effects of (Z)-3-hexenyl acetate (Z3-HAC) on the growth of Leucoagaricus gongylophorus, ants' mutualistic fungi. Ants exhibited delayed rejection for E+ plants while showing no preference for seeds. In vitro, Z3-HAC promoted fungal growth away from volatile sources. These findings suggest that mutualistic fungi of a grass (alien) and its herbivore (native) shape their defense strategies in Pampean grasslands. Z3-HAC's effects extend beyond direct plant-herbivore interactions, signaling antagonistic fungi within the fungal garden without acting as a novel weapon. This study highlights the importance of integrating microbial networks into invasion ecology frameworks to understand the consequences of co-introduced plants and symbionts.