An antifungal terpenoid defends a neotropical tree (Hymenaea) against attack by fungus-growing ants (Atta)

Summary

Foragers of the leafcutting ant, Atta cephalotes L. (Formicidae, Attini) seldom or never attack many of the plant species available to them in nature. In the semideciduous forests of lowland Guanacaste Province, Costa Rica, one of the tree species seldom cut is Hymenaea courbaril L. (Leguminosae, Caesalpinioideae). We tested the hypothesis that this species is avoided by the ants because of the presence of ant-repellent secondary compounds in the leaves. A bioassay to test repellency of leaf extracts was developed to guide the chemical isolation of ant repellents, using a laboratory colony of Atta cephalotes.

The presence of one or more extractable ant repellents was quickly demonstrated. Subsequent chemical isolation and identification revealed that there was essentially only one terpenoid responsible for the repellency: caryophyllene epoxide. Tests with a concentration series of the pure compound demonstrated that the natural concentration of this terpenoid in Hymenaea could fully account for the observed repellency of intact leaves. Field bioassays of the terpenoid in Costa Rica confirmed this result; leaves of a preferred species, Spondias purpurea L. (Anacardiaceae), became as repellent as Hymenaea leaves when treated with caryophyllene epoxide at natural Hymenaea leaf concentrations. Repellency of the epoxide was 20 times greater than that of caryophyllene, its sesquiterpene hydrocarbon precursor, which is also found in Hymenaea leaves.

Attine ants cut leaves to serve as substrate for culturing a specific fungus for food, principally for their larvae. A reasonable hypothesis is that these ants selectively avoid plant species whose leaves contain compounds which are toxic to their fungus. We tested caryophyllene epoxide for antifungal activity and found that it is an extremely potent compound, not only against the attine fungus, but other fungi as well. We speculate that many of the other plant species avoided by these ants in nature may be similarly protected from ant attack by antifungal compounds in their leaves. We further suggest that plant defense against leafcutting ants may be largely an incidental byproduct of selection for fungal resistance in plants.