Ants using formic acid as herbicides

Amazonian ants nesting on tree species Duroia hirsuta have been reported to inject a lethal dose of formic acid into leaves of of other tree species in the vicinity. This resulted in sections of rainforest being comprised of a single species of trees. The ants' actions resulted in less competing species for D. hirsuta and therefore more nesting sites for the ants. More information at - Scientific American: Amazonian Ants Use Own Herbicide to Poison Unwanted Plants