CONTROL OF THE BEHAVIOR OF LEAF-CUTTING ANTS BY THEIR SYMBIOTIC FUNGUS

There is an obligatory relationship between leaf-cutting ants of the g enera Atta and Acromyrmex (Hymenoptera; Attini) and the fungus, Attamy ces bromatificus Kreisel, for which they provide a substrate of cut pl ant material. We show that the ants learn to reject plant material tha t contains chemicals injurious to the fungus. After an initial period of acceptance, ants from laboratory nests stopped harvesting granular bait containing a fungicidal agent (cycloheximide) and orange peel. Th is rejection was maintained for many weeks. These colonies also reject ed control bait containing no cycloheximide. Some generalisation of th e response was observed; colonies rejecting orange granules also rejec ted grapefruit granules, although they still accepted blackcurrant gra nules. Rejection of fungicidal bait by colonies in the field was restr icted to ants on foraging trails exposed to experimental bait. We conc lude that a semiochemical from the fungus, circulated by trophallaxis and grooming, regulates the selection of plant material by foragers. T he fungus gains more advantages from the symbiotic relationship than i s often realised. The ants provide the fungus with housing, sanitation , defence against diseases and predators, pre-selected food, and a mea ns of dissemination, at the cost to itself of providing food for the a nt brood.