The epizootiology of a Metarhizium infection in mini-nests of the leaf-cutting ant Atta sexdens rubropilosa

There is growing interest in the use of entomopathogenic organisms to contr ol leaf-cutting ants (Hymenoptera: Formicidae: Attini). However, the way le af-cutting ants react as a colony to biohazards is poorly understood. We in vestigated the effects of Metarhizium anisopliae (Metschnikoff) (Deuteromyc otina: Hyphomycetes) applied to the foraging arenas of mini-nests (queenles s sub-colonies) of the leaf-cutting ant Atta sexdens rubropilosa (Forel). D ry spores were applied either alone or mixed with citrus powder, at 0.5 g o r 0.05 g per mini-nest. The spores were removed four days after application , and all dead ants removed every three days. Ant numbers near the Metarhiz ium increased as the ants attempted to clean up the biohazard. The ants att empted to place the spores in piles, which they then covered over with othe r material. They were able to deal with the low doses in this way, but the high doses overwhelmed them. All treated mini-nests suffered increased ant mortality during the first ten days after application. This mortality was p articularly high in the media worker caste which had played the major role in attempting to clean up the spores. Foraging activity decreased, as did t he health of the fungus gardens. The mini-nests exposed to the low dose of spores mixed with citrus powder then recovered fully. The health of the oth er treated mini-nests declined gradually until around 26 days after applica tion, when they began deteriorating sharply. However, the decline of these mini-nests after day 26 was not due directly to the pathogenic action of th e Metarhizium, nor to the initial ant mortality it had caused. The results suggest that the social stress caused by even such a short-lived Metarhiziu m epizootic was sufficient to cause the decline and ultimate death of the m ini-nests. This has important implications for the control of leaf-cutting ants. It also demonstrates how important the social homeostasis of the colo ny is to leaf-cutting ants.