Leaf-Cutting Ants, Biology and Control

Abstract

Leaf-cutting ants (Formicidae, Myrmicinae, Attini) are found on the American continent and in the Caribbean and are known to live in symbiosis with a fungus. Among Attini tribe, Atta and Acromyrmex are the two genera, which commonly depend on fresh plant leaves and other plant material for their fungal garden. Overall, these ants are among the most economically damaging herbivorous species. A. octospinosus is classified among the most serious pests of tropical and subtropical America. Due to its foraging activity, it can cause serious damages from 20 to 30 % of crop production. Huge losses were observed in several vegetable and fruit crops, in crop of cacao or citrus orchards and in protected areas where some species may completely disappear due to their endemism. Economic losses due to these ants were estimated at several million dollars per year. Although Acromyrmex octospinosus is one of the most important species of leaf-cutters because of its economic impact there is a lack of review in the literature.

We review here the distribution, biology and different kind of control used and sustainable methods that can possibly help to manage. A. octospinosus is found only in Neotropics from Central America to northern of South America, including parts of West Indies. A colony is divided into males, breeding females, or gyne, and three castes of workers: small workers or ‘minor’, medium worker or ‘media’ and large workers or ‘major’. Pheromones are indispensable supports of social life of A. octospinosus. Foraging is conditioned by trail pheromone and takes place along a ‘line’ involving all castes of the colony. The foraging is divided in several steps. Indeed, the ants cut the plant material, transport it into the nest, lick it, cut it into smaller pieces, chew it, depose fecal fluid on it, cultivate mycelium fragment on this prepared mixture and incorporate it into the symbiotic fungus. Symbiotic fungus, or ‘fungus garden’, is a Basidomycete, Leucocoprinus gongylophorus (Heim) Moeller. It is the exclusive nutrition of both juvenile stages and the queen and is also a supplement of the adult workers diet. The ant also lives with an actinomycete, Pseudonocardia sp. This symbiont grows on the cuticle of all ants of the colony and protects fungus garden against fungal competitors and ants against ubiquist entomopathogenic bacteria and fungi.

The first techniques used to fight against A. octospinosus were mechanical methods which have only local efficiency and do not prevent from the reestablishment of colonies. Synthetic chemical control against A. octospinosus began in the twentieth century with a direct chemical control and afterward with toxic baits. The efficiency of these controls was limited but all of them caused severe injuries on environment and human health. This situation has prompted an increasing interest in alternative methods to control this pest. Laboratory and field tests of biological control were performed with spores of pathogenous fungus or ants predators into nest, with fungal symbiont’s extract and trail pheromone, with entomopathogenic fungus and nematodes. Most of these methods did not give the desired effect or were not confirmed in situ. Biopesticides using insecticidal and fungicidal plants are known to be environmentally safe because of their non-phytotoxicity, biodegradability and renewability. Thus, another alternative to chemical control of A. octospinosus has been explored with the insecticidal or fungicidal activities of plants extracts, which showed various effects on both A. octospinosus workers and Leucocoprinus gongylophorus in vitro cultures. These tests showed very encouraging in vitro preliminary results and may serve as alternatives to synthetic compounds to develop safer control agents of leaf-cutting ants.

Complex relationships and tripartite mutualism are involved between ants, fungus and actinomycete. All studies investigated only one side management. A tripartite management based on a combination of the three strategies should promote a more efficient integrated control and provide some interesting options for the control of this pest.