Atta sexdens (leaf cutting ant)

Atta spp. mostly affect the vegetative plant parts (leaves and young stems), although in some cases they also cut flowers and harvest stored seeds. Young plants are usually totally defoliated, the damage resembling pruning, whereas in mature plants, often only young leaves are removed. Scars, resembling those made with a knife, are frequently left on green parts of the plant stem. Grass blades are often cut near to ground level. Some Atta spp. cut the leaf petioles, whereas those that do not produce more minor damage, recognizable by 'U'-shaped fragments cut from the leaf edges.

Introduction

Atta control is particularly important immediately before and after planting, when the plant is most susceptible to the damage caused and therefore when economic losses are greater.

Cultural Control

Changing the type of land use has been recommended in some situations. Grass-cutter and grassland species, for example, do not survive under the shade of a tree plantation (Mariconi, 1970), whereas forest species die of starvation 5 to 20 months after deforestation (Hernandez and Jaffé, 1995). Another option is to plant species or clones resistant to attack (Laranjeiro and Evans, 1994).

Individual tree seedlings can be protected using plastic or PVC tubes. Similarly, trees can be protected by banding their trunks with grease or polyisobutylene gel, provided that the tree foliage is not touching that of any unprotected plant.

Biological Control

The use of pathogenic fungi, especially Beauveria bassiana and Metarrhizium anisopliae, for control of leaf-cutting ant nests in nature has been not successful, but is still under investigation (Kermarrec et al., 1986).

Arboreal ants of the genus Azteca have been reported as protecting citrus and cocoa trees against attack by Atta spp. (Leston, 1978; Jutsum et al., 1981).

Atta spp. are major pests of many crops and plantations in Latin America, and were considered to be among the five worst pests in seven of the 19 countries surveyed by Cherrett and Peregrine (1976). Pest damage usually causes high mortality to young plants, for example, in Trinidad orchards, 30% of newly-planted citrus trees died due to attack by A. cephalotes (Cherrett and Simms, 1968).

Damage to rubber tree seedlings by A. sexdens and A. laevigata in Brazil resulted in over 70% mortality (Cali and Soares, 1987; Vasconcelos and Cherrett, 1997). In the northeast of Brazil, beans, sorghum, soyabean and rice were considered to be unsuitable for cultivation in areas where A. opaciceps was present, since 71.6-98.9% of the plants were damaged after planting (Sales et al., 1985). Trees in a Brazilian plantation of Pinus caribaea and Gmelina arborea were reported as suffering relatively low mortality due to damage by Atta spp. (Ribeiro and Woessner, 1980). However, in both species, damage caused an increase in branching and a reduction in tree growth. Similarly, in Venezuela, wood production in 8- to 9-year-old plantations of P. caribaea was negatively related to nest densities of A. laevigata (Hernandez and Jaffé, 1995).

The pest potential of grass-cutting Atta spp. to the cattle industry was studied by Robinson and Fowler (1982), and Fowler et al. (1986). Their results indicate that the ants consume at least the same amount of grass as cattle, causing a reduction of 10 to 30% in the number of head of cattle, depending on nest densities. Nesting activities, such as the deposition of excavated soil on the soil surface, results in a loss of area for grass production, as grasses do not grow on the nest mound surface. The area lost varies from 30 to 100 m² per nest (Fowler et al., 1986). In addition, abandoned nests frequently facilitate the invasion of undesirable weeds.

Accidents with cattle and agricultural machinery, due to collapsing nests, have also been reported (Fowler et al., 1986). Similarly, Nogueira and Martinho (1983) documented the fact that leaf-cutter ant nests located under highways can cause parts of the road to collapse.