Efeito de diferentes cepas de Bacillus thuringiensis em larvas e adultos da formigas-cortadeiras Atta sexdens (Hymenoptera: Formicidae)

Abstract

The leafcutter ants Atta sexdens are the most important forest pests in tropical countries and can cause major economic damage mainly during seedling establishment. The management of this forest pests is done predominantly with baits and chemical-based insecticides. However, beyond the toxicity against the environmental and human health these pesticides are not target-specific and can cause death of pollinators, predators and natural enemies. Furthermore, the active principle of this pesticides may also be banned from the market in the coming years, which will cause a demand for efficient products for the successful control of these insects. In this way, sustainable alternatives for the management of forest pests has been the focus of several research centers. The entomopathogenic bacteria Bacillus thuringiensis (Bt) has been used for many decades in agricultural pests’ control and has the advantage to be target specific, safe for mammals and environmentally friendly. In adverse growing conditions, B. thuringiensis produces spores associated with Crystal proteins which have insecticide activity. The toxins produced by Bt bacteria are used successfully in Lepidoptera and Coleopteran larvae insects, these youngest insects cause the largest damage to agricultural crops. However, contrary to this fact, A. sexdens larvae has not mobility and forager adults are the main concern and the focus for the control. Therefore, the present work brings an unprecedented approach, identify differences in susceptibility to the eight strains of B. thuringiensis (T06 - entomocidus, T07 - aizawai, T10 - darmstadienis, T16 - indiana, T23 - japonensis, T24 - neoleonensis, T27 - mexicanensis e T29 -amargiensis) between larvae and foragers of A. sexdens. The efficiency of strains T24, T06 and T16 in controlling larvae was evident, with mortality of 79.6%, 73.7% and 67.6%, respectively. T24 was considered the best strain to control A sexdens larvae due to mortality percentage, mean lethal time (5.000 ± 0.339 days) and mean lethal concentration (7.48x105 spores mL-1). It was also evident that Bt strains did not induce significant mortality in adult forager ants. Furthermore, the present work highlights the main factor that makes adult forager ants less susceptible to Bt spores compared to larvae. The infrabuccal pocket (present in adult forager ant’s head) filters out most of Bt spores before reaching the midgut, the main target of Bt toxins. Bt strains can be used in the biological control of larvae, however, a delivery method for the toxin is required in the field to reach the larvae inside the nest. Alternatively, the solubilization and activation of Bt toxins may enable the use of these strains in the integrated management for both adult foragers and larvae of A. sexdens.