Seleção de isolados de fungos entomopatogênicos e estratégia de aplicação para controle de Atta sexdens Linnaeus, 1758 (Hymenoptera: Formicidae)

Abstract

Leaf-cutter ants cause significant damage to different agroecosystems, with conventional management based on the use of toxic baits or insecticide application in ant nests, restricting their use in agroecological areas and areas protecting native vegetation, highlighting the use of entomopathogenic fungi as a promising alternative. This work aimed to select isolates of the fungi Beauveria bassiana, Metarhizium anisopliae, and Cordyceps spp. considering pathogenicity, virulence, and productivity, changing the control of Atta sexdens via attractive baits. The bioassays were divided in two stages: (1) Pathogenicity, virulence, and productivity of the isolates; (2) use of the selected isolate to impregnate baits based on oat flakes offered to laboratory ant nests containing the queen, workers, and the cultivated fungus. (1) Twelve isolates of B. bassiana, four of M. anisopliae, two of Cordyceps fumosorosea, and one of C. javanica were tested. 1 mL of conidial suspension in distilled water + 0.01% Tween 80® (10⁹ conidia/mL) was sprayed using an airbrush coupled to an air compressor onto foraging and soldier castes collected from ant nests in the field. In the control, only distilled and sterilized water was sprayed. The insects were trapped in gerbox containers (10 insects per reproduction), containing moistened paper at the base and honey solution (10%) (22 ± 2°C, 24 hours in the dark). Isolates that caused mortality were ≥ 85% selected and compared for virulence at different concentrations (10⁶, 10⁷, and 10⁸ conidia/mL). Conidia productivity was also compared in Petri dishes with Potato-Dextrose-Agar (PDA) medium and in parboiled rice. (2) Two-year-old ant colonies maintained in a semi-climate-controlled room (22 ± 2 ºC, 24 hours of darkness) received oat-based baits with the following treatments: pure soybean oil, soybean oil + conidia (1 × 10⁹ conidia/mL), dried conidia (7 × 10¹⁰ conidia/g) and control (pure oats). The bait was offered at 4 times and the colonies were evaluated for 21 days. The parameters evaluated were ant nest weight, percentage of flake loading, attraction frequency, erratic movement, weight of the contents of the garbage chamber and dead individuals outside and inside the garbage chamber. The Unioeste 100 and 104 isolates of B. bassiana were selected, with confirmed mortality between 86 and 98%. Regarding the concentration test, there was little variation in susceptibility among the castes and concentrations evaluated. The Unioeste 100 isolate stood out for its higher productivity in PDA medium (3.24×10⁸ conidia/cm²) and in rice (3.45×10⁹ conidia/g of rice), being selected as the most suitable for the second stage. All treatments included an increase in the weight of the ant nests (from 0.6% to 10.7%) and in the percentage of infestation over time (from 10% to 49%). There was a higher occurrence of grooming, erratic movement, and dead ants outside and inside the garbage chamber in the treatments containing entomopathogenic fungus, with no difference in the weight of the garbage chamber contents. One of the ant colonies treated with oat bait and dried conidia died.