Effectiveness of an oil-based <i>Beauveria bassiana</i> formulation for controlling the neotropical brown stink bug, <i>Euschistus heros</i> (Heteroptera: Pentatomidae) on soybean crops

The fungus <i>Beauveria bassiana</i> (Unioeste 76) was tested against the soybean pest <i>Euschistus heros</i> in laboratory, greenhouse, and field. In the laboratory, insects were sprayed with pure conidia (TC) suspended in distilled water or in an oil dispersion formulation (OD; vegetable oil) at a concentration of 10⁹ conidia/mL. The UV-B radiation and heat tolerance of the conidia were also assessed. After 12 days, the mortality rates in the laboratory were 70% for the TC treatment and 80% for the OD treatment. In the greenhouse pre-infestation bioassay, which used soybean plants in cages, the fungal treatments resulted in 52% and 47% mortality for the TC and OD formulations, respectively. In the post-infestation bioassay, both fungal treatments caused 83% mortality. In the field trial conducted on soybean plots (14 × 18 m), the treatments included: (i) biological: OD (10⁹ conidia/mL); (ii) chemical insecticide; (iii) biological + chemical, all applied at 150 L/ha. Insect numbers were evaluated using beating-sheet sampling. In the final population sample, the biological treatment showed a population density similar to the chemical treatment (0.94 and 0.83 insects/m, respectively), both below the economic threshold. Conidia tolerance to UV-B radiation was similar across both treatments, but conidia in oil were less tolerant to heat. These results suggest that strategically combining both approaches (<i>B. bassiana</i> with chemical insecticides), with careful consideration of application intervals, could provide a sustainable and effective method for managing natural populations of <i>E. heros</i>.