The fungus Beauveria bassiana (Unioeste 76) was tested against the soybean pest Euschistus heros 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 (B. bassiana with chemical insecticides), with careful consideration of application intervals, could provide a sustainable and effective method for managing natural populations of E. heros.