Bacillus thuringiensis Berliner is a highly efficacious bioinsecticide used to control lepidopteran pests in the field. Unfortunately, it has limited residual activity on plants because sunlight inactivates spores and crystals and they can be washed off by rain. To minimize loss of activity, formulations must contain UV protectants, stickers, or both. We tested ≈80 formulations and determined optimal combinations of ingredients and spray drying conditions for improving B. thuringiensis residual activity after simulated rain and simulated sunlight. B. thuringiensis stability, after simulated sunlight (xenon light/8 h) and rain (5 cm/50 min), was improved using formulations based on lignin, corn flours, or both, with up to 20% of the active ingredient, when compared with technical powder or Dipel 2× in laboratory assays. Two formulations, made with corn flours or lignin pregelatinized corn flour (PCF), killed 51.6 and 75.3% of Ostrinia nubilalis (Hübner) neonates after rain, respectively, versus 27% for technical powder. When the insecticidal activity was tested after simulated sunlight, corn flour-based formulations killed 78.5% of test larvae, and the lignin PCF formulation killed 70.4%, in contrast to technical powder which caused an average of 29% mortality. Formulations made with Dipel 2× rather than technical powder, caused 62.5% mortality (corn flour-based formulations), and 72.3% mortality (lignin PCF), versus 53.4% for Dipel 2× after rain. When tested after simulated sunlight, formulations killed 95% of the larvae (average of both formulations) versus 82% for Dipel 2×. In a field test, formulations were applied to cabbage and insecticidal activity was determined against Trichoplusia ni (Hübner) neonates exposed to treated leaves. Insecticidal activity of the corn flour-based formulations was comparable to Dipel 2× for 4 d after treatment, but was significantly better than Dipel 2× 7 d after application. A lignin and PCF-based formulation showed significantly higher residual activity than Dipel 2×, 4 and 7 d after application.
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Vol. 93 • No. 2