Dual life stage effects of spray adjuvants combined with phagostimulants on <i>Drosophila suzukii</i> (Diptera: Drosophilidae) egg viability and adult survival in blueberries

Spray adjuvants are typically considered "inert" ingredients that play a crucial role in enhancing the efficacy of chemical pest control by improving pesticide absorption, spread, penetration, and longevity. This study evaluates the toxicity of organosilicone-based spray adjuvants on Drosophila suzukii Matsumura (Diptera: Drosophilidae), an invasive fruit fly species that poses a significant threat to the soft skin and stone fruit. Experiments were conducted under controlled conditions using 2 primary application methods of dipping and spraying egg-infested blueberries to assess toxicity to D. suzukii eggs and adults. Egg viability was assessed by adults that emerged, while adult mortality was evaluated following exposure to spray adjuvants alone and adjuvants combined with phagostimulants, applied on fresh blueberries. No significant reduction in egg viability was observed for either the dipping or spraying method. However, adult mortality increased significantly when adjuvants were combined with phagostimulants. After 72 h of exposure, D. suzukii populations were reduced by up to 83.0% with Syl-Coat (Wilbur-Ellis; wilburellisagribusiness.com) combined with a phagostimulant and 75.6% with Dyne-Amic (Helena Agri-Enterprises, LLC, Tennessee, United States) combined with a phagostimulant. No significant toxicity was recorded in adults exposed to adjuvants alone, although a numerical reduction in D. suzukii survival was noted with increasing exposure time. These findings demonstrate that combining a phagostimulant with an appropriate adjuvant provides a promising, reduced-risk approach to suppress Drosophila suzukii populations while reducing dependence on conventional insecticides. The results also highlight the importance of using spray adjuvants in combination with phagostimulants to enhance feeding-based management strategies and improve overall control efficacy.