Influence of Rainfall and Drying Time on the Efficacy of Insecticides for the Control of Grapholita molesta (Busck) (Lepidoptera: Tortricidae) in Apple Orchards

Field experiments were conducted to evaluate the effects of rainfall and drying time on the efficacy of two chemical insecticides used to control the oriental fruit moth (Grapholita molesta; Lepidoptera: Tortricidae) in apple orchards. The study was carried out at the Temperate Climate Fruit Experimental Station of the Brazilian Agricultural Research Corporation (Embrapa Uva e Vinho) in Vacaria, RS, Brazil, in a 0.4‑hectare orchard with 7‑ and 8‑year‑old Gala and Fuji apple trees during the 2018/2019 and 2019/2020 production cycles. A Swanson‑type rainfall simulator was employed to reproduce rainfall events under commercial orchard conditions. Simulated rainfall at intensities of 10 or 50 mm h⁻¹ did not reduce the overall efficacy of insecticide control within 1 hour after precipitation. Notably, control efficacy increased during the first 30 minutes of rainfall, likely due to the redistribution of pesticides across plant surfaces. A drying period of 6 hours for both phosmet and chlorantraniliprole was sufficient to maintain insecticide performance, regardless of rainfall intensity. These findings provide practical guidance for apple growers, supporting informed decision‑making regarding the optimal timing for insecticide reapplication under varying rainfall conditions.