Field-simulated rainfall impacts the efficacy of foliar insecticides against thrips in cotton

Tobacco thrips (Frankliniella fusca) and western flower thrips (F. occidentalis) are seedling pests of cotton that cause significant economic losses annually in the USA. Growers often rely on foliar-applied insecticides to reduce thrips populations. However, environmental factors such as rain can affect insecticide efficacy. To evaluate thrips insecticide efficacy after rain events, we conducted field studies in Jackson Springs, NC in 2023, and in Lewiston, NC in 2024. We sprayed at the 1st true leaf stage with Orthene® 97 (acephate) and Intrepid Edge® (methoxyfenozide + spinetoram) at concentrations of 0.5 lb/acre and 6 oz/acre, respectively, along with untreated plots as a control. We simulated 2.54 cm rain events at 1, 4, or 8 hours after insecticide application. We also had a control with no simulated rain. We visually assessed cotton seedling injury at 1 and 2 weeks following application, using a standardized 1-5 rating scale. We also counted the number of thrips on plants and recorded aboveground plant biomass. Our results show that rain events can negatively impact thrips control, especially for Intrepid Edge, which had reduced effectiveness under simulated rain compared to Orthene. However, in the absence of rain, Intrepid Edge provided better thrips control. Our findings emphasize the need to optimize spray timing in relation to rainfall to preserve thrips insecticide performance.