Evaluating the role of microbial degradation and analytical methods in monitoring imidacloprid persistence for white grub management in turfgrass

White grubs (Coleoptera: Scarabaeidae) are the most destructive insect pests of cool- and warm-season turfgrasses worldwide (Vittum, 2020). Neonicotinoids, particularly products containing the active ingredient imidacloprid, have been widely used to control grubs since the 1990s. Many turfgrass managers make preventive applications near egg hatch in summer to control larvae upon emergence and avoid larval feeding damage. Recently, turfgrass managers in the northeastern United States report white grub control failures at sites where imidacloprid has been applied annually for over a decade. We investigated the potential of microbial degradation at sites with a sustained history of imidacloprid use. Additionally, we explored the use of enzyme-linked immunosorbent assay (ELISA) kits to reliably quantify imidacloprid residues in soils and translocation in turfgrass over time to assist in future surveillance efforts. Field-collected soils were split between sterilized and non-sterilized treatments for use in greenhouse studies. Lolium perenne was planted in each treatment group, and the concentration of imidacloprid in various tissues was quantified over a period of 56 days and compared with estimated obtained through high-performance liquid chromatography (HPLC). ELISA provided reliable estimates of relative imidacloprid concentration, though it provided significantly higher estimates than those determined by HPLC. Significant degradation of imidacloprid was observed between 28 and 56 days after treatment, though differences between sterilized- and non-sterilized soil treatments were less than 5 ppb. To understand whether these concentrations would amount to failure, imidacloprid was applied at two rates and three timings relative to oviposition in field studies. Concentrations in roots remained relatively stable, with treatments ranging between 37 and 59 ppm at oviposition. All treatments provided complete control of larvae. Significant degradation was only observed after 100 days. Given the concentration differences due to microbial degradation observed in greenhouse studies, it is unlikely that microbial degradation is the sole contributor to product failure.