Control of mosquito larvae in catch basins using pyriproxyfen and the mechanism underlying residual efficacy

Abstract

Storm water catch basins are a major source of dengue vector mosquito larvae (Diptera: Culicidae) in the urban and residential areas of Japan. In this study, the mechanisms underlying the residual efficacy of the insect growth regulator pyriproxyfen in treated catch basins were examined. One tablet of the formulation of pyriproxyfen, SumiLarv^®Tablet, was applied to a catch basin at 1-month intervals in Takarazuka, Japan. Simultaneously, one disk of the matrix-release formulation of pyriproxyfen, SumiLarv^®2MR, was applied to another catch basin. Weekly bioassays of water sampled from the treated basins showed insufficient efficacy against fourth-instar larvae of laboratory-reared Aedes albopictus. However, over 6 months, mortality rates greater than 95% and 100% were observed in pupae collected from the basins treated with SumiLarv^®Tablet and SumiLarv^®2MR, respectively. Laboratory experiments were conducted to clarify the difference in the efficacy between the water sampling method and the pupal collection method. Bioassay using water samples containing added antibiotics indicated that biodegradation of the active ingredient in water caused the low observed efficacy when using the water sampling method. By contrast, the active ingredient that was retained by organic matter persisted for several weeks and was gradually released to the water. In addition, larval ingestion of organic matter that had absorbed the active ingredient appeared to enhance mortality rates. Thus, organic matter in the water was responsible for the high efficacy of the pupal collection method. This study confirmed the importance of organic matter as a reservoir of pyriproxyfen in water.