Environmental Factors Affecting Methoprene Concentrations for West Nile Virus Control in a Storm Sewer System

To address vector-borne West Nile virus in Canada, chemical larvicides (methoprene) are applied to storm sewer system catch basins (CBs) to control mosquitoes. This study assessed the fate and transport of methoprene in such systems over time relative to precipitation. Rainfall and methoprene concentration patterns revealed the effect of dilution, dissolution, and fl ushing of the larvicide. In the summer and fall of 2003 to 2005, fi eld monitoring studies were conducted in Toronto, Ontario on two CBs, each treated with a control dose of methoprene, supplied in pellet or ingot formulation. Furthermore, in 2005, concentrations at the storm sewer outfall were measured during nine rainfall events. Based on daily monitoring, fi ndings indicate that (1) the methoprene concentration at the CBs fell below the minimum lethal concentration or LC 50 one or two weeks after treatment, and remained below LC 50 concentrations over 70% of the time; (2) rainfall fl ushed methoprene from the CBs to the storm sewer outfall at concentrations higher than the levels specifi ed by Ministry of Environment, which may cause ecosystem damage; (3) based on the number of cycles per diem within each CB in each study period, there was no conclusive pattern in the fl ushing susceptibility of pellets versus ingots; (4) the mean concentration of methoprene increased with reduced CB sump volume; (5) less total precipitation resulted in higher average methoprene concentrations and a higher number of days above the LC 50 based on ingot-dosed CBs; (6) counter-intuitive to (4) and (5), larger sump water volumes and greater rainfall resulted in lower mean concentrations and fewer days above the LC 50 ; and (7) a single ingot dosage was comparable in performance to a three pellet dosage.