Abstract
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 flushing of the larvicide. In the summer and fall of 2003 to 2005, field 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, findings indicate that (1) the methoprene concentration at the CBs fell below the minimum lethal concentration or LC50 one or two weeks after treatment, and remained below LC50 concentrations over 70% of the time; (2) rainfall flushed methoprene from the CBs to the storm sewer outfall at concentrations higher than the levels specified 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 flushing 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 LC50 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 LC50; and (7) a single ingot dosage was comparable in performance to a three pellet dosage.
Exploring vector-borne infection ecology in multi-host communities: A case study of West Nile virus