Evaluation of Storm-Water Impacts on Larviciding Chemicals at Catch Basins for West Nile Virus Control

Methoprene larvicide is a biochemical insect growth regulator used in North America to control the mosquito population. It can be incorporated in clay pellets or chalks and dropped into a catch basin sump. The slow release mechanism of methoprene has the potential to inhibit the growth of mosquito larvae into pupae in the sump and reduce the emergence of adult mosquitoes. As a result, the vector transport of the West Nile virus can be minimized. The main concern is whether or not the methoprene pellets or ingots will remain in a catch basin sump or be flushed during storm events, resulting in ecological impacts on the receiving waters. A full-scale physical model was constructed to investigate the flushing characteristics of methoprene pellets and chalks in a catch basin. It was found that (1) the sump water could provide a cushion against the hydraulic turbulence of inflows and trap the pellets and ingots; (2) flushing of pellets or ingots was more sensitive to the critical inflow magnitude than the time to reach this flow; (3) ingots would require a higher inflow to be flushed than would pellets; and (4) sediments and leaves at the sump might inhibit the flushing of pellets. Using the measured critical inflow rate at which pellets and ingots were flushed in the experiments, it was found that a 3-month storm and 1.4-year storm in Toronto might have the potential to flush out pellets and ingots respectively in a catch basin filled with sediments. Additionally, the risk of flushing pellets over the summer months may be high when the sump water depth of a catch basin is shallow. On the other hand, the risk of flushing ingots over the summer months is low. As a result, ingots should be used in larviciding catch basins.