Efficacy and Potential for Non-Target Effects of Larvicides for West Nile Virus Control in Seatttle Catch Basin

Abstract

In anticipation of the spread of West Nile Virus, Seattle officials promoted a city-wide research effort over the summers of 2006 and 2007 designed to investigate the efficacy and fate of four common larvicides: Mosquito Dunks® and Bits® (Bacillus thuringiensis [Bti]), VectoLex® WSP (Bacillus sphaericus [Bs]), VectoLex® CG (Bs), and Altosid® Briquets (Methoprene). All treatments resulted in a rapid reduction in number of pupae (Bti and Bs treatments) or emergence success of pupae (Methoprene). The Bs treatment was the most efficacious in 2006. From these results, VectoLex® CG was chosen for city-wide application in 2007. Efficacy was realized in most cases for 7 weeks, though a relatively high number of precipitation events confounded the direct effect of the larvicide. The larvicide was detected above background levels at least one week post-treatment in each of the basins chosen to monitor larvicide fate; subsequent precipitation events likely decreased the Bs concentrations. Larvicide was detected in three urban creeks (Longfellow, Pipers, Thornton) connected to treated basins. Results from toxicity tests suggest that concentrations of Bs and Bti detected in each of the watersheds pose little hazard to juvenile salmonids.