263 - Efficacy testing of a residual mosquito larvicide on flood irrigated ranches in Teton County, Wyoming

Flood irrigated ranch lands present unique challenges for mosquito management in western Wyoming given the overwhelming numbers of floodwater nuisance mosquitoes produced throughout the summer. In Teton County it is common practice for private ranches to flood large areas (400+ acres) of grassland throughout the season, creating ample, long-lasting, anthropogenic habitat for mosquito breeding. Not only can this extend the natural breeding season for floodwater species, but because they are often flooded all summer, breeding sites for semi-permanent and permanent mosquito species such as the West Nile Virus vector Culex tarsalis are created. The repeated flooding and continuous movement of water makes it difficult to know whether insecticides designed with long residual times remain efficacious throughout the season. We conducted 5-week efficacy tests to determine the duration of effective control achieved after treatment with a 42-day residual methoprene larvicide (MetaLarv). Control and treatment larval samples were collected weekly from five treatment and control polygons in the field and reared in the lab in triplicate. Daily observations of larval development and mortality rates were recorded for each sample. Analysis of results will be discussed and will inform a cost-benefit approach to product purchasing and management decisions among flood irrigated ranches for future mosquito seasons.