This summer, the District participated in a collaborative study to evaluate a new formulation of methoprene, a biorational mosquito larvicide, in catch basins. While District mosquito control operations has used methoprene products for many years as part of its Integrated Pest Management program (IPM), this collaborative effort introduced our staff to new sampling and testing methods specific to catch basin/storm drain applications. Instead of using traditional cup-shaped dippers to collect water samples, District staff used fish nets attached to long poles to sample the often hard-to-reach underground water sources.
Water samples were quickly evaluated in a white tray in the field to estimate the abundance and stages of mosquito larvae present. It was sometimes quite a challenge to get a good mosquito count with not only hundreds of wriggling larvae, but lots of detritus from the basins floating in the samples as well.
Because of the unique mode of action, special tests are required to evaluate methoprene and other insect growth regulators. This material acts not by directly killing mosquito larvae, but by preventing successful development into biting adults. Therefore, the metric for success in a methoprene treatment is not whether or not you see mosquito larvae in a source, but whether those larvae successfully molt through the pupal stage into flying, biting adult mosquitoes. To assess if the treatment was working, mosquitoes in the pupal stage were brought back to the lab in their source water and allowed to emerge. If no adult mosquitoes flew up to the top of these emergence chambers after a few days in the lab, the treatment was deemed successful. In the coming months, data from District sampling will be combined with results from other districts across the state to provide a balanced assessment of this control method.