Methoprene, an insecticide known as an insect growth regulator, is a true analog of a mosquito’s own growth or juvenile hormone. This insecticide works by inducing damaging physical changes to second, third and fourth stage (instar) mosquito larvae preventing the successful emergence of adult mosquitoes from the pupal stage.

Unlike most insecticides, methoprene is not a direct toxin. This insecticide is target specific, affecting only mosquitoes when used at Federally and State mandated label rates for mosquito control. Like the larvicide’s Bti (Bacillus thuringiensis var. israelensis serotype H-14) and Bacillus sphaericus, this insecticide must be applied to all of the water areas that have larval mosquitoes to be effective. Methoprene has a very limited residual effect, being light sensitive and breaking down within three to four days in an aquatic environment. Timing of the applications of Methoprene is critical since this insecticide has no effect on pupal or adult mosquitoes. Therefore, it must be applied on a regular basis to those areas, which have continuous mosquito breeding problems. The effectiveness of Methoprene is limited by the density of vegetative cover, organic content of the water and the density of the larval mosquito population.

Methoprene is routinely used and is applied to those water sites that contain the larval or immature stages of mosquitoes. This material does not affect the pupal or adult stages and thus requires proper timing of the application in order to be effective.