Replicated field trials of commercial granular formulations of chlorpyrifos, fenthion and temephos were conducted in Colorado against the larvae of Culicoides variipennis. Test sites were littoral habitats polluted with decomposing animal manure, ideal conditions for larval development. Efficacy was evaluated by counts made prior to and after treatments. Samples taken at 72 hr post-treatment showed that mean reductions in larval populations by chlorpyrifos at 0.20, 0.10 and 0.05 ppm were 100%, 50% and 63.7% respectively; by fenthion at 1.00,0.50 and 0.25 ppm were 94.5%, 99% and 90.7%, respectively, and by temephos at 2.0, 1.0 and 0.5 ppm were 99.5%, 99.7 and 99.3%, respectively.