The duration of the efficacy of 4% methoprene pellets against emergence of chironomid midges, Chironomus stigmaterus Say, Goeldichironomus amazonicus (Fittkau) and Tanypus imperialis Sublette, in man-made lakes supplied with tertiary effluent water in Palm Desert, California, was dose dependent. For C. stigmaterus and T. imperialis, the rate of 4.5 kg/ha of the controlled release pellet formulation of 4% methoprene yielded > 90% control for 3 wk and 75% control in the 4th week, the rate of 3.4 kg/ha provided > 90% control for 2 wk, and the rate of 2.25 kg/ha provided control for 1 wk. Goeldichironomus amazonicus was under 90% control for 2 wk at the rate of 4.5 kg/ha and 1 wk at the rates of 3.4 kg/ha and 2.25 kg/ha.