What environmental condition can enhance the effectiveness of surface oils against mosquito larvae?

Surface oils are designed to create a film over the water's surface to inhibit the respiration of mosquito larvae. When the water surface is calm, this film can spread more uniformly and effectively cover the larvae, reducing their access to air.

In turbulent or choppy water, as caused by wind or rain, the surface oils can break apart or be agitated, which diminishes their effectiveness. Calm conditions allow for better persistence of the oil film on the water, ensuring that larvae are effectively smothered and denied the oxygen they need to survive. Thus, the presence of a tranquil water surface significantly enhances the efficacy of surface oils in mosquito control.