Why must spray applications dry onto a surface before rain?

Rainfall on pesticide applications before they have dried can lead to runoff, which diminishes the effectiveness of the pesticide and can contaminate nearby water sources. When pesticides are still wet, they have not yet adhered to the target surface, and rain can wash them away, resulting in insufficient coverage for pest control. Proper drying allows the pesticide to bond with the plant or soil, maximizing its efficacy and minimizing unintended impacts on the environment.

The principle behind needing pesticides to dry is rooted in their mode of action—once dried, they form a protective barrier, allowing for the desired pest management effect to take place effectively without being disrupted by environmental factors like rain.