Why must spray applications dry onto a surface before the rain?

When pesticide applications are made, it is crucial for them to dry onto the targeted surface before exposure to rain. This is primarily because if the spray has not dried, rain can lead to runoff, washing away the pesticide and significantly reducing its effectiveness. When pesticides are applied, they need time to adhere to the surface they are intended to protect or treat. If rain occurs before the drying process is complete, it can wash away not just the pesticide, but also the protective benefits it would have provided against target pests.

While other options mention aspects related to dilution or potential chemical reactions, the core reason centers around the actual physical process of adherence and the potential loss of the product through runoff caused by rain. Hence, ensuring that applications dry adequately before precipitation is vital to achieving the desired pest control outcomes.