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

The reason spray applications must dry onto a surface before rain falls is primarily to ensure that the pesticide adheres properly to the treated area. If the pesticide has not fully dried, rain can wash it away, leading to runoff. This runoff can reduce the effectiveness of the application, as the pesticide may not remain where it needs to act against the targeted pests.

By ensuring that the spray is dry before rain occurs, the pesticide can create a strong enough bond with the surface or plant to provide its intended protective efficacy without the risk of being washed away. This bonding is crucial for the successful control of pests and achieving the desired results from the application.

While other factors like dilution or reactions with rainwater can have some effects, the primary concern regarding effective pesticide application is preventing runoff, making it essential for the product to dry first.