Why is it not recommended to apply pesticides before rainfall?

Applying pesticides before rainfall is not recommended primarily because rain can wash away the pesticide, which not only reduces its effectiveness but also increases the risk of runoff. When pesticides are applied to a surface and then rain occurs shortly thereafter, a significant portion of those chemicals can be carried off the target area, leading to incomplete pest control. This runoff can also pose a risk to nearby water sources, as the pesticides may contaminate soil and water bodies rather than remaining in place to perform their intended function against pests.

The effectiveness of pesticides is highly dependent on their ability to stay on the target area long enough to be absorbed by plants or ingested by pests. If rainfall occurs soon after application, the pesticide may not have adequate time to work, resulting in less control over the pest population and greater environmental concerns due to potential runoff. Thus, timing pesticide applications in relation to rainfall is critical for both efficacy and safety.