Why might re-applications of pesticide be necessary?

The necessity for re-applications of pesticides is often tied to environmental factors and the characteristics of the pesticides themselves. In the context of the chosen answer, contact herbicides indeed can dilute quickly, especially when exposed to rain or irrigation, which might wash away the active ingredients and reduce their effectiveness. This dilution can undermine the initial application, leading to the need for additional treatments to achieve the desired pest control results.

While contact herbicides may require re-application due to their rapid dilution, it's important to recognize that other factors also contribute to the need for follow-up treatments. Pesticides, including systemic types, may become less potent after their initial application due to degradation over time, exposure to sunlight, or interactions with other environmental factors, which also necessitate re-applications to maintain efficacy.