Which of the following can lead to reduced efficacy of pesticides?

Storing pesticides in direct sunlight can lead to reduced efficacy because exposure to UV rays and heat can degrade the chemical components of the pesticide. Pesticides are formulated to be stable under specific conditions, and prolonged exposure to extreme temperatures or light can break down their active ingredients, making them less effective or even ineffective when applied.

In contrast, closely following label instructions ensures that the product is used as intended, maximizing its effectiveness. Applying pesticides before rain can wash the chemicals away before they effectively take action, but proper application itself does generally promote effectiveness when conditions are suitable. Using fresh products within their expiration date ensures that the pesticides maintain their potency, further supporting their efficacy. Therefore, only storing pesticides in direct sunlight is a clear factor that directly undermines their effectiveness.