Which of the following indicates that a pesticide is working effectively?

A reduction in pest populations over time is a clear indicator that a pesticide is working effectively. When a pesticide is applied, it is designed to target specific pests, and its effectiveness can be measured by observing a decline in the numbers of those pests. This decrease generally occurs as the pesticide takes effect, leading to fewer individuals capable of causing damage.

In the context of pest management, the primary goal of using pesticides is to control or eliminate pest populations that threaten crops or the environment. The visible signs of pest damage on plants may occur even when a pesticide is effective, as there can be a delay between application and noticeable changes in pest behavior or population. Similarly, a delay in the expected action time could suggest that the pesticide is taking longer to show results, but it does not necessarily reflect its effectiveness. Increased populations of the target pest would indicate the opposite of effectiveness, suggesting that the pesticide may not be functioning as intended. Therefore, observing a consistent reduction in pest populations over time is the most reliable measure of a pesticide's successful application.