Which of the following is a common effect of pesticide application on non-target organisms?

The application of pesticides can lead to decreased populations of beneficial insects, making this the correct choice. Pesticides are designed to control pests, but they often do not differentiate between harmful pests and beneficial organisms, such as pollinators and natural pest predators. When pesticides are applied, they can inadvertently affect these non-target organisms, leading to a decline in their populations. This reduction can disrupt ecosystems and negatively impact agricultural practices, as beneficial insects play crucial roles in pollination and pest management.

In contrast, the other options suggest outcomes that are generally not associated with pesticide application. Improved biodiversity is unlikely when beneficial species are harmed by chemicals. Similarly, while some chemical fertilizers may enhance soil fertility, most pesticides can have the opposite effect by harming soil-dwelling organisms essential for maintaining soil health. Lastly, enhanced water quality is not a typical result of pesticide use; in fact, runoff from pesticide applications can lead to water contamination, adversely affecting aquatic life and overall water quality.