Which of the following is an advantage of pesticide residues?

Pesticide residues offer the advantage of less frequent reapplication needed. When residues remain on crops after application, they can continue to provide protective effects against pests. This means that the crop is less likely to suffer damage from pest infestations for an extended period, which can lead to increased efficiency in agricultural practices. By reducing the need for constant reapplication, farmers can save on labor and pesticide costs while also minimizing the overall environmental impact of using pesticides.

The other options do not present advantages. More frequent applications required refers to the increased labor and cost involved with needing to apply pesticides often, which is not beneficial. Reduced effectiveness over time suggests that the pesticides may lose their potency, resulting in a need for more frequent applications or higher dosages, which is again not advantageous. Lastly, harmful effects on aquatic life point to environmental concerns that arise from pesticide use, which is considered a significant disadvantage rather than an advantage.