Which of the following statements about insecticides is true?

Insecticides can target specific pests, which is a critical characteristic of many formulations. This targeted action is often achieved through the chemical properties of the insecticide or the mode of action against certain biological systems that are unique to specific insects. For example, some insecticides might interfere with the nervous system of insects, which does not affect other organisms in the same way, making them safer for non-target species. This specificity helps in controlling pest populations effectively while minimizing harm to beneficial insects and the environment.

The other statements do not hold true in general terms. Not all insecticides are safe for humans, as many can be harmful if misused or not handled properly. While some insecticides are designed for quick knockdown of pests, not all will eliminate pests immediately, as effectiveness can depend on several factors, including the type of pest, the method of application, and environmental conditions. Additionally, insecticides can have environmental impacts, influencing ecosystems beyond just the targeted pest, leading to issues like pollinator decline or water contamination. Therefore, the assertion that they have no environmental impact is misleading.