What characterizes contact pesticides?

Contact pesticides are defined by their mode of action, specifically the requirement that they must physically come into contact with the pest to be effective. This characteristic means that the pesticide needs to be applied directly onto the target organism, such as insects or weeds, in order for it to initiate its toxic effects. This direct contact facilitates the absorption of the chemical, leading to pest control.

In contrast, the other options reflect characteristics that do not align with the nature of contact pesticides. For instance, the idea that a pesticide can be applied once and remain effective indefinitely pertains more to systemic pesticides, which are capable of being absorbed by the plant and providing ongoing protection from within. Similarly, the notion that contact pesticides penetrate the plant and act internally describes a different category of pesticides entirely—specifically systemic pesticides that distribute throughout the plant's tissues. Lastly, stating that contact pesticides are only effective in dry conditions is misleading, as their effectiveness is contingent upon direct application rather than environmental moisture conditions. Therefore, the key feature that clearly distinguishes contact pesticides is their requirement to physically touch the pest.