Which type of pesticide is effective only on exposed parts of pests or plants?

The type of pesticide that is effective only on exposed parts of pests or plants is contact pesticides. This category of pesticides functions by coming into direct contact with the target organism, leading to its effectiveness on the surfaces that are treated. When a contact pesticide is applied, it requires the pest or weed to be present on the treated area to initiate its mode of action.

In comparison, translocated pesticides are absorbed by the organism and then moved throughout its system, allowing them to affect parts that are not directly contacted during application. Preemergence herbicides work by targeting weeds before they germinate, usually affecting the soil environment rather than exposed parts of plants or pests. Postemergence herbicides address weeds after they have emerged, but they can either be contact or systemic depending on their formulation, and thus do not strictly refer to exposed parts only. Therefore, contact pesticides are specifically designed for direct application to surfaces where the pests reside.