How does a systemic insecticide act on the pest?

A systemic insecticide operates by being absorbed by the plant it is applied to, becoming integrated into its tissues, and ultimately affecting any pests that consume parts of the plant. When insects feed on the treated plant, they ingest the systemic insecticide along with the plant material. This method of action allows for targeted pest control, as the insecticide is delivered directly within the plant, making it effective even if pests do not come into direct contact with the insecticide itself. This is particularly useful for managing pests that are difficult to reach or control through contact insecticides alone.

In contrast, the other options describe actions that do not align with the unique mechanism of systemic insecticides. For instance, the option suggesting that a systemic insecticide enters the pest and attacks a specific body system misrepresents how these insecticides function, as they work primarily through ingestion rather than direct entry into the pest's body. Additionally, the assertion that systemic insecticides kill pests on contact or have no effect on them does not align with the scientific understanding of how these products are designed to work. Systemic insecticides are specifically formulated to act via the plant, making option A the accurate description of their function.