What is a primary function of systemic pesticides?

Systemic pesticides serve the primary function of being absorbed by plants and transported throughout their tissues. This characteristic allows them to target pests that feed on the plant, acting not only on the exterior but also internally, where pests might be more challenging to control with contact pesticides. When systemic pesticides are applied, they move into the plant's vascular system, making them available in the leaves, stems, and roots. This systemic action is particularly useful for managing pests that bore into or reside within plant tissues, as they can effectively reach pests that are otherwise protected from topical applications.

Other options suggest functions that are not inherent to systemic pesticides. Instant knockdown refers to the rapid efficacy seen with contact pesticides, where the pesticide must come into direct contact with the pest to be effective. Delivering nutrients is not a role that pesticides play; instead, that function is associated with fertilizers. Lastly, the functionality of only acting on contact excludes the inherent capability of systemic pesticides, which interact internally after being absorbed. Thus, the ability of systemic pesticides to be absorbed and then transported within the plant is their defining feature.