How does a systemic pesticide function?

A systemic pesticide functions by being absorbed into the plant or organism and then circulated throughout its tissues. This means that when a pest tries to feed on the part of the plant where the pesticide has spread, it ingests the toxic substance, leading to the pest's control. This method is particularly effective for pests that feed on plant sap or other internal tissues, as the systemic nature ensures that the pesticide is present where the pests are active.

The other options do not accurately describe the functionality of systemic pesticides. For instance, a pesticide that kills pests solely on contact would not be classified as systemic since it doesn't require absorption or circulation. A systemic pesticide doesn't act independently of the organism; instead, it relies on the host plant's ability to absorb and distribute the pesticide. Additionally, limiting the effects of a pesticide to just the roots of plants would not capture the full capability of systemic pesticides, which can distribute throughout the entire plant, affecting all parts, including leaves and stems.