What characteristic defines systemic insecticides?

Systemic insecticides are characterized by their ability to be absorbed by plants and distributed throughout their tissues. When these insecticides are taken up by the roots or leaves of a plant, they move within the plant and become part of its fluid systems, allowing them to be present in the sap and other plant tissues. This mechanism is particularly effective because it means that pest insects that feed on the treated plant consume the insecticide as they ingest plant material. This mode of action enables systemic insecticides to effectively target pests that might otherwise evade traditional surface treatments, which rely on the insect coming into direct contact with the pesticide.

The other options do not correctly define systemic insecticides. While some systemic insecticides may be derived from natural sources, this is not a defining characteristic since there are also synthetic options. The idea that they affect pests regardless of the application method is misleading; they must be absorbed by the plant to work effectively, and their efficacy largely depends on the plant's uptake. Lastly, the statement regarding the frequency of application is not universally applicable, as systemic insecticides can vary in their persistence and duration of effectiveness, thus influencing how often they might need to be reapplied.