What is a systemic insecticide primarily used for?

A systemic insecticide is designed to be absorbed by plants and then distributed throughout their tissues. This type of insecticide works effectively because it targets insects that feed on the plant, as they ingest the insecticide when they consume the plant material. This method not only effectively eliminates the targeted pests but also helps protect the plant from further insect damage during its growth cycle.

In contrast, options that suggest killing insects on contact or creating barriers against insect movement do not accurately describe the mechanism of systemic insecticides, as these actions are characteristic of contact insecticides or barrier treatments, respectively. Additionally, repelling insects involves different chemical properties and modes of action that do not align with the characteristics of systemic insecticides. Therefore, the focus on how systemic insecticides operate specifically through absorption by plants and subsequent impact on feeding insects is what makes this answer correct.