It must be eaten by the insect. Move unto and around plant circulatory system, so they are effective not only against chewing but also against sucking insects. Useful against nematodes. Usually done professionally. Moth balls are example of a slow- releasing fumigant agent.
Insecticides are generally more hazardous to humans than are herbicides.
Infants and pets are at most risk. Toads are natural predators of insects, and so are some birds and other insects such as the praying mantis. Caterpillars and some beetles can be removed this way. Helps to get rid of aphids, spider mites and some scales. After treatment, water your plant abundantly.