How do stomach insecticides primarily work?

Stomach insecticides primarily function by being ingested by the target insect. When these insecticides are consumed, they disrupt the insect's normal physiological processes. This can lead to a variety of effects, such as interfering with the digestion of food or causing toxicity that can ultimately kill the insect. The mechanism relies on the active ingredients being processed in the insect's digestive system, where they exert their harmful effects.

Other methods listed, such as contact through the exoskeleton or absorption through the leaves, describe different types of insecticides. Contact insecticides primarily act when they come into direct contact with the insect's body surface, while systemic insecticides are absorbed by the plant and then consumed by the insect when it feeds on the plant. Vapor exposure refers to products that release fumes into the environment to affect insects, which is a different mode of action altogether. Therefore, the ingestion mechanism is specifically characteristic of stomach insecticides, making it the correct choice.