Which of the following pesticides effectively control maize stem borers?

The correct answer is "systematic insecticides". Maize stem borers are insects that bore into the stem of the maize plant, causing damage and reducing yield. Systematic insecticides are pesticides that are absorbed by the plant and transported to all parts of the plant, including the stem. This makes them effective in controlling insects that bore into the stem, such as maize stem borers. Contact poisons, on the other hand, are pesticides that kill insects on contact with the plant surface, and may not be effective against insects that are inside the plant. Nematicides are pesticides used to control nematodes, which are not the same as stem borers. Fumigants are pesticides that are inhaled by insects and can be used to control pests in stored grains or in the soil, but may not be effective against pests that are actively feeding on the plant.