Which chemical substance is primarily used to control insects and rodents?

The correct choice is pesticide, which is specifically designed to control a variety of pests, including insects and rodents. Pesticides include chemical substances that target organisms harmful to crops and human health. They can take many forms, including insecticides for insects, rodenticides for rodents, and other formulations that can address various pest issues.

In contrast, herbicides are chemical agents used to kill or inhibit the growth of unwanted plants, particularly weeds. While they are important in agriculture, their primary function is not to control insects or rodents. Fungicides are used to eliminate or inhibit the growth of fungi and are focused on preventing plant diseases caused by fungal infections. Fertilizers, while essential for plant growth by providing nutrients, do not have pest control properties. Thus, pesticides stand out as the appropriate substances for managing insects and rodents.