Pesticides Pesticides is somewhat of an umbrella topic. A pesticide is classified according to what its job is to control. There are four main types, or classes, of pesticides. They are herbicides, fungicides, rodenticides, and insecticides. Herbicides are used to prevent the growth of, or to kill existing, unwanted plants. Such plants are, crab grass, dandelions, and other weeds in parks, ponds, farms, and residential areas. Fungicides are to get rid of unwanted fungi that may cause diseases infect plants, animals, and even humans. Some fungi can infect wood used in building houses, causing dry rot, so fungicides are very often used in lumber. Rodenticides are used to kill ro