Any of a large group of substances used to kill insects. Such substances are mainly used to control pests that infest cultivated plants and crops or to eliminate disease-carrying insects in specific areas. Inorganic insecticides include arsenic, lead, and copper compounds. Some organic insecticides are natural, such as rotenone, pyrethrins, and nicotine (see toxin). Others are synthetic, such as chlorinated hydrocarbons (e.g., DDT, dieldrin, lindane); carbamates, related to urea (e.g., carbaryl, carbofuran); and parathions, organic phosphorus esters. Insect hormones may be included as a class. Insecticides may affect the nervous system, inhibit essential enzymes, or prevent larvae from maturing (e.g., juvenile hormone). Some are stomach poisons, some inhalation poisons, and others contact poisons. Agents such as inert oils act mechanically, simply blocking the breathing pores. Insecticides vary widely not only in effectiveness against target insects (which may develop resistance) but also in toxicity to nontarget species (including humans) and environmental effects; many of the worst (e.g., DDT) have been banned or their use curtailed.