An insecticide is a substance used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed to be a major factor behind the increase in agricultural 20th century's productivity. Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans; some concentrate along the food chain. Insecticides can be classified in different ways: Systemic insecticides are incorporated by treated plants. Insects ingest the insecticide while feeding on the plants. Contact insecticides are toxic to insects when brought into direct contact. Efficacy is often related to the quality of pesticide application, with small droplets (such as aerosols) often improving performance. Natural insecticides, such as nicotine, pyrethrum and neem extracts are made by plants as defenses against insects. Plant-incorporated protectants (PIPs) are systemic insecticides produced by transgenic plants. For instance, a gene that codes for a specific Bacillus thuringiensis biocidal protein was introduced into corn and other species. The plant manufactures the protein which kills the insect when consumed. Inorganic insecticides are contact insecticides that manufactured with metals and include arsenates, copper and fluorine compounds, which are now seldom used, and sulfur, which is commonly used. Organic insecticides are contact insecticides that comprise the largest numbers of pesticides available for use today. The mode of action describes how the pesticide kills or inactivates a pest. It provides another way of classifying insecticides. Mode of action is important in understanding whether an insecticide will be toxic to unrelated species, such as fish, birds and mammals. For products that repel rather than kill insects see insect repellents.