Pesticides and herbicides have a major impact on crop production. The 20th century brought many breakthrough developments in pesticide production. Pesticides control troublesome pests, such as mites, insects, rodents and bacteria. Herbicides are actually a type of pesticide that protects herbs from weeds and other harmful or invasive plant species. Despite their beneficial effects, pesticide use has come under critical evaluation in the last 50 years. In 1960, the World Health Organization initiated the WHO Pesticide Evaluation Scheme (WHOPES) to increase awareness of the dangers of pesticides and to encourage their proper use.
The World Health Organization estimates that 1 to 5 million cases of pesticide poisoning occur every year. Pesticide exposure affects the immune system, and scientists speculate that impaired immune system function, commonly seen in underdeveloped countries, could be linked to pesticide use. Additionally, 10 per cent of pesticides are linked to cancer; for example, Dr. Aaron Blair of the National Cancer Institute has discovered links between the herbicide 2, 4-D to non-Hodgkin's lymphoma.
Pesticides also affect the environment. They can damage ecosystems which may result in the extermination of species that are not necessarily intended to be their targets. Pets and native beneficial species may also be harmed by the strong chemicals in many pesticides. As reported at the International Symposium on Urban Tree Health, held in Paris in 1999, certain herbicides can leach into the soil and cause damage when used in high doses. This effect is known as pesticide residue and is a primary reason for closer scrutiny of pesticides in recent years.
Widespread herbicide use has resulted in some species forming resistance to their chemical make-up. In areas that have used the same herbicides for many years, the targeted plant species have developed immunity to the herbicides, which has made weed control much more difficult. Herbicide use should always be a last resort in order to avoid the development of "resistant biotypes."