pesticide pesticide pesticides are substances that are meant to control pests. this includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampricide. the most common of these are herbicides, which account for approximately 50% of all pesticide use globally. most pesticides are intended to serve as plant protection products (also known as crop protection products), which in general, protect plants from weeds, fungi, or insects. as an example, the fungus alternaria solani is used to combat the aquatic weed salvinia. a crop-duster spraying pesticide on a field a lite-trac four-wheeled self-propelled crop sprayer spraying pesticide on a field in general, a pesticide is a chemical (such as carbamate) or biological agent (such as a virus, bacterium, or fungus) that deters, incapacitates, kills, or otherwise discourages pests. target pests can include insects, plant pathogens, weeds, molluscs, birds, mammals, fish, nematodes (roundworms), and microbes that destroy property, cause nuisance, or spread disease, or are disease vectors. along with these benefits, pesticides also have drawbacks, such as potential toxicity to humans and other species. definition type of pesticide target pest group algicides or algaecidesalgae avicidesbirds bactericidesbacteria fungicidesfungi and oomycetes herbicidesplant insecticidesinsects lampricideslampreys miticides or acaricidesmites molluscicidessnails nematicidesnematodes rodenticidesrodents slimicidesalgae, bacteria, fungi, and slime molds virucidesviruses the food and agriculture organization (fao) has defined pesticide as: any substance or mixture of substances intended for preventing, destroying, or controlling any pest, including vectors of human or animal disease, unwanted species of plants or animals, causing harm during or otherwise interfering with the production, processing, storage, transport, or marketing of food, agricultural commodities, wood and wood products or animal feedstuffs, or substances that may be administered to animals for the control of insects, arachnids, or other pests in or on their bodies. the term includes substances intended for use as a plant growth regulator, defoliant, desiccant, or agent for thinning fruit or preventing the premature fall of fruit. also used as substances applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport. pesticides can be classified by target organism (e.g., herbicides, insecticides, fungicides, rodenticides, and pediculicides – see table), chemical structure (e.g., organic, inorganic, synthetic, or biological (biopesticide), although the distinction can sometimes blur), and physical state (e.g. gaseous (fumigant)). biopesticides include microbial pesticides and biochemical pesticides. plant-derived pesticides, or "botanicals", have been developing quickly. these include the pyrethroids, rotenoids, nicotinoids, and a fourth group that includes strychnine and scilliroside.: 15 many pesticides can be grouped into chemical families. prominent insecticide families include organochlorines, organophosphates, and carbamates. organochlorine hydrocarbons (e.g., ddt) could be separated into dichlorodiphenyl ethanes, cyclodiene compounds, and other related compounds. they operate by disrupting the sodium/potassium balance of the nerve fiber, forcing the nerve to transmit continuously. their toxicities vary greatly, but they have been phased out because of their persistence and potential to bioaccumulate.: 239–240 organophosphate and carbamates largely replaced organochlorines. both operate through inhibiting the enzyme acetylcholinesterase, allowing acetylcholine to transfer nerve impulses indefinitely and causing a variety of symptoms such as weakness or paralysis. organophosphates are quite toxic to vertebrates and have in some cases been replaced by less toxic carbamates.: 136–137 thiocarbamate and dithiocarbamates are subclasses of carbamates. prominent families of herbicides include phenoxy and benzoic acid herbicides (e.g. 2,4-d), triazines (e.g., atrazine), ureas (e.g., diuron), and chloroacetanilide (e.g., alachlor). phenoxy compounds tend to selectively kill broad-leaf weeds rather than grasses. the phenoxy and benzoic acid herbicides function similar to plant growth hormones, and grow cells without normal cell division, crushing the plant's nutrient transport system.: 300 triazines interfere with photosynthesis.: 335 many commonly used pesticides are not included in these families, including glyphosate. the application of pest control agents is usually carried out by dispersing the chemical in an (often hydrocarbon-based) solvent-surfactant system to give a homogeneous preparation. a virus lethality study performed in 1977 demonstrated that a particular pesticide did not increase the lethality of the virus. combinations that included surfactants and the solvent clearly showed that pretreatment with them markedly increased the viral lethality in the test mice. pesticides can be classified based upon their biological mechanism function or application method. most pesticides work by poisoning pests. a systemic pesticide moves inside a plant following absorption by the plant. with insecticides and most fungicides, this movement is usually upward (through the xylem) and outward. increased efficiency may be a result. systemic insecticides, which poison pollen and nectar in the flowers, may kill bees and other needed pollinators. in 2010, the development of a new class of fungicides called paldoxins was announced. these work by taking advantage of natural defense chemicals released by plants called phytoalexins, which fungi then detoxify using enzymes. the paldoxins inhibit the fungi's detoxification enzymes. they are believed to be safer and greener. history since before 2000 bc, humans have utilized pesticides to protect their crops. the first known pesticide was elemental sulfur dusting used in ancient sumer about 4,500 years ago in ancient mesopotamia. by the 15th century, toxic chemicals such as arsenic, mercury, and lead were being applied to crops to kill pests. in the 17th century, nicotine sulfate was extracted from tobacco leaves for use as an insecticide. the 19th century saw the introduction of two more natural pesticides, pyrethrum, which is derived from chrysanthemums, and rotenone, which is derived from the roots of tropical vegetables. until the 1950s, arsenic-based pesticides were dominant. paul müller discovered that ddt was a very effective insecticide. chlorinates such as ddt were dominant, but they were replaced in the u.s. by organophosphates and carbamates by 1975. since then, pyrethrin compounds have become the dominant insecticide. herbicides became common in the 1960s, led by "triazine and other nitrogen-based compounds, carboxylic acids such as 2,4-dichlorophenoxyacetic acid, and glyphosate". the first legislation providing federal authority for regulating pesticides was enacted in 1910. during the 1940s, manufacturers produced large amounts of synthetic pesticides and their use became widespread. before the first world war, germany was the world's leading chemical industry and exported most of the dyes and other chemicals that were used in the united states. war implemented tariffs that stimulated the growth of the chemical industry in the u.s., which made chemistry a prestigious occupation as this industry expanded and became profitable. money and ideas flowed back from europe after the u.s. entered wwi, changing the way americans interacted with themselves and nature, and the industrialization of war hastened the industrialization of pest control. some sources consider the 1940s and 1950s to have been the start of the "pesticide era." although the u.s. environmental protection agency was established in 1970 and amendments to the pesticide law in 1972, pesticide use has increased 50-fold since 1950 and 2.3 million tonnes (2.5 million short tons) of industrial pesticides are now used each year. seventy-five percent of all pesticides in the world are used in developed countries, but use in developing countries is increasing. a study of usa pesticide use trends through 1997 was published in 2003 by the national science foundation's center for integrated pest management. in the 1960s, it was discovered that ddt was preventing many fish-eating birds from reproducing, which was a serious threat to biodiversity. rachel carson wrote the best-selling book silent spring about biological magnification. the agricultural use of ddt is now banned under the stockholm convention on persistent organic pollutants, but it is still used in some developing nations to prevent malaria and other tropical diseases by spraying on interior walls to kill or repel mosquitoes. development available pesticides are not sufficient and new developments are needed. continued research into the basic biology of pests may identify new vulnerabilities and produce new pesticides; it may also yield pesticides with better financial and environmental characteristics than those presently used. plant-derived pesticides, or "botanicals", have been developing quickly. these include the pyrethroids, rotenoids, nicotinoids, and a fourth group that includes strychnine and scilliroside. in 2010, the development of a new class of fungicides called paldoxins was announced. these work by taking advantage of natural defense chemicals released by plants called phytoalexins, which fungi then detoxify using enzymes. the paldoxins inhibit the fungi's detoxification enzymes. they are believed to be safer and greener. fungicide resistance is increasing the proportion of inactive enantiomers in fungicide applications: the evolution of resistance necessitates research and discovery of new active ingredients, which trends away from already-discovered classes and toward more complex chemical structures. these tend to have more chiral centers more often which means more off products during synthesis. insecticide development is being discouraged and slowed down by public sentiment surrounding the worldwide colony collapse disorder crisis. although ccd is a serious problem, there are indications that other facts are involved, especially cox-foster et al. 2007's discovery that a virus is substantially to blame. (