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Insecticides

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Insecticides

For other uses, see Insecticide (disambiguation).

An insecticide is a chemical used against insects. They include ovicides and larvicides used against the eggs and larvae of insects, respectively. Insecticides are used in agriculture, medicine, industry, and general home use. The use of insecticides is believed to be one of the major factors behind the increase in agricultural productivity in the 20th century.[1] Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans; and others are concentrated in the food chain.

The classification of insecticides is done in several different ways:

  • Systemic insecticides are incorporated by treated plants. Insects ingest the insecticide while feeding on the plants.
  • Contact insecticides are toxic to insects brought into direct contact. Efficacy is often related to the quality of pesticide application, with small droplets (such as aerosols) often improving performance.[2]
  • Natural insecticides, such as nicotine, pyrethrum, and neem extracts are made by plants as defenses against insects. Nicotine-based insecticides are still being widely used in the US and Canada, however they are barred in the EU.[3]
  • Plant-incorporated protectants (PIPs) are insecticidal substances produced by plants after genetic modification.[4] For instance, a gene that codes for a specific Baccilus thuringiensis biocidal protein is introduced into a crop plant's genetic material. Then, the plant manufactures the protein. Since the biocide is incorporated into the plant, additional applications, at least of the same compound, are not required.
  • Inorganic insecticides are manufactured with metals and include arsenates, copper compounds and fluorine compounds, which are now seldom used, and sulfur, which is commonly used.
  • Organic insecticides are synthetic chemicals that comprise the largest numbers of pesticides available for use today.
  • Mode of action—how the pesticide kills or inactivates a pest—is another way of classifying insecticides. Mode of action is important in predicting 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.

Classes of insecticides

Organochlorides

The insecticidal properties of the best-known representative of this class of insecticides, DDT, was made by the Swiss Scientist Paul Müller. For this discovery, he was awarded the Nobel Prize for Physiology or Medicine in 1948.[5] DDT was introduced on the market in 1944. The contemporary rise of the chemical industry facilitated the large-scale production of DDT and related chlorinated hydrocarbons. DDT functions by opening the sodium channels in the nerve cells of the insect.[6]

Organophosphates and carbamates

The organophosphates are another large class of synthetic insecticides. These also target the insect's nervous system. Organophosphates interfere with the enzymes acetylcholinesterase and other cholinesterases, disrupting nerve impulses, killing or disabling the insect. Organophosphate insecticides and chemical warfare nerve agents (such as sarin, tabun, soman, and VX) work in the same way. Organophosphates have an accumulative toxic effect to wildlife, so multiple exposures to the chemicals amplifies the toxicity.[7]

Carbamate insecticides have similar toxic mechanisms to organophosphates, but have a much shorter duration of action and are, thus, somewhat less toxic.

Pyrethroids

In order to mimic the insecticidal activity of the natural compound pyrethrum another class of pesticides, pyrethroid pesticides, has been developed. These compounds are nonpersistent sodium channel modulators, and are much less acutely toxic than organophosphates and carbamates. Compounds in this group are often applied against household pests.[8]

Neonicotinoids

Neonicotinoids are synthetic analogues of the natural insecticide nicotine (with a much lower acute mammalian toxicity and greater field persistence). These chemicals are nicotinic acetylcholine receptor agonists. Broad-spectrum—systemic insecticides, they have a rapid action (minutes-hours). They are applied as sprays, drenches, seed, and soil treatments—often as substitutes for organophosphates and carbamates. Treated insects exhibit leg tremors, rapid wing motion, stylet withdrawal (aphids), disoriented movement, paralysis, and death.[9] Imidacloprid may be the most commonly used neonicotinoid. It has recently come under scrutiny for its deleterious effects on honeybees,[10] and its potential to increase the susceptibility of rice to planthopper attacks.[11]

Ryanoids

Ryanoids are synthetic analogues with the same mode of action as ryanodine, a naturally occurring insecticide extracted from Ryania speciosa (Flacourtiaceae). They bind to calcium channels in cardiac and skeletal muscle, blocking nervous transmission. Only one such insecticide is currently registered, Rynaxypyr, generic name chlorantraniliprole.[12]

Insect growth regulators

Insect growth regulator (IGR) is a term coined to include insect hormone mimics and an earlier class of chemicals, the benzoylphenyl ureas, which inhibit chitin (exoskeleton) biosynthesis in insects. Diflubenzuron is a member of the latter class, used primarily to control caterpillars that are pests. The most successful insecticides in this class are the juvenoids (juvenile hormone analogues). Of these, methoprene is most widely used. It has no observable acute toxicity in rats, and is approved by WHO for use in drinking water cisterns to combat malaria. Most of its uses are to combat insects where the adult is the pest, including mosquitoes, several fly species, and fleas. Two very similar products, hydroprene and kinoprene, are used for controlling species such as cockroaches and white flies. Methoprene has been registered with the EPA since 1975, and there are virtually no reports of resistance. A more recent type of IGR is the ecdysone agonist tebufenozide (MIMIC), which is used in forestry and other applications for control of caterpillars, which are far more sensitive to its hormonal effects than other insect orders.

Biological insecticides

Many plants exude substances to prevent insects from eating. Premier examples are substances activated by the enzyme myrosinase. This enzyme converts glucosinolates to a variety of compounds that are toxic to herbivorous insects. One product of this enzyme is allyl isothiocyanate, the pungent ingredient in horseradish sauces.

The myrosinase is released only upon crushing the flesh of horseradish by the herbivore (or preparer of horseradish sauce). Since allyl isothiocyanate is harmful to the plant as well as the insect, it is stored in the harmless form of the glucosinolate, separate from the myrosinase enzyme.[13]

In general, tree rosin is considered a natural insecticide. To be specific, the production of oleoresin by conifer species is a component of the defense response against insect attack and fungal pathogen infection.[14]

Bacterial insecticides

Bacillus thuringiensis is a bacterial disease that affects Lepidopterans and some other insects. Toxins produced by different strains of this bacterium are used as a larvicide against caterpillars, beetles, and mosquitoes. Toxins from Saccharopolyspora spinosa are isolated from fermentations and sold as Spinosad. Because these toxins have little effect on other organisms, they are considered more environmentally friendly than synthetic pesticides. The toxin from B. thuringiensis (Bt toxin) has been incorporated directly into plants through the use of genetic engineering. Other biological insecticides include products based on entomopathogenic fungi (e.g., Beauveria bassiana, Metarhizium anisopliae), nematodes (e.g., Steinernema feltiae) and viruses (e.g., Cydia pomonella granulovirus).

Environmental effects

Effects on nontarget species

Some insecticides kill or harm other creatures in addition to those they are intended to kill. For example, birds may be poisoned when they eat food that was recently sprayed with insecticides or when they mistake an insecticide granule on the ground for food and eat it.[7]

Sprayed insecticides may drift from the area to which it is applied and into wildlife areas, especially when it is sprayed aerially.[7]

DDT

Main article: DDT

The development of insecticides such as DDT has been motivated by desire to replace more dangerous or less effective alternatives. DDT was introduced to replace lead and arsenic-based compounds, which were in widespread use in the early 1940s.[15]

Some insecticides have been banned due to the fact that they are persistent toxins that have adverse effects on animals and/or humans. An oft-quoted case is that of DDT, an example of a widely used (and maybe misused) pesticide, which was brought to public attention by Rachel Carson's book Silent Spring. One of the better-known impacts of DDT is to reduce the thickness of the egg shells on predatory birds. The shells sometimes become too thin to be viable, causing reductions in bird populations. This occurs with DDT and a number of related compounds due to the process of bioaccumulation, wherein the chemical, due to its stability and fat solubility, accumulates in organisms' fatty tissues. Also, DDT may biomagnify, which causes progressively higher concentrations in the body fat of animals farther up the food chain. The near-worldwide ban on agricultural use of DDT and related chemicals has allowed some of these birds, such as the peregrine falcon, to recover in recent years. A number of the organochlorine pesticides have been banned from most uses worldwide, and globally they are controlled via the Stockholm Convention on persistent organic pollutants. These include: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, and toxaphene.

Pollinator decline

Insecticides can kill bees and may be a cause of pollinator decline, the loss of bees that pollinate plants, and colony collapse disorder (CCD),[16] in which worker bees from a beehive or Western honey bee colony abruptly disappear. Loss of pollinators will mean a reduction in crop yields.[16] Sublethal doses of insecticides (i.e. imidacloprid and other neonicotinoids) affect foraging behavior of bees.[17] However, research into the causes of CCD was inconclusive as of June 2007.[18]

Individual insecticides

Pyrethroids

Neonicotinoids

Ryanoids

  • Chlorantraniliprole
  • Cyantraniliprole
  • Flubendiamide

Insect growth regulators

Plant-derived

Biologicals

Other

See also

References

Further reading

  • McWilliams, James E., "‘The Horizon Opened Up Very Greatly’: Leland O. Howard and the Transition to Chemical Insecticides in the United States, 1894–1927," Agricultural History, 82 (Fall 2008), 468–95.

External links

Template:Americana Poster

  • InsectBuzz.com - Daily updated news on insects and their relatives, including information on insecticides and their alternatives
  • International Pesticide Application Research Centre (IPARC)
  • Pestworld.org – Official site of the National Pest Management Association
  • Classification of insecticides
  • Streaming online video about efforts to reduce insecticide use in rice in Bangladesh. on RealPlayer
  • How Insecticides Work – Has a thorough explanation on how insecticides work.
  • University of California Integrated pest management program
  • Using Insecticides, Michigan State University Extension
  • Example of Insecticide application in the Tsubo-en Zen garden (Japanese dry rock garden) in Lelystad, The Netherlands.
  • Home Insecticides Brands [1] at Insectia.info


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