Insecticides are substances 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 the 20th-century’s agricultural productivity. Nearly all insecticides have the potential
to significantly alter ecosystems; many are toxic to humans and/or animals; some become
concentrated as they spread along the food chain. Insecticides can be classified into two major
groups: systemic insecticides, which have residual or long term activity; and contact
insecticides, which have no residual activity. Furthermore, one can distinguish three types
of insecticide. 1. Natural insecticides, such as nicotine, pyrethrum
and neem extracts, made by plants as defenses against insects. 2. Inorganic insecticides, which are metals. 3. Organic insecticides, which are organic chemical
compounds, mostly working by contact. 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. Insecticides may be repellent or non-repellent. Social insects such as ants cannot detect
non-repellents and readily crawl through them. As they return to the nest they take insecticide
with them and transfer it to their nestmates. Over time, this eliminates all of the ants
including the queen. This is slower than some other methods, but
usually completely eradicates the ant colony.Insecticides are distinct from non-insecticidal repellents,
which repel but do not kill.==Type of activity==Systemic insecticides become incorporated
and distributed systemically throughout the whole plant. When insects feed on the plant, they ingest
the insecticide. Systemic insecticides produced by transgenic
plants are called plant-incorporated protectants (PIPs). For instance, a gene that codes for a specific
Bacillus thuringiensis biocidal protein was introduced into corn (maize) and other species. The plant manufactures the protein, which
kills the insect when consumed. Contact insecticides are toxic to insects
upon direct contact. These can be inorganic insecticides, which
are metals and include the commonly used sulfur, and the less commonly used arsenates, copper
and fluorine compounds. Contact insecticides can also be organic insecticides,
i.e. organic chemical compounds, synthetically produced, and comprising the largest numbers
of pesticides used today. Or they can be natural compounds like pyrethrum,
neem oil etc. Contact insecticides usually have no residual
activity. Efficacy can be related to the quality of
pesticide application, with small droplets, such as aerosols often improving performance.==Biological pesticides==Many organic compounds are produced by plants
for the purpose of defending the host plant from predation. A trivial case is tree rosin, which is a natural
insecticide. Specifically, the production of oleoresin
by conifer species is a component of the defense response against insect attack and fungal
pathogen infection. Many fragrances, e.g. oil of wintergreen,
are in fact antifeedants. Four extracts of plants are in commercial
use: pyrethrum, rotenone, neem oil, and various essential oils==
Other biological approaches=====Plant-incorporated protectants===
Transgenic crops that act as insecticides began in 1996 with a genetically modified
potato that produced the Cry protein, derived from the bacterium Bacillus thuringiensis,
which is toxic to beetle larvae such as the Colorado potato beetle. The technique has been expanded to include
the use of RNA interference RNAi that fatally silences crucial insect genes. RNAi likely evolved as a defense against viruses. Midgut cells in many larvae take up the molecules
and help spread the signal. The technology can target only insects that
have the silenced sequence, as was demonstrated when a particular RNAi affected only one of
four fruit fly species. The technique is expected to replace many
other insecticides, which are losing effectiveness due to the spread of pesticide resistance.===Enzymes===
Many plants exude substances to repel insects. Premier examples are substances activated
by the enzyme myrosinase. This enzyme converts glucosinolates to various
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. 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.===Bacterial===
Bacillus thuringiensis is a bacterial disease that affects Lepidopterans and some other
insects. Toxins produced by 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).==Synthetic insecticide and natural insecticides
==A major emphasis of organic chemistry is the
development of chemical tools to enhance agricultural productivity. Insecticides represent a major area of emphasis. Many of the major insecticides are inspired
by biological analogues. Many others are completely alien to nature.===Organochlorides===
The best known organochloride, DDT, was created by Swiss scientist Paul Müller. For this discovery, he was awarded the 1948
Nobel Prize for Physiology or Medicine. DDT was introduced in 1944. It functions by opening sodium channels in
the insect’s nerve cells. The contemporaneous rise of the chemical industry
facilitated large-scale production of DDT and related chlorinated hydrocarbons.===Organophosphates and carbamates===
Organophosphates are another large class of contact insecticides. These also target the insect’s nervous system. Organophosphates interfere with the enzymes
acetylcholinesterase and other cholinesterases, disrupting nerve impulses and 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 a cumulative toxic effect
to wildlife, so multiple exposures to the chemicals amplifies the toxicity. In the US, organophosphate use declined with
the rise of substitutes.Carbamate insecticides have similar mechanisms to organophosphates,
but have a much shorter duration of action and are somewhat less toxic.===Pyrethroids===
Pyrethroid pesticides mimic the insecticidal activity of the natural compound pyrethrum,
the biopesticide found in pyrethrins. These compounds are nonpersistent sodium channel
modulators and are less toxic than organophosphates and carbamates. Compounds in this group are often applied
against household pests.===Neonicotinoids===
Neonicotinoids are synthetic analogues of the natural insecticide nicotine (with much
lower acute mammalian toxicity and greater field persistence). These chemicals are acetylcholine receptor
agonists. They are broad-spectrum systemic insecticides,
with rapid action (minutes-hours). They are applied as sprays, drenches, seed
and soil treatments. Treated insects exhibit leg tremors, rapid
wing motion, stylet withdrawal (aphids), disoriented movement, paralysis and death. Imidacloprid may be the most common. It has recently come under scrutiny for allegedly
pernicious effects on honeybees and its potential to increase the susceptibility of rice to
planthopper attacks.===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 nerve transmission. The first insecticide from this class to be
registered was Rynaxypyr, generic name chlorantraniliprole.==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 World Health Organization (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 was registered with the EPA in
1975. Virtually no reports of resistance have been
filed. 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.==Environmental harm=====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. Sprayed insecticide may drift from the area
to which it is applied and into wildlife areas, especially when it is sprayed aerially.===DDT===The development of DDT was 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.DDT was brought to public
attention by Rachel Carson’s book Silent Spring. One side-effect of DDT is to reduce the thickness
of shells on the eggs of predatory birds. The shells sometimes become too thin to be
viable, reducing bird populations. This occurs with DDT and 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 organochlorine pesticides have
been banned from most uses worldwide. 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), in which worker bees from a beehive
or Western honey bee colony abruptly disappear. Loss of pollinators means a reduction in crop
yields. Sublethal doses of insecticides (i.e. imidacloprid
and other neonicotinoids) affect bee foraging behavior. However, research into the causes of CCD was
inconclusive as of June 2007.===Bird decline===
Besides the effects of direct consumption of insecticides, populations of insectivorous
birds decline due to the collapse of their prey populations. Spraying of especially wheat and corn in Europe
is believed to have caused an 80 per cent decline in flying insects, which in turn has
reduced local bird populations by a third to two thirds.==Alternatives==
Instead of using chemical insecticides to avoid crop damage caused by insects, there
are many alternative options available now that can protect farmers from major economic
losses. Some of them are: Breeding crops resistant, or at least less
susceptible, to pest attacks. Releasing predators, parasitoids, or pathogens
to control pest populations as a form of biological control. Chemical control like releasing pheromones
into the field to confuse the insects into not being able to find mates and reproduce. Integrated Pest Management- using multiple
techniques in tandem to achieve optimal results. Push-pull technique- intercropping with a
“push” crop that repels the pest, and planting a “pull” crop on the boundary that attracts
and traps it.==Examples====See also==Integrated pest management
Fogger Index of pesticide articles
Endangered arthropod Pesticide application