Introduction
Insectides are substances used to kill insects. They include ovicides and larvicides used against insect eggs, larvae and pupae respectively. They are used in agriculture, medicine, industry and by general public. Insecticides play a vital role in the recent increase in agricultural productivity. It is important to note that 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.
Classification of Instecides
Insecticides can be classified as follows:
- Classification based on mode of action
- Classification based on Sources
- Chemical Classification of Synthetic insecticides
1. Classification based on mode of action thus:
- Stomach poisons
- Contact poisons
- Repellent and Non-repellent
A. Stomach poisons which work by ingestion and have residual or long term activity.
They cause death by interference with respiratory system, by depression or overstimulation and paralysis of the nervous system. The dry poison may be applied as dust, or in solution or in suspension form by spraying with power-sprayers or by air.The poison remains as a residual coat on the surface of plant parts. Chewing insects eat and ingest the plant parts with the poisons which is effectively absorbed through the gastrointestinal tract.
Examples are acid lead acetate, Calcium arsenate used on tomatoes and Paris green. Other forms of stomach poisons are plant systemic.Theyare readily absorbed through both the roots and shoots of Plants.They remain within plant tissues and give prolonged protection against sucking insects e.g Schradan, Dimefox, for the protection against aphids, mites and other sucking insects.
B. Contact poisons, which work by touching the body of insects with no residual activity.
These substances may be applied as dusts, sprays or aerosol to have a direct contact with the insects. It is possible for insects not touched by appropriate amount of the poison to be unaffected and develop resistance and become difficult to control e.g Flies and mosquitoes.
Contact insecticides may be further classified as
- Organic Contact insecticides which are natural extracts made by plants as defenses against insects such as powdered tobacco leaves, nicotine and its salts, pyrethrum flowers, pyrenthrins, rotenone and neem.
- The synthetic organic insectides including DDT and related compounds, benzene hexachloride, Aldrin,dieldrin, the organic phosphorus insecticides such as malathion, parathion and fluorophosphates.
- Inorganic contact insecticides, which are metals or metallic salts. Examples include the commonly used sulfur, and the less commonly used arsenates , copper and fluorine compounds. Diatomite, borate and borax are also included.
C. Insecticides may be repellent or non-repellent
Insect repellent is a means of preventing insect attack. Social insects such as ants cannot detect non-repellent insectides 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. Citronella oil was a popular repellent replaced by Dimethyl phthalate, Ethohexadiol and Butopyronoxy in U.S.P XV.
2. Classification based on Sources
A. Biological insectcides
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. Antifeedants are organic compounds produced by plants to inhibit attack by insects and grazing animals
Four extracts of plants are in commercial use. These are
- pyrethrum
- rotenone
- neem oil, and
- various essential oils e.g Anethole (mosquito larvae) and Asimina (pawpaw tree seeds) for lice
Other biological approaches
i. 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.
ii. 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 .
Biosynthesis of antifeedants by the action of myrosinase.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.
iii 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 fromfrom 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 and Granulovirus).
3. Chemical Classification of Synthetic Insecticides
Many of the major insecticides are inspired by biological analogues. Many others are completely alien to nature. Examples are Organochlorides
A. 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.
The development of DDT was motivated by desire to replace more dangerous or less effective alternatives. It was introduced to replace lead and arsenic -based compounds, which were in widespread use in the early 1940s. Rachel Carson’s book ‘Silent Spring’ brought this insecticide to public attention
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 bio-accumulation, wherein the chemical, due to its stability and fat solubility accumulates in organisms’ fatty tissues. A number of organochlorine pesticides have been banned from 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.
B. 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. They 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.
A carbamate is an organic compound derived from carbamic acid (NH 2 COOH). A carbamate group, carbamate ester (e.g., ethyl carbamate ), and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes .
SYNTHESIS
Carbamic acids are unstable, but the salts of these elusive acids are well known. For example, ammonium carbamate is generated by treatment of ammonia with carbon dioxide
2 NH 3 + CO2 → NH 4[H 2NCO2]
Carbamates also arise via alcoholysis of chloroformamides:
R 2NC(O)Cl + R’OH → R 2NCO2R’ + HCl
Alternatively, cabamates can be formed from chloroformates and amines:
R’OC(O)Cl + R2 NH → R 2NCO2R’ + HCl
Carbamates may be formed from the Curtius rearrangement, where isocyanates formed are reacted with an alcohol.
RCON 3 → RNCO + N 2
RNCO + R′OH → RNHCO 2R′.
C. Pyrethroids
Pyrethroid pesticides mimic the insecticidal activity of the natural compound pyrethrum, the biopesticide found in pyrethrins. These compounds are non-persistent sodium channel modulators and are less toxic than organophosphates and carbamates. Compounds in this group are often applied against household pests.
D. 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.
E. 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.
F. 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.
Effects of Instecides
The effect could be:
- Environmental harm
- Effects on nontarget species
A. 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.
B. 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.
C. 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 to Insecticides Use
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.