Insecticide
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Classes of agricultural insecticides
The classification of insecticides is done in several different ways:- The systematic insecticides which are incorporated into the plants and the insects take up the insecticide while feeding on the plants. The contact insecticides which take action when they are brought into direct contact with the insect by aerosol distribution.
- The natural insecticides which are made from plants as defence against insects like nicotine and pyrethrum. In contrast to the inorganic insecticides like arsenates copper- and fluorine compounds, which are now seldom used and the organic insecticides which is now the biggest group.
- The point of action is also a good way of sorting the insecticides. The neurotocic insecticides, the acetylcholinesterase blocking and moulting blockers. The neurotoxic and coleterinas blocker are difficult because they are also toxic for other organisms. Where as the skinning blockers and the other insecticides which take action only in metabolism of insects are less likely to be highly toxic on mammals.
- The contribution as aerosol (liquid or solid) is mostly used for contact insecticides, as solid for the uptake by the plant roots is preferred for systematic insecticides and fumigation with phosphine or hydrogen cyanide is mostly used for sensitive products after the harvest.
Classes of insecticides, a short A series of classes of insecticides have existed, as time progressed one class has largely replaced the one before it. These trends in the classes of compounds in some ways has mirrored the development of chemical warfare agents.
Heavy metals, eg lead, mercury, arsenic and plant toxins such as nicotine have been used for many years. Various plants have been used as folk insectides for centuries, including tobacco and pyrethrum.
Chlorine based agents, with the rise of the modern chemical industry it was possible for form organochlorines. The substances used in chemical warfare tend to be more potent electrophiles than those used as insecticides. For instance mustard gas (sulfur mustard, HD) is a potent alklating agent which uses neighbouring group participation of the sulfur to make the alkyl chloride a stronger electrophile. While a typical insecticide such as DDT or lindane does not depend on an electrophile/nucleophile reaction to kill the insect. It is likely that the chlorine is used to tune the lipophilicity of the compound, and to alter the shape and electrostatic effects involved in the interactions of the insecticide and the biomolecules in the target organism. For insatnce DDT works by opening the sodium channels in the nerve cells of the insect.
The next large class was the organophosphates, both the insecticides and the chemical warfare agents (such as sarin, tabun, soman and VX) work in the same way. All these compounds bind to the neurotransmitter acetylcholinesterase and other cholinesterases. This results in disruption of nervous impulses, killing the insect or interfering with its ability to carry on normal functions.
Recent efforts to reduce broad spectrum toxins added to the environment have brought biological insecticides back into vogue. An example is the development and increase in use of Bacillus thuringiensis, a bacterial disease of Lepidopterans and some other insects. It is used as a larvicide against a wide variety of caterpillars. Because it has little effect on other organisms, it is considered more Environmentally friendly than synthetic pesticides.
Environmental effects
One of the bigger drivers in the development of new insecticides has been the desire to replace toxic and irksome insecticides. The notorious DDT was introduced as a safer alternative to the lead and arsenic compounds which had been used before. It is the case that when used under the correct conditions that almost any chemical substance is 'safe', but when used under the wrong conditions even the ' safest ' substance can be a threat to life, limb and/or the environment.
Some insecticides have been banned due to the fact that they are persistent toxins which have adverse effects on animals and/or humans. A classic example which is often quoted is that DDT is an example of a widely used (and maybe misused) pesticide. 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 fat. 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.
The longlived organochlorine Dieldrin, if used with care, can prevent termites from destroying telephone cables, but it is clearly not suitable for aerial spraying to control insects in a field close to a school playground.
While the overuse of DDT lead to a reduction in its use, opponents of traditional environmentalism often cite it as an example of environmentalism going too far and interfering with malaria eradication, going so far as to estimate the cost of human lives resulting from the DDT ban; for instance the novelist Michael Crichton states in his bestselling book, State of Fear:
- "Since the ban, two million people a year have died unnecessarily from malaria, mostly children. The ban has caused more than fifty million needless deaths. Banning DDT killed more people than Hitler."
- "The outcome of the treaty is arguably better than the status quo going into the negotiations over two years ago. For the first time, there is now an insecticide which is restricted to vector control only, meaning that the selection of resistant mosquitoes will be slower than before." [link].
- "Correlating the use of DDT in El Salvador with renewed malaria transmission, it can be estimated that at current rates each kilo of insecticide added to the environment will generate 105 new cases of malaria."
Application methods
Insect repellent, commonly referred to as "bug spray", comes in a plastic bottle or aerosol can. Applied to clothing, arms, legs, and other extremities, the use of this pesticide will tend to ward off nearby insects. This is not an insecticide.Insecticide used for killing pests—most often insects, and arachnids—primarily comes in an aerosol can, and is sprayed into the air or nest as a means of killing the animal. Fly sprays will kill house flies, blowflies, ants, cockroaches and other insects and also spiders.
Active ingredients of many household insecticides include permethrin and tetramethrin, which act on the nervous system of insects and arachnids.
Bug spray should be used in well ventilated areas only, as the chemicals contained in the aerosol and pesticide can be harmful or deadly to humans.
Individual insecticides
Chlorinated
Several are now banned because of their ecological persistence:
Organophosphorus
Chemically similar to Nerve agents
Plant toxin derived
External links
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