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Insecticides

In the early period of human civilization, it was realized that pests harm crops and transmit diseases both to men and animals. The use of chemicals to kill pests dates back to 70 A.D, when arsenic was recommended to kill insects.
In the sixteenth century, the Chinese used arsenic disulphide as an insecticide.

Paris Green, or copper acetoarsenite was extensively applied to pools in the tropics for controlling Malaria transmitting mosquitoes. Due to the very harmful effect of arsenic and its damage to the crops, as well as soil, arsenic is completely banished from the pesticide scenario for long now.

Pesticides is the general term used for insecticides, acaricides, rodenticides, herbicides, fungicides, etc.

 

What are Insecticides?

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Insecticides are type of pesticide used against the insects. Insecticides kills or deactivates an insect. There are many insecticides depending upon the type of pest to be dealt with. The use of Insecticides have increased the agricultural productivity.

Many insecticides like, DDT, etc. are banned due to their highly toxic nature. Some of the chemical component have entered the food chain of mammals due to their presence in the naturally occurring food liek vegetable, pulses, etc. Organic farming is a method adopted in many countries now, which avoid the use of insecticides due to its harmful effect.

Examples of insecticides are BHC, DDT, Nicotine, etc.

Natural Insecticides

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Plant based insecticides, available in nature are said to be natural insecticides.
Many plants like neem, Pepper, etc have been used in age old preparations to curb the infiltration of pests and insects. Some of the very commonly used natural insecticides, without a chemical base, are:

1. Rotenone


Rotenone is a compound extracted from the roots of tropical plants such as Derris elliptica and Lonchoncarpus nicou. Rotenone is relatively non toxic to mammals because it is absorbed poorly, although exposure of the lungs to dust is a little more dangerous. This compound is very effective against insects, as it readily passes into their breathing tubes.

2. Limonene


Limonene is another naturally found insecticide, which is extracted from lemon oil. It is also extracted from other citrus fruits, like oranges and are very effective against insects.

3. Neem


Neem has been used against pests for many centuries now. The bark, leaves, fruit, etc, are all used against many types of insects.

Organic Insecticides

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Insecticides are classified as Organic and inorganic insectides. The major class of insecticides are Organic in nature, like organophosphorous compounds, organochlorine compounds, etc.

Organic insecticides also include the naturally available compounds present in leaves of neem, basil, pepper, etc.

Types of Insecticides

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The basic classification of an insecticide is
  1. Natural insecticide
  2. Man made insecticide.
Before the advent of chemical formulations, the farmers as well as in household, pests were controlled by natural means. Many insecticides like lime, pepper, ginger, Basil leaves, etc, were being used in various combinations against the insects investing both crops as well as against household pests.

The synthetic variety of insecticide are also classified depending upon their structure.

Four major types of insecticides, based on their structure are
  1. Chlorinated Hydrocarbons
  2. Organophosphates
  3. Carbamates
  4. Pyrethroids

1. Chlorinated hydrocarbons: Example - DDT


Chlorinated hydrocarbons like DDT, BHC, etc are used as insecticides. They act on the nervous system of the insects. They can be effectively called as Nerve poisons.

DDT is 2, 2, -di- (p-chlorophenyl)-1, 1, 1- trichloroethane. DDT is an effective insecticide, particularly for mosquitoes, flies and crop pests. It i responsible for the almost complete eradication of malaria -carrying mosquitoes and thus making the world free from malaria. But, it is toxic to humans and plays havoc with several species of useful birds and fishes.

Degradation of DDT

The major microbial metabolic steps are the reductive dechlorination reaction, oxidation and dehydrochlorination. The degradation product, DDD is more toxic than DDT. It is even manufactured as a pesticide. The chlorinated hydrocarbons like DDT and BHC are also degraded by the process of dehydrochlorination.

2. Organophosphates : Example - Malathion, mercarbam, etc


These are organic phosphorous insecticides, which act on the important protein of the insects, acetyl cholinesterase. They act as phosphorylating agent, thereby hindering the protein activity. Malathion is an organophosphorus drug, widely used against insects investing in crops, etc.

Mercarbam, on the other hand, is used against mites, plant hoppers, etc, which damage rice and other fruit trees.

Degradation

Degradation of organophosphates like Malathion takes place by hydrolysis.

3. Carbamates: Example - Aldicarb, XMC,carbofuran etc.


Carbamates are insecticides with nitrogen in them. XMC or 3, 5 dimethyl phenyl methyl carbamate. It has a chemical formula of sructure of: It is also used against all types of hoppers, especially against the tea tree hoppers which damage tea crops.

Carbofuran has a chemical formula of C12H15NO3. The name given by I.U.P.A.C for this insecticide is 2, 3 - dihydro-2, 2, -dimethyl benzofuran - 7-yl-methyl carbamate. This is used against soil investing insects and nematodes. Carbamates are also degraded by the process of hydrolysis.

4. Pyrethroids: Example - Pyrethrum, allethrin, etc.


Pyrethroids are another class of insecticides, which acts on the central and the peripheral nervous system of the insects. These are poisonous in nature.

Pyrethrum is a group of insecticide, with many isomers ranging from C21H28O3 to C22H30O5. It is used against pests affecting stored good and also against goods stored at home. These range of insecticides are not usually used in farms.

Insecticide Synergists

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Synergists are compounds used with an insecticide, mostly with the pyrethroids. These help in the insecticidal action, by blocking the resistance causing activity of the insects, when insecticide acts on it. These compounds usually block the metabolic systems in an insect, which may otherwise metabolize or deactivate the insecticide.

Most of these synergists used are non toxic in nature. Only very few synergists are toxic themselves. Example of synergists are piperonyl butoxide, permethrin, etc.

Damage Caused by Synthetic Insectides

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Insecticides are found widespread in various parts of the environment. Their impact on the environment is based on their property.
  1. Tendency to vaporize
  2. Tendency to dissolve in water and other solvents
  3. Resistance to various degradation processes
Insecticides are applied as aerial sprays, dusts to foliage or directly to the soil. Finally they reach the soil which serves as a reservoir. However, from the land they move to air and ware or are degraded.

The ultimate depository is water bodies, particularly for persistent insecticides. The bio degradation of insecticides in aquatic and terrestrial environments is important for environmental quality. The biodegradability, however, varies depending on the chemical nature of the insecticides itself.

In soil, degradation of insecticides can be carried out by microorganisms like actinomycetes, fungi and bacteria. They can degrade insecticides through oxidation, ether cleavage, ester and amide hydrolysis, etc.

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