Types of Pesticides
Since the dawn of human civilization, man has been trying to improve the agriculture. Modern agriculture employs a number of chemicals for enhancing crop yield and protecting the same. Synthetic fertilizers are added to replenish the various nutrients and maintain the soil fertility. We have already discussed various fertilizers that provide nitrogen, potassium and phosphorus to crops in India. These chemical fertilizers are added to the soils in order to overcome the deficiency of minerals and to provide extra chemicals required for proper growth of high yielding varieties. Plant development pattern is highly modified by addition of plant growth regulators. Growth regulators or hormones stimulate or retard the plant growth and affect several other characters. Growth regulators are required in low concentrations. Many growth regulators like malic hydrazide, methyl ester of naphthalene acetic acid (NAA) prolong storage. Still others like 2/4D and 2, 4, 5T prevent premature fruit drop and are widely used as weedicides. Ethylene induces early ripening of fruits. Some other physiological effects of growth regulators are rooting of stem cuttings, enhanced vegetative growth, prevention of flowering etc.
Agricultural crops are mainly destroyed by insects. Various types of fungi and bacteria cause diseases in plants. According to an estimate, there is an annual loss of 30 percent in agricultural production due to insect pests and plant diseases. If only 50 percent of this loss could be saved from pests, the food problem of our country can be solved to a great extent. A pest may be defined as any organism that causes an economic loss or a damage to the physical well being of human beings. It may destroy our crops, cause diseases in them or in human beings etc.
There are a number of chemicals which can kill or destroy these pests. These chemicals are called as pesticides (cides means to kill). Pesticides are sprayed over crops, human dwellings etc. Few familiar pesticides are baygon spray, finit (flit), DDT, BHC which are widely used in houses to kill mosquitoes, flies, ants, cockroaches etc. During the Second World War, two synthetic pesticides i.e. DDT (dichloro diphenyl trichloroethane) and 2-4 D (2, 4 dichlorophenoxy acetic acid) were mainly used.
The first pesticide i.e. Bordeaus mixture was developed by Prof. Millardet in 1882. The mixture consists of copper sulphate and lime (calcium hydroxide) in a 4 : 4 ratio dissolved in 50 gallons of water. Prof. Millardet from University of Bordeaux (France) noticed in 1878 that downy mildew disease of grapes caused by Plasmorora viticola was absent on grape vines where sprays were made of mixture containing copper sulphate and lime.
Types of pesticides
Pesticides are of several types depending upon the types of pests killed or controlled. Thus they may be:
About 30% of agricultural produce in India is lost every year due to pests and diseases. Chemical pesticides are toxic chemicals used in killing pests. On the basis of chemical structure, major pesticides are grouped into :
(a) Organochlorines :
These are basically organic compounds that have been chlorinated. Organochlorines are lipophilic and show much affinity for fatty tissue of animals. Organochlorines have very low bio-degradation, get accumulated in environment posing serious problems. Important examples of organochlorines are (1) DDT, (2) BHC, (3) Aldrin and (4) Endosulphan.
(1) DDT (Dichlorodiphenyl trichloroethane) C14H9Cl5
DDT was first synthesised by a German chemist Othnar Zeidler in 1874 and its insecticidal value was discovered by Paul Muller in 1939. DDT is the most famous pesticide of the world and is a nonbiodegradable pollutant. Spraying of DDT on crops produces pollution of air, soil and water. In India, as a result of prolonged use of DDT, 13-31 ppm of DDT can be detected in the body fat of the people, highest in the world. DDT concentrates from water into the body and magnified in higher members of the food web. DDT tolerance level is 10 ppm for Daphnia (a freshwater crustacean) and this means Daphnia will die beyond that concentration. DDT has become ineffective for killing mosquitoes because of the development of adaptive resistance. DDT does not inhibit cholinesterase activity and is relatively non-toxic to mammals, but in oil solution it is absorbed by skin. Pesticide (DDT) is banned now a days.
(2) BHC (Benzene hexachloride) C6H6Cl6 : Benzene hexachloride is incorrect from a chemical standpoint, its correct name is Hexachloro cyclohexane or HCH. BHC was first synthesized by Michael Faraday in 1825 and its insecticidal value was independently discovered by Dupire (1941) in France and Leicester (1942) in England. The most common pesticide used in India is BHC; it represents about 50% of total volume of pesticides used in India. BHC is more toxic to insects than DDT and is used mainly in public health programmes.
(3) Aldrin (Octalene) C12H8Cl6
• Aldrin is an insecticide applied to foundations of buildings to prevent termites.
• Aldrin has been successfully used in control of locusts and grasshoppers in India.
• Aldrin, Dieldrin and Endrin are very poisonous pesticides.
(4) Endosulphan (Thiodan) C9H6Cl6O2S : Endosulphan is a pesticide and is useful in the control of aphids, caterpillars, plant bugs and borers.
(b) Organophosphates :
The insecticidal properties of organophosphates were discovered by Schrader in Germany during World War II. Organophosphates are the pesticides most toxic to verterbrates. Organophosphates inhibit cholinesterase, an enzyme essential for transmission of nerve impulse across synapse. Major organophosphates used in India are Malathion, Parathion and Fenitrothion. Malathion is one of the two active ingredients in Flit, the second being Pyrethrin. Malathion is also employed widely in anti-malarial programmes. Mosquito-repelling coils contain pyrathrin.
(c) Carbamates :
Carbamates are derivatives of carbamic acid and have an – OCON = group in the molecule. Some commonly used carbamates are Carbofuran (Furadan), Propoxur (Baygon) and Aldicarb (Temik). Derivatives of carbamates are also used as herbicides (phenylcarbamates, thiocarbamates) and Fungicides dithiocarbamates. Carbamates are useful in the control of nematodes and snails. Mode of action of carbamates is quite similar to that of organophosphates. Being structurally similar to acetylcholine, these have high affinity for the enzyme cholinesterase. Methyl isocyanate gas which caused Bhopal gas tragedy on 3rd Dec. 1984, is used as a raw material for synthesizing Carbaryl (trade name Selvin). The ingredient which killed hundreds of people in Bhopal gas tragedy was Methyl isocyanate. It is also called MIC gas.
(d) Pyrethroids :
Pyrethroids are synthetic derivatives of Pyrethrin, a chemical produced by grinding of flowers of the plant Chrysanthemum cinerarifolium. Examples of pyrethroids are Allethrin, Cyclethrin and Barthrin which are quick-acting broad spectrum insecticides. Pyrethroids are highly toxic and quite expensive, not used on a large scale in India at present.
(e) Triazines :
Triazines (Simazine, Atrazine, etc.) are a group of herbicides derived from urea. Triazines are used for controlling weeds in tea, tobacco and cotton.
(ii) Bordeaux Mixture : Bordeaux mixture was discovered by Millardet in France in 1882. Bordeaux mixture is prepared by dissolving 40 g. of copper sulphate and 40 g. of calcium hydroxide in 5 litres of water. Bordeaux mixture is used primarily as a fungicide, it was first used to control downy mildew disease of grape-wine caused by a fungus, Plasmopara viticola. The first pesticide to be used commercially was Bordeaux mixture.
(iii) Mode of Action of Pesticides : Most insecticides attack the nervous system, interfering with the conduction of nerve impulses. Most herbicides attack the Photosystem II (photolysis of water and oxygen evolution) in photosynthesis and also translocation of organic substance in plants. Pesticides zinc phosphide is used for Rodents
(iv) Advantages of Pesticides : High yielding varieties of crops are very susceptible to pests and require the use of pesticides. Pesticides help in improving crop yields and in public health programmes. Pesticides are used to control carriers of vector borne diseases like malaria, filarial, sleeping sickness, dengue fever, yellow fever, etc.
(v) Hazards of Pesticides : Being non-specific, pesticides kill ‘non-target’ species also. Pesticides kill both harmful and useful insects. Most of the pesticides, especially organochlorines, are nonbiodegradable and accumulate in the environment resulting in pollution. Pesticides also enter the food chain; their concentration goes up as they move up in the food chain. This is called biomagnification or bioconcentration.