AGRICULTURAL PESTICIDE

 

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.

Pesticides

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.

Types of pesticides : Pesticides are of several types depending upon the types of pests killed or controlled. Thus they may be:

(1) Fungicides : Destroy fungal pathogens. Inorganic fungicides include Bordeaux mixture, Burgandy Mixtue, sulphur, mercuric chloride, etc. Organic fungicides are dithane S-21, dithane M-22, dithane Z-78 (all carbamates), oxanthiins (e.g., vitavax), mercury compounds (e.g., agrosan, tillex), benzimidiazole derivatives (e.g., benlate).

(2) Weedicides/Herbicides : They kill weeds in agriculture/horticulture and unwanted plants. Depending upon the mode of action, there are selective and nonselective herbicides, contact herbicides, translocated herbicides, foliage applied and soil applied herbicides. Herbicides can be triazines (e.g., atrazine, simazine) carbamates (e.g., thiocarbamates, phenyl carbamates) and auxin derivatives (e.g., 2 : 4 – D, 2 : 4 : 5 – T).

(3) Nematicides : They kill or repel nematodes, e.g., methyl bromide (MB), ethylene dibromide (EDB), chloropicrin.

(4) Rodenticides : Pesticides that kill or repel rodents, e.g., sodium fluoroacetate, warfarine, red squill, zinc sulphate.

(5) Insecticides : They kill or repel insects. Insecticides can be stomach poisons (poisonous on ingestion), contact poisons or fumigants (inhaled). Natural insecticides include Azadirachta indica (Margosa/Neem), Boenighausenia albiflora, Peganum harmala, Derris (rotenone) and Chrysanthemum (pyrethrum). First commerical bioinsecticide is sporeine developed in Germany. Synthetic insecticides are organochlorines, organophosphates, carbamates and pyrethroids, Ethylene dibromide (EDB) is a volatile liquid (fumigant) used in controlling insect pests in stored grains and fruits.

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 : (i) Organochlorines, (ii) Organophosphates, (iii) Carbamates (iv) Pyrethroids and (v) Triazines.

(i) 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 causing serious problems. Important examples of organochlorines are (a) DDT, (b) BHC, (c) Aldrin and (d) Endosulphan.

(a) DDT (Dichlorodiphenyl trichloroethane) C₁₄H₉Cl₅ : DDT was first synthesized 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.

(b) 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.

(c) 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.

(d) Endosulphan (Thiodan) C₉H₆Cl₆O₂S : Endosulphan is a pesticide and is useful in the control of aphids, caterpillars, plant bugs and borers.

(ii) 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.

(iii) Carbamates : Carbamates are derivatives of carbanic 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 3^rd 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.

(iv) 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.

(v) Triazines : Triazines (Simazine, Atrazine, etc.) are a group of herbicides derived from urea. Triazines are used for controlling weeds in tea, tobacco and cotton.

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.

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

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.

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.

Biological methods of Pest control

Biopesticides are living organisms of their products used for killing pests of interfering with their biological processes.

Bioherbicides : Biological control of weeds involves

(1) Utilization of insects which would feed selectively on weeds and (2) use of certain microorganisms which produce diseases in weeds and eliminate them. Much of the work using insects for biological control has been done in North America. In India and Australia, the overgrown of cacti was checked by the introduction of cochineal insect (Cactoblastis cactorum). The first bioherbicide was mycoherbicide, based on the fungus Phytophthora palmivora, and was developed in 1981.

Bioinsecticides : Bioinsecticides include :

(1) Pathogens, parasites and predators

(2) Sterilization strategy

(3) Insect hormones

(4) Natural insecticides

Aphids have been controlled by the use of ladybugs or praying mantis. Screw-worm was eradicated by releasing sterile males (sterilized by irradiation) at the time of mating to compete with natural fertile population. Introduction of juvenile hormones at inappropriate time results in the early death of insect pests.

Natural insecticides are obtained mostly from plants and occasionally from microbes.

Table. 10.4-1 : Botanical insecticides and their sources

S.No.	Insecticide	Source
1.	Nicotine	Tobacco (Nicotiana tabacum)
2.	Pyrethroids	Chrysauhemum cinerarifolium
3.	Rotenone	Roots of Derris elliptica
4.	Sabadilla	Seeds of Schoenocaulom officinale
5.	Ryania	Roots and stem of Ryamia speciosa
6.	Azadirachtin	Seeds of Azadirachta indica (Neem or Margosa)

Rotenone is a natural insecticide.

One of the earliest pesticides employed by human was Margosa (Neem) leaves.

 

Pest control by Bacteria.

The spore-forming bacteria have been particularly useful in controlling certain pests. Bacillus popilliae is used for the control of Japanese beetle. The bacterium Bacillus thuringenesis produces protein toxins. One of these, thurioside, is active against different groups of insects. Sporeine was the first bioinsecticide developed on commercial scale in Germany. Sporeine kills insects by inhibiting ion transport in the midgut. Genes for some of these toxins have been isolated and transferred to host through recombinant DNA technology (transgenic plants). Transgenic plants of tomato showing resistance to horn worm larvae have been obtained.

Integrated pest Management (IPM)

Integrated pest management is the selection, integration and implementation of pest control based on predicted economic, ecological and sociological consequences. IPM is based on the assumption that no single safe pest control. Method will be successful. IPM, therefore, seeks to use a variety of biological, physical and chemical methods integrated into a cohesive scheme designed to provide long-term protection. Biological methods include using natural predators of pests, using resistant varieties, crop rotation, intercropping, etc. Mechanical methods include manual destruction of eggs of pests, removing weeds, etc. Use of chemical pesticides is carefully timed.

 

 

 

 

 

 

 

?  Organophosphates is most toxic, synthetic but degardable.

?  Insecticides : Either as respiratory poisons or prevent nerve transmission. Depending upon mode of entry, insecticides are of three types – contact poisons, stomach poisons and fumigants.

?  Dioxin is highly toxic and carcinogen present as impurity in 2, 4, 5-T.

?  Methyl Isocyanate is a raw material for synthesising carbaryl. (an insecticide of carbamate group).

?  30% agriculture product is lost due to pest.

?  Inorganic, oil or plant extracts are first generation of pesticides.

?  Synthetic organic compounds are second generation of pesticides.

 

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