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Pesticides: Common Agrochemical Poisons To Kill Pests

Pesticides being sprayed on crops
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Agrochemical poisons are the chemical substances used in agriculture to get rid of the various pests that harm the crops. These are now called pesticides. “Pesticides” are named because of their nature to kill various pests (insects, fungus, rodents, weeds, etc.) that are harmful for the crops. It is an umbrella term that includes the following terms:

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  1. Insecticides The chemicals which kill all the insects that cause harm to the crops. Example-   paraquat,  atrazine, simazine, nitrofen, etc.
  2. Herbicides– They are also called weedicides as they kill or prevent the growth of harmful weeds in the crop field.
  3. Rodenticides– The chemicals which kill the rodents (rats, rabbits) that destroy the crops. Example- aluminium and zinc phosphide, alpha-naphthyl-thiourea, cholecalciferol, etc.
  4. Fungicides– There are various fungi organisms that are destructive to the crops. To kill them fungicides are sprayed on the crops. Example- thiocarbamates, captafol, vitavax, benzimidazole, etc.
  5. Fumigants– These are the pesticides in the form of vapors or fumes sprayed on the crops. Example- ethylene dibromide, methyl bromide, etc.
  6. Repellants– This is the category of pesticides that repel the pests from invading the crops. Example- naphthalene, paradichlorobenzene, etc.

History of Pesticides

In the past, the Chinese used arsenic to kill the pests, whereas the Romans used salt to control weeds and sulphur to kill the pests. Later in the 18th century pyrethrin was discovered of having insecticidal properties which was extracted from the flower of Pyrethrum cinerariifolium or chrysanthemum.

Various natural insecticides were discovered in the 1940s and 1950s. Then a lot of synthetic pesticides were manufactured in the later years which are continued till today.

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Pesticides

Pesticides can be categorized as inorganic and organic. Inorganic pesticides are the one which lack carbon in them and are obtained from the mineral ores. Examples include- copper sulphate, ferrous sulphate, etc.

On the other hand organic pesticides are the ones which have carbon in their chemical structure and are extracted from plants or are synthesized from various combinations. Example- pyrethrin, glyphosate, etc.

On the basis of the mechanism of actions pesticides are classified as:

  • Organophosphates
  • Carbamates
  • Organochlorides
  • Pyrethroids 

1. Organophosphates

They are the most commonly and widely used pesticides in India and all over the world. Chemically they are the phosphoric acid esters or thiophosphoric acid esters. They are available as granules, dust or liquids. Some of them are required to be dissolved in water first and then used, while some are burned to produce the smoke that kills the insects.

Its common examples are Malathion, Methyl parathion, Chlorpyrifos, Diazinonphosmet, Azamethiphos, Azinphos-methyl, etc.

Organophosphate
Mechanism of Organophosphates

Organophosphates are known to be powerful inhibitors of acetylcholinesterase enzymes. The enzyme is responsible for the hydrolysis of acetylcholine into choline and acetic acid which is an essential process of propagation of action potential.

Organophosphates bind to acetylcholinesterase molecule and phosphorylate serine moiety which leads to formation of the conjugate which is more stable than acetylcholine-acetylcholinesterase conjugate.

This results in continuous build up of acetylcholine at the nerve terminals which causes receptors to desensitize that is responsible for cell death and ultimately paralysis. The reaction of acetylcholinesterase and organophosphates is irreversible due to which the effects are severe.

Fatal Dose– Malathion, Phenthoate, Trichlorfon, etc. have slightly (LD₅₀: >5000 mg/kg) to moderately toxic (LD₅₀: 501-5000 mg/kg) effects, whereas Ethion, Fenthion, Methyl parathion etc. have highly (LD₅₀: 51-500 mg/kg) to extremely toxic (LD₅₀: 1-50 mg/kg) effects on human body.

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Fatal Period– For fatal doses it is 3 hours and for non-fatal doses the period is 36 hours to 3 weeks.

Human Contact– It causes an attack of bronchial asthma, headache, dimness of vision, giddiness, frequent urination, abdominal cramps, muscular twitching, etc.

2. Carbamates

They are similar to organophosphates which are derived from the carbamic acid and aryl or alkyl esters of carbamic acid. Common examples include –Aldicarb, Bendiocarb, Aminocarb, Bufencarb, Methiocarb, Pinnicarb, Carbofuran, etc.

Carbamates

Carbamates are also inhibitors of acetylcholinesterase similar to organophosphates but the reaction of carbamate and acetylcholinesterase is reversible which causes less severe effects as compared to organophosphates.

Also carbamates do not penetrate the Central Nervous System, therefore the toxicity is also less. They are rapidly metabolized and hydrolyzed  by the liver into carbamic acid and toxic phenolic compounds. 

Fatal Dose– Aldicarb, Bufencarb, Isoprocarb, etc are slightly to moderately toxic, whereas, Bendiocarb, Aminocarb, Carbofuran, etc. are highly to extremely toxic.

Human Contact– Effects are similar to organophosphates

3. Organochlorides

These are the chlorinated hydrocarbons available in the form of dusting powders, wettable powders, emulsions, granules and solutions. Examples include- DDT( dichlorodiphenyltrichloroethane), Benzene hexachloride, Aldrin, Endosulfan, Dieldrin, Toxaphene, etc.

Organochlorides

They interfere with the nerve impulse transmission and first stimulate the Central Nervous System but depresses it afterwards. They are partially metabolized in the liver and excreted through urine, feces and milk. 

Fatal Dose– Dieldrin is extremely toxic i.e. LD₅₀: 1-50mg/kg and DDT is highly toxic i.e. LD₅₀: 51-500 mg/kg.

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Human Contact– It causes nausea, vomiting, froth from mouth and nose, tonic and clonic convulsions, coma and death.

4. Pyrethroids

They are the synthetic analogues of pyrethrins. They are available as dusts, powders, mats and coils. Examples are Allethrin, Bifenthrin, Cypermethrin, Flumethrin, Permethrin, Decamethrin, etc. 

Pyrethroids

They act at the places where electric signals are produced. They prolong the inactivation of the sodium channel by binding to it in the open state due to which signals are not able to enter the channels.

Fatal Dose– 1 gm/kg

Human Contact– It causes dermatitis, blistering, nausea, altered mental state, convulsions, coma.

Pesticides are used in households/gardens, agriculture, veterinary practices(removing termites), public health programs (spray for removing mosquitoes), etc.

Conclusion

Pesticides are the poisons and are easily available to the people especially who are engaged in farming. Therefore, most of the cases encountered in pesticide poisoning are the suicides conducted by the farmers.

However, people other than farmers also have access to these substances. Accidental poisoning is also common by the pesticides. Homicidal poisoning is encountered but the percentage is less as compared to suicides.

The most commonly encountered pesticides in poisoning deaths are the insecticides.

For the analysis, pesticides are extracted from the samples by the method of liquid-liquid extraction, micellar extractions, universal trace residue extraction, accelerated solvent extraction, superfluid critical extraction, clean-up methods, etc.

The extracted pesticides are directed to the detection methods which involve the TLC (Thin Layer Chromatography), HPLC (High Performance Liquid Chromatography), GC (Gas Chromatography), GC-MS(Gas Chromatography-Mass Spectrometry), etc.

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