What are the main uses of pesticides?

Chemicals which prevent, destroy or mitigate any pest (insects, fungus, bacteria, viruses, etc.) are called pesticides.

Pesticides are classified on the basis of (i) chemical structure (ii) biological action (iii) class of pests they combat.

Some chemicals, however, fall into more than one category. For example, parathion, while being an insecticide, is also a nematicide and acaricide.

The main groups of pesticides of our concern are insecticides. With increase in population growth man had to increase agricultural productivity.

Use of insecticides, like DDT has not only improved the productivity of agricultural goods by protecting crops from pests but has also contributed in curbing diseases, like malaria which once was an epidemic.

However, the extensive use of insecticides has proved that its use is not entirely in the interest of man due to its harmful side effects. There are three main groups of pesticides namely organochlorines, organophosphates and carbamates.

A great variety of insecticides, under different technical and trade namfes are used. Although the targets of pesticides are insects, fungus, nematodes and rodents which damage crops, they also target non-target beneficial organisms like earth worms, fish, birds, mammals and man.

The killing of such species disturbs the balance of natural cycle and also the evolution of new species, necessitating more severe environmental control measures.

The harmful effects of insecticides thus, have been a great cause of concern and persistence of insecticides in the environment after application is not at all desirable from environmental considerations.

The most widely used organochlorine pesticide is DDT (dichloro diphenyl trichloroethane), which has controlled insects carrying diseases like malaria (mosquitoes), plague (fleas) and typhus (body lice).

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The organochlorine insecticides are highly toxic to insects, but acute human toxicity is low. It is the bio-accumulation of organochlorine pesticides such as DDT in the food chain which creates toxicity.

Such type of insecticides persist longer in the environment before undergoing degradation to other substances and are quite soluble in hydrocarbons and, easily accumulate in the fatty tissues.

And so, because of their build up in the food chain, they get magnified to higher trophic level. Birds are high on the food chain and thus, more accumulation of DDT has adverse effects, interfering with enzyme regulating distribution of calcium, resulting in thinning of egg shell that supports its weight.

The resulting difficulty to reproduce due to high levels of DDT has brought the hunting birds like bald eagle, ospreys and brown pelicans on the verge of extinction.

The other organochlorines widely used are chlordane, aldrin, endrin, dieldrine, kepone, etc. Some of these types of insecticides show birth defects in mice, hamster and some cause liver cancer. Kepone, a class of plant insecticide, finds its application in controlling tobacco wire worm, ants and cockroaches.

Exposures to this insecticide can cause severe neurological damage and when they undergo de­composition and are discharged into water, incorporated into the food chain.

As far as the effect of organ chlorines is concerned, they pose a serious threat to the eco-system, human health and moreover the long term use of such chemicals is no longer found to be effective in controlling the pests due to the biological assistance development in these pests against the pesticides.

Thus, another class of insecticides which are not persistent has been developed and is being replaced. Some examples of this class are Malathion, Phosdrin, Guthion, Diorom, etc. Although these insecticides are not accumulated in the food chain, they pose direct threat.

They get absorbed rapidly through skin, lungs and gastro-intestinal tract and become acutely toxic than the organ chlorines that they have replaced.

Excessive amount of such types of insecticides cause chest discomfort, vomiting, headache, nervous disorder, etc.

The third category of insecticides, carbamates, is derived from carbamic acid (NH₂ COOH). These insecticides are not persistent like organo chlorines, but they get absorbed as organo phosphate insecticides, causing acute toxicity.

Some of the carbamate insecticides are aldicarb, carbonyl, etc. Exposures to such chemicals cause blurred vision, nausea, vomiting. The accumulation of DDT in the body tissue of an average Indian is much greater than anyone in the world.

Some other pesticides commonly used to kill pests are:

Fumigants:

The insects die inhaling poisonous gaseous or fumigants like HCN vapour, CH₃Br, PH₃, para dichlorobenzene, nicotine, chloropicrin, etc.

Fungicides:

Fungus on plants and crops are killed by quinones, organomercury and its compounds and phenols.

Bactericides or antibiotic:

Bacterial growth in plants is checked or removed completely by streptomycin spray.