The use of insecticides in the field of agriculture has proved very useful in protecting crops from pests and, thereby, significantly improving the yield. Besides agriculture, the role of insecticides in protecting human life by controlling the spread of epidemics and providing human life hygienic environment is also well-known.
The most familiar example is that of DDT (Dichlorodiphenyltrichloroethane), the use of which has saved millions of human lives by successfully checking malaria. On the agricultural side too, the crop yield has been considerably increased by the use of DDT. Hence, its popularity. Paul H Muller, the inventor of DDT was awarded the Nobel Prize in 1948 for his invention.
The extensive use of insecticides became a cause of concern, however, when some of their harmful side-effects were observed. On both the fronts, i.e., that of agriculture and human health, on the basis of quantitative results, it became evident that in the long range, the use of insecticides is not in the overall interest of man
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Rachel Carson alerted the nation against the use of pesticides in her book Silent Spring.
Chemical pesticides are primarily classified by referring to the type of pest they control, and, thus, we speak of acaricides, fungicides, insecticides, nematicides, rodenticides and weed killers or herbicides. Some chemicals fall into more than one of these categories: par- athion, for example, while being an insecticide, is also a nematicide and an acaricide.
Insecticides are commonly classified as organo-phosphates and organo-chlorines. A great variety of them is in use, either singly or in combination, under different technical and trade names. They have their specific applications and characteristic harmful effects from the point of view of pollution About 50,000 pesticide formulations are currently available for use on a, world-wide scale. The harmful environmental effects of insecticides are outline in the following paragraphs.
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In general, every chemical pesticide is a poison to some living thing and no poison has yet been discovered that will affect only one kind of insect or weed. If analyzed, it is found that the results are good on a short term basis but in the long run, the results are disappointing. An example of the use of DDT over a long period of time would serve as a good illustration.
In 1949, the experiments on the use of DDT were carried out on cotton fields. Initially, the yield increased from 440 lbs/acre, to 648 lbs/acre after 3 years it was found that the DDT was no longer effective in controlling the pests; the first pests were killed but DDT resistant strains had risen. Afterwards, it was found that six types of pests had appeared, and ultimately after 5 years, it was observed that the yield of cotton reduced to 1/3 of the original.
From the preceding example, it is evident that the overall effect is not beneficial, even from the point of view of yield. Here, another important point to be noted is that while attempting to kill one species of pest, other species are also killed and some of them may be natural predators to the very species we want to kill.
It is reported that in practically every spray operation, thousands of non-target insects are also killed. Further, many new insect species develop which are resistant to the insecticides.
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The killing of ‘non-target’ species disturbs the balance in the natural cycle and the evolution of new species necessitates more severe environmental control measures.
As far as human beings are concerned, the harmful effects of insecticides have been a cause of concern. The people working on farms, involved in the handling, etc., are exposing directly and are likely to become victims of the toxic effects of insecticides which may be lethal. Organophosphates decompose more rapidly than the organ chlorines but some of them have atoxic strength which is 300 times higher.
Organophosphates build up residues on foliage after application. In one case, 90 peach pickers were poisoned while working in orchards, subjected to heavy spraying of parathion, over a period of several months. The toxic effect of some of the insecticide.
The persistence of insecticides in any environment is harmful. Besides, they, sometimes, undergo metabolic and other degradation processes and, thereby, produce more harmful products than the parent substance.
The harmful effects of the presence in DDT, the most evidently used insecticide at one time in the environment, have been reported from different parts of the world.
The presence of DDT in water, air and soil results in its presence in various species that live in those surroundings. DDT’s concentration builds up in the body and accumulates in the fat. Further, as it goes up the food chain, it concentrates more and more, and the effect is cumulative.
The quantity or extent to, which it gets accumulated in the body, varies with the kind of species. illustrates this aspect.
In the human body also, the presence of DDT has been detected. In some cases, the level has exceeded the normal limit and people are facing its harmful consequences. It is found that most of Americans have 8-10 ppm DDT in their body. In this connection, it must be noted that prior to the use of DDT, it was never detected in the human body. With its increased use, its concentration in the body has also increased.
DDT affected vegetables, fish, milk, etc. There have been reports that even breast fed babies get 70% more than the accepted amount of DDT Like human beings, other species are also affected adversely in various ways, e.g., thinning of egg shells, etc.
Therefore, precautions have to be taken for the presence of pesticide residues on foods to ensure that the quantities remaining on a fruit, vegetable, grain or animal product are reduced to such levels that do not pose any health hazard to human beings.
The adverse effects of using insecticides have become of vital concern these days, to the extent that opinions favors to limiting the use of insecticides.
The harm caused by DDT has been well-established beyond all doubt and EPA of America has banned the use of DDT since 1972. In future, environmental considerations would be the guiding factor in recommending a particular insecticide for general purposes.
There is a definite trend towards finding suitable alternatives for those insecticides which have adverse effects on the environment. In general, the organ phosphorous based insecticides show greater promise than organ chlorine based ones.
Lastly, it would be appropriate to point out that use of insecticides would have to be carried out with greater care and control, than has been common in the past.