Pesticides
Pesticides have been used since the evolution and development of agriculture. Pesticide is from Latin word pestis which means the destructive agent or plague. Thus pests are the living creatures which cause economic loss, destruction or injury to agricultural or other household articles.
The pests affect the reproductive potentiality of the products over which they feed.
The pesticides are the chemicals which can kill these pests. These are generally oily or waxy substances. These may be in the form of powder, granules, pallets, emulsified concentrates, aerosols, soluble powders, wettable powders etc.
Uses of pesticides
(1) The insects harmful for crops are killed by the use of pesticides (insecticides). This increases the crop yield manifold.
(2) Use of insecticides also destroys the insects which are the carrier of disease to many humans and household animals.
(3) Certain fungus causing diseases to the crops or stored grains or food products are also killed by the use of fungicides (pesticides).
Hazards of pesticides
(1) Pesticides kill the insects and other pests by which ecosystem gets disturbed. Because these pests and insects also make food chain in the ecosystem and if any component of food chain is disturbed or eliminated, the ecosystem is disturbed.
(2) The pesticides left after killing the pest, their excess amount causes toxicity to other non-pest useful organisms.
(3) The pesticides are soluble in water. When they come through rains in water streams, they cause water pollution of different nature.
(4) Pesticides are harmful to man. Regular exposure of pesticides to man in gaseous form or smoke form or in fumigation form can cause different lungs and body disorders.
(5) The pesticides are making pest resistant to these killers. The house flies are now resistant to the DDT. This DDT was very much effective when it was used for the first time.
Types of pesticides
According to chemical nature, pesticides are classified as follows :
(1) Organochlorines : These are slowly decomposing chlorinated organic compounds. These compounds are lipophilic and show much affinity for the fatty tissue of animals. e.g. D.D.T., B.H.C., aldrin, endosulphan etc. D.D.T. has been the most famous chemical in the world. It is the oldest synthetic pesticide. B.H.C. is used in the public health programmes. Aldrin is applied to foundations of buildings as antitermite treatment. D.D.T. and BHC are not highly toxic, whereas aldrin, dieldrin and endrin are highly poisonous and toxic chemicals. These chemicals are banned in western countries.
In 1971, the brown pelican bird had completely become extinct from Louisiana—once nicknamed the Pelican state—and only eight of this species were left in Anacapa island near California. In 1972, a ban was put by the U.S.A. on the use of D.D.T., and in 1983, over 3,000 pelicans had come to breed in Anacapa island. Thus the brown pelican has been removed from the list of endagered species.
(2) Organophosphates : Most pesticides affect the nervous system. They are organic esters of phosphoric, thiophosphoric and other phosphoric acids. e.g. malathion, parathion, fenitrothion etc. Malathion is one of the two active ingredients in Flit. It is also used in anti-malaria programmes.
(3) Carbamates : They are organic esters of the hypothetical carbanic acid. They structurally resemble acetylcholine very much and, therefore, they have a high affinity for the enzyme acetylcholine esterase, e.g. carbaryl, carbofuran, aldicarb propoxur etc. Carbofuran is sold as Furadon, propoxur as Baygon and aldicarb as Temik.
(4) Pyrethoids : These are the synthetic derivatives of pyrethrin. Pyrethrin is obtained from a plant botanically called Chrysanthemum cinerarifolium. These compounds are the fastest growing groups of chemicals today. They are very quick acting, broad spectrum insecticides. They are highly toxic and quite expensive. In diluted form pyrethrins are used as mosquitoes repellent.
(5) Triazines : These are the urea derivatives and are known as a group of herbicides. They are used for controlling weeds in tea, tobacco and cotton. e.g. simazine, atrazine etc.
Biopesticides
Animals or plants used willfully by man to destroy pests are called biopesticides. These are—
(1) Bioherbicides : Bioherbicides are pesticides destroying herbs. Generally they are selective feeder. e.g. cochineal insect cactoblastis cactorum eats only cactus of Opuntia variety. In 1981, the first bioherbicide was developed. It is a mycoherbicide based on the fungus Phytophthora palmivora. It controls the growth of milk weed vines in citrus orchards.
(2) Bioinsecticides : Insect killing other insects is called a bioinsecticide e.g. praying mantis eats aphids.
Fungicides : Most common fungicide used in our country is Bordeaux mixture (a mixture of 40 gm copper sulphate and 40 gm calcium hydroxide dissolved in 5 litres of water) which is used to control plant diseases caused by fungi.
Predators : Larvae of mosquitoes are eaten by gambusia fish.
L.D. 50 : It is the lethal dose or quantity of pesticide required to kill 50% of pests on which it is applied.
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