Insecticides & Pesticides | Biology for Grade 12 PDF Download

AGRICULTURAL CHEMICAL – PESTICIDES

Pesticides – Pesticides are those substances, which are used to kill, control or repel pest.

Pesticide Spray

Pests – Pests are the organisms (viz - disease causing microbes, insects, mites, nematodes and weeds) those cause harm to human interests by destroying the agricultural crop, stored plant products as well as bring about diseases in domestic animal and human beings. Pests cause loss in agricultural productivity up to about 30%.

TYPES OF PESTICIDES :-

(1) Algicide :- Destroy algal pathogens & algal bloom e.g Y copper sulphate

(2) Fungicide :- Destroy fungal pathogen e.g – 

• Bordeaux mixture (It is first discovered fungicide)

• Burgandy mixture (CuSO₄⁺ Na₂CO₃⁺ water)

• Mercuric chloride

• First pesticide to be used commercially was bordeaux mixture (CuSO₄ ⁺ Ca(OH)₂⁺ H₂O). It was discovered by Millardet (1822).

At the university of Bordeaux, France. The fungicide was named after the name of university.

(3) Weedicide/Herbicide :- They kill weeds in agriculture. e.g Y Triazines Y 2, 4–D, 2, 4, 5–T(Auxin derivatives).

(4) Insecticide :- They kill insects. e.g. Y DDT, Malathion, Aldrin.

(5) Nematicide :- They kill nematodes e.g. Y Chloropicrin, Methyl bromide.

(6) Rodenticide :- They kill rodents e.g. Y Warfarine, Zinc sulphate, Zinc phosphide(Used against field rats).

Classification Of Pesticides On The Basis Of Chemical Structure:

(1) Organochlorines :- They are chlorine containing hydrocarbons. Some important organochlorines are :-DDT, Aldrin, Dialdrin, Endosulphan, BHC.

• Organochlorines are lipophilic in nature ; hence show great affinity for the fatty tissues of animals. These are non-biodegradable. They decompose very slowly, therefore get accumulated in environment and posing serious problems.

• DDT (Dichloro-diphenyl trichloro ethane) is the most famous pesticide of the world. DDT was  discovered by Dr. Paul Muller.

DDT is banned for agricultural use in India in 1985. 

•  BHC (Benzene hexa chloride) is the most commonly used pesticide in India

•  Aldrin – It is added to building foundation to prevent termites.

(2) Organophosphates :- These are organic esters of mostly phosphoric acid and triphosphoric acid.

These are most toxic pesticides to vertebrates. e.g Y Malathion Y Parathion * Malathion is one of the most widely used organophosphate

•  It is an ingredient of "Flit".

It is widely used in antimalarial programmes.

•  Organophosphate acts on central nervous system. They inhibit normal functioning of enzyme cholinesterase.

(3) Carbamates :- These are salts of carbamic acid.(NH₂COOH)

•  Structurally, these pesticides are similar to acetylcholine and therefore they bind with the enzyme acetylcholinesterase. They also affect the nervous system e.g. Y Carbaryl Y Propoxur (Baygon)

•  Carbamates are effective insecticides, but derivatives of carbamates are used as fungicides and herbicides Fungicides® dithiocarbamates like diethane – M – 22 Herbicide- phenyl carbamates & thio carbamates

•  Methyl isocyanate gas (MIC) is raw material for synthesizing carbaryl, which caused Bhopal gas tragedy on 3 Dec, 1984.

(4) Pyrethroids :- These are synthetic derivatives of "pyrethrin" - a chemical extracted from floral heads of Chrysanthemum cinerarifolium and C. marschallii (family Asteraceae).

•  Pyrethroids are largest group of insecticides of plant origin. Used commercially now a days.

(5) Triazines :- This group of pesticides is derived from urea. eg. Atrazine, Simazine

•  These are commonly used herbicides for controlling weeds in tobacco, tea and cotton plant

(6) Auxin derivative :- 2, 4 – D, 2, 4, 5 – T, Delapon, TCA (Trichloroacetic acid) etc. are used as a selective herbicide or weedicide Mode of action of pesticides :-

• Most of the herbicides have affect on photosystem-II of photosynthesis (disturbing the photolysis of water and oxygen evolution). Some herbicides block sieve tube thus disturb translocation of organic food.

•  Most of the insecticides kill insects by affecting their nervous system. Some insecticides arrest the respiratory system and kill the insects.

Advantages of pesticides: 

•  Increase in yield of food and other crop plants.

• Pesticides control vectors or carriers of different disease causing organisms.

Harmful effects(hazards) of pesticides :-

(1) Environmental pollution :- As pesticides are non selective in their mode of action, so these also kill useful organisms along with harmful organisms thus equilibrium state of ecosystem is disturbed.

(2) As pesticides are poisonous or toxic, so cause serious health hazards.

(3) Excessive and prolonged use of pesticides lead to resistance in pest. Hence more money is to be spent in controlling to these resistant pest, this effect is called"Pesticide treadmill"

(4) Due to their non biodegradable nature, their concentration increases in the body of organisms with the rise in trophic level is called biomagnification or bioconcentration. 

• Most organochlorines are fat soluble and undergo biomagnification.

• In India as a result of prolonged use of DDT, 13 – 31 ppm of DDT can be detected in the body fat of people  of India (highest in the world).

• Development of resistance to pesticides :- When pesticides are sprayed, some pest individuals having :-

(i) Less permeable cuticle

(ii) Capability of faster storage of toxin in fat.

(iii) Better enzyme equipment for metabolising the toxin.

•  So due to above reasons pest individuals develop resistance to pesticide

Biological pest control :- biopesticide 

• Biopesticides are pesticides of biological origin. The living organisms used to destroy the undesirable organisms are know as biopesticides. They are of two types:

(1) Bioherbicides

(2) Bioinsecticides.

(1) Bioherbicides :-The first bioherbicide was "Devine" is a mycoherbicide. Devine is based on a fungus Phytophthora palmivora. 

•  It is being used since 1981 to control Morrenia odorata(milkweed vine) in Citrus orchards. 

•  Similarly "Collego" is an another mycoherbicide and is based on fungus Colletotrichum gloeosporioides.

• Prickly pear cactus (Opuntia) in Australia and India was effectively controlled by Cactoblastis cactorum (Cochineal insect).

• Eichhornoa crassipes has been successfully controlled in Florida (USA) with the application of a fungus Cercospora rodmanii.

• Smoother crop :- The crop which does not allow weeds to grow eg. sunflower, soyabean, barley, rye, sorghum.

(2) Bioinsecticides :– 

(I) Use of parasite, predators and pathogens 

• Control of aphid by use of praying mantis or lady bug.

• Insect affecting maize, cotton, cabbage, sunflowers etc are controlled by mutant strains of Bacillus thuringiensis bacteria.

• The bacterium, Bacillus thuringiensis produce protein toxin. eg. Thurioside Cry - protein.

• Thurioside is active against different group of insects.

• Sporeine :- First commercial bioinsecticide, developed in Germany.

• Sporeine kill, the insect by inhibiting ion transport in the midgut.

• Mosquito larvae can be easily controlled by fish Gambusia.

(II) Sterilization technique :- This is the modern method of  biological pest control, In this technique male insects are sterilized with the help of radiations and these sterile male are released at the time of mating and hence their multiplication is checked. e.g. "Screw worm" and " Red weavil" was eradicated by this technique by Kipling.

(III) Use of insect hormones (Sex attractants) :- The insect hormones are called pheromones and these are useful in controlling insects. synthetic pheromones are used in different ways :-

(a) Pheromone trap :- Hollow cylinders coated inside with sticky substance and synthetic sex pheromones of the insect are kept at different places in the infected fields. The male of insect fly is trapped and get immobilized. Male insects are not available for reproduction, so that their population is controlled. Gypsy moth has been controlled by this method.

(b) Confusion technique :-

• In this technique large amount of hydrophobic papers containing the sex attractant are dropped over a cropped area.

• The male insects get confused and fail to locate their female. Thus population is checked.

(c) Juvenile hormone and the moulting hormone (ecdysone) :-

• These hormone are needed for proper metamorphosis of young ones to the adult insect.

• Use of juvenile hormones at inappropriate times results in the early death of insects pests

(IV) Natural insecticide :- These are insecticide of biological origin.

• They are generally obtained from plant and some times from microbes.
  (a) Rotenones :- Obtained from roots of Derris eliptica. Insecticidal properties of rotenones were first discovered by Chinese.
  (b) Nicotine :- Obtained from Nicotiana tobacum (Tobacco) leaves.
  (c) Pyrethroids :- (Pyrathrum & Cinerin). 

• Obtained from "Chrysanthemum cinerarifolium".
  (d) Azadirachtin :- It is an antifeedant compound extracted from neem (Azadiracta indica) spray of neem extract repel Japanese beetles and leaf eating pests.
  (e) Thurioside :- Obtained from a bacterium "Bacillus thuringiensis".

• Thurioside is the microbial insecticide

INTEGRATED PEST MANAGEMENT (IPM)

•  It is technique of pest control, which involves natural methods to a greater extent.

• IPM involves use of different pest control method, which are ecologically sound i.e. do not cause hazards to environment. e.g – biological control method, better agricultural practices like- crop rotation, sanitation etc., use of resistant varieties.