PESTICIDES
Definition
pesticide/pesticides is/are any agent (A synthetic or natural chemical) that fights off pests by either controlling them or killing.
Classification Of Pesticides
Pesticides are divided into two groups on basis of use and authorization:1) General Pesticides:
They are comparatively safe and should be used under the directions given on the label.2) Restricted Pesticides:
These are used by skilled and properly trained staff with special instruments & applicators.Another classification is based upon the pest being killed or controlled.it includes
- Insecticides
- rodentidices
- Repellents
- Herbicides
INSECTICIDES:
These are the chemicals which are used to control the insects. These are named according to the species of insects against which these are used. These are also named according to the stage of life cycle of insect which has been affected by these chemicals e.g.Ovicides: It kills the insects at egg stage.
Larvicides: It acts on the larval stage.
Muscicides: It kills house flies e.g. Musca domestica
Pediculocides: Chemicals which are used to control body lice e.g. Pediculus corporis.
Scabicides/ miticides: Chemicals which are used to control scabies, mites etc.
Generally insecticides on base of action are divided into two groups viz.
- Stomach poisons
- Contact Poisons
A) STOMACH POISONS:
I. As it is indicated by the name, they are absorbed from the stomach and are used to control the chewing insects.II. Death of insect occurs due to respiratory failure and paralysis of neurotransmitter centre.
III. Stomach poisons are applied as dusting powders, solutions or suspensions for spray locally or by aeroplane.
IV. After spray stomach poisons forms a protective cover over the leaves and other plant parts. Death of insect occurs due to chewing of leaves by insects.
V. As the stomach poisons are used before the onset of pest attack so they are also called as protective insecticides. They remain on leaves until washed out.
VI. They are sensitive to oxidation, therefore anti oxidants and stabilizers are added before spraying.
1)ACIDIC LEAD ARSENATE
It is used only for the control of insects on growing plants. Basic lead arsenate is less toxic to the insects but safer to the foliage.2) CALCIUM ARSENATE (Ca3As2O4)
It is used as protective for cotton, tomato, potato. It damages the leaves of other plants.3) ARSENIC TRIOXIDE (AS2O3):
It is used in the form of poisonous pulls to control grass hoppers, armyworms and other chewing insects.4) PLANT SYSTEMIC INSECTICIDES:
These chemicals are sprayed on the foliage and are also mixed with soil. From these places the poison is absorbed by the plant and is distributed to all parts of the plant specially leaves. Such insecticides are more effective against aphids, mites & other sucking insects. It gives prolonged protection.a. DEMETON & METHYL DEMENTON:
Chemically these are thioethers containing phosphorothioates and absorb only through foliage. It is also taken up by roots when mixed with soil. It is used to kill sucking insects such as plant lice, scale insects, leaf hoppers & mites. It is employed to control insects on potatoes, apples, cotton, walnut and certain ornamental flowers.
b. OCTAMETHYL PYROPHOSPHAMIDE (OMPP):
It is plant systemic insecticide which is absorbed both from roots and foliage.
c. THIAMATE AND DISYSTON:
Chemically they are phosphorothioate. These are applied to soil but are toxic to membrane of plant. Therefore these chemical agents are not used for edible crops.
5) HOUSEHOLD INSECTICIDES:
Insecticides used within the house include;
a) Na Fluorosilicate: It is used in the form of dusting powder. Its main use is to kill cockroaches. The powder adhere to different parts of the insect and it carries the chemical to nesting places where it adds to the food of other insects and upon ingestion, the insect dies.
b) Na Arsenate, Lead Arsenate, Calcium arsenate: These are used in the form of poisonous baits for fly control, cutworms and as larvicides for Anopheles mosquitoes. They are also used in the form of dust and sprays for controlling a variety of chewing insects.
c) Na Fluoride: It is used as dusting powder to control ants and other invader insects. It is also used for controlling of lice on poultry and is applied as powder.
B) CONTACT POISONS:
These insecticides produce their poisonous effects when they touch the body of insect. These may be in the form of dusting powders, solutions/ suspensions for sprays.DISADVANTAGES: Some time insect is attached to the lower side of leaves so the drug should be applied effectively on both sides of leaves.Sometime the insect develop resistance if they are exposed to small doses of poisons.
Contact poisons can be divided into two classes:
2) Synthetic contact poison
1) NATURAL CONTACT POISONS
a. Powder tobacco leavesb. Pyrethrum flowers
c. Cinerus
d. Rotenons & rotenoids
e. Sabadilla & white hellibore etc.
2) SYNTHETIC CONTACT POISONS
This one is a huge list in itself.Here we will focus on Natural contact poisons.Following are some of The common natural contact poisons.
1) NATURAL CONTACT POISONS - TOBACCO LEAVES:
BOTANICAL SOURCE:
Nicotiana tobaccum, N. Persian, Virginia tobaccum.
FAMILY: SolanaceaePart Used: Cured, dried and powdered leaves.
Preparation: Nicotine is obtained from dried leaves, stem and midribs of the tobacco plant by the process of steam distillation.
Cured tobacco leaves were used as insecticides for long time until 1828 when nicotine was discovered which replaced cured tobacco leaves as insecticides.
CONSTITUENTS:9% Nicotine, small amount of nicotianine (which is produced during the curing, also called tobacco camphor. Small amount of nor-nicotine.
Description Of Nicotine: Nicotine is an alkaloid, colourless to yellowish in colour and turns brown on exposure to atmosphere. It is poisonous and local irritant with unpleasant & strong taste having pyridine like odour.
Actions Of Nicotine as natural contact poison:
I. It kills the insects because it is toxic to CNS and causes paralysis.II. It is available as nicotine sulphate (10% soln.). The toxicity of this solution increases when alkalinized. Usually soap solution is added to this which removes the salt, thus liberating free nicotine which is more toxic and mainly acts as contact poison.
III. Nicotine also causes stimulation of cholinergic receptors resulting in twitching of muscle and death.
IV. Nicotine is also toxic to chewing & sucking insects. But the use of nicotine has been declined due to the availability of more safer and effective insecticides.
V. Free nicotine is used for fumigating green houses b/c it is immediately volatilized.
VI. Used to kill plant lice & also used as a stomach poison for some pests.
NATURAL CONTACT POISONS - PYRETHRUM FLOWERS:
BOTANICAL SOURCE:
Chrysanthemum cinerariaefolium (Major source), C. coccineum, C. marschilli (Golden flowers).
Preparation: Only the flowers of the plant are collected b/c they contain about 90% of the insecticidal activity. The flowers are usually collected from 2-6 years old plant. After collection they are carefully dried and preserved (b/c these are unstable in air). After drying some of the flowers are baled for export but the majority are made either into powder or standardized liquid extract.
CONSTITUENTS:
a) Esters of the chrysanthenic acid (Chrysanthemum monocarboxylic acid) e.g. Pyrethrin I, Jasmolin I, Sinerin I.
b) Esters of pyrethric acid (Chrysanthemum dicarboxylic acid) e.g.Pyrethrin II, Jasmolin II, Sinerin II.
These esters are oily in nature and are soluble in organic solvents but insoluble in water. They are highly unstable even if present in solid form, therefore they are usually used in combination with anti oxidants & stabilizers.
NATURAL RODENTICIDES - STRYCHNINE:
Botanical Source: Strychnos nux vomicaFamily: Loganiaceae
Part Used: Seeds
Preparation: It has a bitter taste and is water soluble. It is available in the form of salts. They are used in the form of poison.
Mechanism Of Action: Strychnine causes excitation of all parts of the central nervous system; it increases the level of neuronal excitability by interfering with inhibitory influences on the motor neurons. The site of action of strychnine is the postsynaptic membrane. The convulsant action of the substance is due to interference with the postsynaptic inhibition that is mediated by glycine. Glycine is an inhibitory transmitter to motor neurons and interneurons in the spinal cord. Strychnine acts as a selective competitive antagonist to block the inhibitory effects of glycine at the glycine receptors.
Dose: 10mg /Kg for rats.
Natural Contact poisons Rotenones & Rotenoids:
Botanical Source: Derris elliptica, D. chinensis, Lonchocarpus utilis (Derris root & Cube roots)Family: Fabaceae
Part Used: Root
Preparation: Cube roots are the major source of the prepared derris powder, which consists of either Derris 4 6% of rotenones.
Constituents: Rotenones
Description: It is odourless and relatively stable than pyrethrum. It is soluble in alcohol, chloroform and Carbon tetrachloride.
Action: It produces the same knock down & paralyzing effects as seen by pyrethrum but its onset of action is somewhat slow.
Uses:
i. Usually it is used in 5% solution. It is also used as powder. The powder is diluted with talc in 1% concentration to form dusting powder.
ii. It is also used as a spray in which it is combined or solubilized in the oil, mixed with emulsifying agent and these sprays are used as household sprays and cattle sprays.
iii. It is also used to control both chewing and sucking insects.
iv. Rotenoids do not leave any harmful residues so they are safely used as garden plants and garden and kitchen crops.
v. Commercially it is available as rotenoid powder which is diluted to 1% before use.
also see:
classification of pest
