Chemical Control of Pests:
Chemical control of pests in agriculture and horticulture is a method of controlling pests through chemical materials poisonous for pests. It includes use of insecticides that directly kill the insects or attractants and repellents that avoid pest infestation onto the crop.
Insecticides are chemical compounds which kill the insects through various mechanisms. Insecticides are classified on the basis of mode of entry into the insects, mode of action and their chemical nature.
Classification on the Basis of Mode of Entry into the Insects:
On the basis of mode of entry into the insect, insecticides are classified into four types.
Stomach poisons are used against insects having “chewing type mouth”. These poisons are directly swallowed by insects through their month. Thus poisons are either applied on natural food of insect (foliage of plant), mixed with substances attractive for insects or dusted/sprayed on run ways of insects. A good stomach poison must be tasteful to insects, spread and stick with plants, cheap, available in large quantities and must be active. Some good stomach poisons are P, Hg, S, OC, & Paris Green.
Contact poison is used against insects having “piercing type mouthpart”. It kills the insects when come in contact through any mean. It enters the body either directly or through respiratory system of the insect. It causes sleepiness, paralysis, corrosion of insect skin or tissues and ultimately death. ROSIN SOAP is a good contact poison.
These chemicals are used as spray or mixed with irrigation water or applied as seed treatment or injected to plant. They are absorbed readily by the plant and become part and parcel of plant cell. They are translocated to other parts of the plant. Thus, the whole plant possesses insecticidal action. The plant becomes lethal for insects feeding or crops. It has the advantage of being specific to insects and does not kill other natural enemies of the harmful insects. Demeton, menazone & phorate are some of the many systemic poisons.
Poisonous gases, either derived from solids or liquids used in the form of gases, smoke or vapour to kill the insects are called fumigants and the operation is called fumigation. Fumigation is mostly done in places not involving living plants like stores, mills, collection of plants and animals, factories, jails, hospitals etc. Vacume fumigation is done in specially constructed chambers for bales of cotton, bags of wheat flour and other gains. Soil fumigation is done for control of soil insects like black ants. The chief advantages of fumigation over sprays or dust is that it penetrates deep into the cracks and crevices, where ordinary poison applied through sprays or dust cannot reach. Various fumigants, available in the market are: HCN gas, phosphene gas, sodium cyanide, carbon bi sulfide etc.
Classification of Insecticide on the Basic of Mode of Action:
On the basis of mode of action, insecticides are classified into four types.
These insecticides kill bacteria by some physical action as excluding air (through mineral oil) or by abrasive or surptive actions resulting in loss of water (using silica aerogel).
These insecticides kill the insects by precipitating their proteins.
These insecticides interfere with the normal metabolism of the insects. They can be carbohydrate cycle inhibitor, respiratory poisons or Amine metabolism inhibitors.
These poisons effect nerve receptors or ion permeability off the insects. Nicotine, DDT, carbamide are some of the many available nerve poisons for the insects.
Classification of Insecticides On the Basis of their chemical Nature:
On the basis of chemical nature, insecticides can be classified into two broad groups; organics & inorganics.
Inorganics are used less frequently than organic insecticides.
LEAD ARSENATE— Fruit Insects
SODIUM FLUORIDE – Bats & Ants
SULFUR (Dust/liquid Sulfur line) Broad spectrum Phosphorous – Cockroach.
Organic insecticides are divided into oils, botanicals and synthetic chemicals.
Oils are phytotoxic in nature and being used in emulsion form. Oils are of various types including summer oil, dormant oil, emulsible oil, miscible oil, tank oil etc. They are cheap, safe, easy to mix and have good spreading capacity. Oils have the disadvantage of being phytotoxic in nature and can cause injury to plants. They are instable to store and are generally low in toxicity to insects.
Botanicals are insecticides of plant origin (plant products). They act as attractants, repellants and solvents towards various insects. PYRETHRUM is toxic to large variety of insects. Nicotine plants and cotton seed oil are also toxic to insects. NEEM& DHAREK contain AXADIRACHTIN which is toxic to insects.
Synthetic organic Insecticides:
Synthetic organic insecticides are the most abundantly used insecticides now days. These insecticides are classified as.
These insecticides are compounds of chlorine being the active ingredient & organic molecules. Famous organochloride insecticides are DDT (Dichlorodiphenyltrichloroethane), BHC (Benzene Hexachloride), Chlordane, & Aldrin etc.
Organophosphorons insecticides are compounds of phosphorous and organic materials. Parathion, malathion, chlorothion are organaphosphorous insecticides.
Carbonates are esters of unstable carbonic acid. These insecticides break down readily, giving no harmful residues. Methomyl, and Furadan are carbamate insecticides.
Insect Control Regulators (IGR):
These chemicals cause metabolic disturbances, leading to death of insect. Methoprene & Dimicin are IGRs.
Some chemicals have the property of greatly increasing toxicity of certain insecticides. When the increased toxicity is markedly greater than the sum of two used separately, it is called a synergistic action.
2+2 = 10.
Pesticide A kills 20% insects
Pesticide B kills 30% insects.
Pesticide A+ pesticide B 20+30=50%
Pesticide A+Pesticide B → 100% Synergistic action.
Synergistic action is important because it provides a measure for a more effective insecticide at less cost.
Attractants & Repellents:
PHEROMES – the attractants produced by insects and chemical repellents, which repel the insects from their food.
The insects which lured into traps, may be killed with insecticides or may be sterilized with chemosterilants.