Classification Pesticide use in Agriculture-The pesticides can be grouped in various ways,viz. according to the mode of their entry (stomach poisons, contact poisons and fumigants), classification pesticide use in Agriculture their mode of action ( physiological poisons, protoplasmic poisons, respiratory poisons and nerve poisons ) or according to their chemical composition. As an insecticide may enter and act in an insect body in more than one way, it may be desirable to group them according to their chemical nature.
Chemically ,the types of pesticides are :
(1) elements, such as sulphur, phosphorus,thallium,mercury etc.
(2) inorganic compounds such as lead arsenate, sodium fluosilicate, zinc phosphide, paris green, etc;
(3) Organic compound (a) compounds of plant origin such as pyrethrum, nicotine, rotenone, etc ; (b) animal and mineral oils such as fish oil, diesel oil, etc; (c) synthetic organic compounds such as DDT, malathion, carbaryl, etc,;
(4) poisonous gases such as hydeogen cyanide,ethylene dichloride, carbon tetrachloride, methyl bromide, phosphine, etc, used as fumigants.
Classification Pesticide use in Agriculture
Based on the target organisms, the ‘classificatio pesticide’ are grouped into insecticides, fungicides, weedicides nematicides
, rodenticides, etc.
For those interested in insect control, however, it is more meaningful to list these compounds according to their mode of entry in the insect body and lethal action on the pests. list of pesticides used in agriculture
The above Classification pesticide mentioned compounds could, therefore, be rearranged under the following categories :
1. STOMACH POISONS
They are generally used against insects with chewing type of mouthparts and under certain conditions against those with sponging, siphoning,lapping or sucking mouthparts. Some of these poisons are also mixed with food for killing higher animal pests, such as rodents, jackals and birds.
For the stomach poisons to be acceptable in common use, they should have certain prerequisites :
- The concentration which is applied to the plant for insect control should not be injuries to the foliage.
- The non-systemic compound should generally be insoluble in water, otherwise it might be absorbed through leaves and roots and thus, poison the plant.
- It should be potent so as to kill the pest quickly and, thus, save the crop.
- It should be inexpensive and be easily available in large quantities.
- The compound should not be distasteful to the insect, so that they may not be repelled.
- Chemically , the compound should be stable, so that it does not lose its toxicity while in shipment, in storage or after it has been applied to the plants.
- It should also not break down too easily when mixed with other chemicals .
- The insecticides should have the physical properties of adhering to the plant surface on application and yet after a certain period it should disintegrate or be washed away by rain etc., so that it does not persist on the plants for long periods and, thus, endanger the health of farm animals or of human beings.
2. SYSTEMIC POISONS
An insect with piercing mouthparts sucks cell-sap through its proboscis which it embeds into the plant or animal tissue. The only way a poison can reach its stomach its through the plant or animal system. It is only in insecticide when applied to seeds, roots,stems or leaves of plants, is absorbed and translocated to various parts of the plant in amounts lethal to insects which feed on them. Some of them are quite specific to certain groups of insects. The systemic insecticides have a great value in an integrated approach to pests control, because through their application the pesticides, predators and other natural enemies of the pests escape damage. Most of these insecticides act primarily as stomach poisons, and they include oxydemeton methyl, dimethoate, phosphamidon,aldicarb,phorate,carbofuran and thers.
3. INERT DUSTS:-
Fine dusts of various compounds have been used for a long time to control insect pests of crops and stored grains. These dusts are mixed with the grain or dusted on a surface from where the insect pick up the fine particles which cause irritation and abrasions in the cuticle, particularly at the body articulation. It results in excessive loss of moisture from the body of an insect which ultimately dies of desiccation. These dusts are based on fine sand, powdered cowdung ash (which also acts as a hygroscopic compound), calcium carbonate, silica, aerogel, etc.
4. CONTACT POISONS:-
The other most effective and most prevalent method of insect control lies in the use of contact poisons, a large number of which are now available. The contact poisons are applied as sprays or dusts, either directly onto the body of insects or to the places frequented by them. These poisons kill the insect either by clogging spiracles and respiratory system or by entering through the cuticle into the blood and acting as nerve or general tissue poisons. It has been shown that the insect cuticie possesses very high absorptive properties, so that the lethal does applied externally is almost the same as the quantity required to kill by injecting it into the body. The contact insecticides, even though applied in the form of emulsion, suspensions or dusts are highly lipophilic and are readily absorbed by the lipids present in the epicuticle of insect exoskeleton. The residual film of a contact insecticide may kill an insect by its action on the sensory organs present in the tarsi of the legs.
A wide spectrunm of new synthetic organic compounds have revolutionzed the modern practices of pest control. The major groups include chlorinated hydrocarbons (DDT, HCH, aldrin, etc.),carbamates (carbaryl, aldicarb, carbofuran,etc.), and organophosphates (malathion, oxydemeton-methyl, monocrotophos,etc.).
5. FUMIGANTS :-
Poisonous gases, derived from either solids or liquids, are used as fumigants to kill insect pests of stored grains and other products in warehouse, museums, godowns, etc. They are also used to kill insect pests found in animal sheds or human dwellings; soil-infesting grubs and nematodes, the scales infesting nursery stock , the borers founds in trees or wooden structures ; to control all kinds of greenhouse pests and even worms found inside the intestines of animals. Since practically all the fumigants are deadly poisons, great care is needed in their use. Some of the common fumigants, both old and new include nicotine, hydrogen cyanide, carbon bisulphide, sulphur dioxde, paradichlorobenzene, naphthalene, ethylene dichloride and carbon tetrachloride mixture, methyl bromide, phosphine, DBCP,D-D(dichloropropane-dichloropropene) mixture etc.
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