Various pests, fungi, weeds and rodents cause much harm to the production and storage of agricultural food. This has given rise to the use of various chemical agents to combat and control these harmful agents. Some of these chemical agents are non-toxic to human beings, some are only mildly toxic, some are moderately and some are highly toxic.
Agents harmful to agriculture being different, chemical agents used to control them are also different and the chemical agents are classified according to their nature of action. Thus, they may be-
1. Fungicidal – Used mostly to preserve seeds for seedling purpose. Ex. Alkyl mercurial compounds like phenyl mercuric acetate. In human beings, consumption of seeds preserved with this agent causes mercurial erethism and encephalopathy (other ex. captan, captafol).
2. Herbicidal or weed-killer – Growth of undesirable herbs or weeds in the field is a major cause of low production. Hence, herbicidal or weed-killing agents are useful and available. These are –
(a) 12% sulphuric acid – For human being it is a corrosive agent.
(b) Chlorate of sodium – It is an irritant.
(c) Arsenite of sodium – It is also an irritant. Those who handle it regularly and carelessly, may develop signs of chronic arsenic poisoning.
(d) Cyanate of potassium – It causes tissue anoxia.
(e) Dinitrocresol products – These are metabolic poisons.
(f) Paraquat (Dipyridyl herbicide)
3. Insecticidal – These are used to kill insects which destroy plants or plant products. Some of these are ovicidal which kill ova of the insects. Insecticidal agents –
(a) Organophosphorus compounds or organic polyphosphates. These agents are most popular among the Indian cultivators. Some of the preparations are also used as domestic insecticidal agents. Two chemical groups are available.
(i) Alkyl compounds – e.g. Tetrathyl pyrophosphate (T.E.P.P.), Hexaethyl tetraphosphate (H.T.P.), Octomethyl pyro- phosphoramide (O.M.P.A.), Systox, dimefox, isopestox, dipterex, sulfotep, malathion.
(ii) Aryl compounds – Chlorothion, dia- zinon, paraoxon, parathion methyl parathion
(b) Halogenated hydrocarbons – A good number of preparations of chlorinated hydrocarbons are available in the market for use as insecticidal agents in different fields. These are –
Aldrin, dieldrin, endrine, chlordane, D.D.T, gammexane; heptachlor, methoxy- chlor.
(c) Carbamate compounds – Carbaryl, aldicarb, carbofuran, propoxur.
(d) Miscellaneous preparations — Apart from organic polyphosphates and chlorinated hydrocarbons, a good number of other preparations of insecticidal agents are also available. These are –
Salts of mercury and arsenic, fluorides, cyanides, phenolic compounds – particularly the chlorinated preparations like pentachlorphenol, plant products like pyrethrum, nicotine, etc.
4. Rodenticidal – In India and many parts of the world, huge quantity of grains is eaten up and destroyed by rats and other rodents. Hence, for preservation of grains, use of rodenticidal agents has become imperative. They may be –
(a) Inorganic preparations like, barium carbonate, phosphorus, thallium, zinc phosphide.
(b) Organic preparation like fluoroacetate compound.
(c) Convulsant, like strychnine.
(d) Anti-coagulant, like warfarin.
5. Acaricide – To destroy mites, ticks and spiders, e.g. chlorobenzilate, axobenzene.
6. Nematicide – To destroy nematodes, e.g. Ethylene-di-bromide.