The chemical substances used to kill the pests are known as pesticides. A pest is a living organism which destroys plants, foods or animals. Fungus, insects, rodents and various plants are the major pests. The pests reduce the crop yield significantly. The pests interface the life of many plants and animals. They also degrade the food quality of harvested food and grains. Herbicides, fungicides, insectides and rodenticides are common pesticides.
The pests are the organism which may damage the economic and physical well being of human. A variety of unwanted, and even harmful insects, weeds, micro-organism (bacteria, fungi), rodents nematodes and other organisms come under the categories of pests. Without pest control food production could decrease.
Pesticides are those substances which are used to kill, control or repel pests. Depending upon the type of target organism, the pesticides are classified as Fungicides (for fungi), Herbicides (for weeds), Insectides (for insects), Nematocides (for nematodes), Bactericides (for bacteria), Rodenticides (for rodents).
Chemical pesticides are toxic chemicals used for killing pests. On the basis of chemical composition, chemical pesticides are grouped into:
1. Organochlorides: The pesticides which contain organic chlorides are DDT, BHC, Aldrin ,Deildrin, chlordane, toxaphene, etc
2. Organophospates: The pesticides which contain organic phosphates like parathion, malathion,diazion, phenothane guthion,etc.
3. Carbamates: These pesticides are pyrethrins, baygon, temik, zectran, Carbaryl, Aldicarb, Carbofuran etc.
4.Pesticides of plant origin are pyrethroids from Chrysanthemum flower, nicotine from tobacco leaf and other like neem plant, Artemisia leaf, etc.
Insecticides and rodenticides are Zinc phosphide, Arsenic compounds, Thallium sulphate, etc. Nematicides are aliphatic halogen compounds, organophosphates and carbamates. Herbicides are phenoxy acids (2-4D and 2, 4, 5-T), Paraquat, Diquat and Triazoles. Fungicides are HCN, ethylene oxide, acetaldehyde and methyl bromide.
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MODES OF ACTION:
Insecticides are either nervous or respiratory poisons. The Organochlorides are nervous poisons. They either inside the insects body either through the integument (cuticle) or spiracles. If the pesticides are in solution or absorbed form, they are taken orally and reach the nerve fibres where they inhibit NA+, K+ and Mg++ ATPase (Adenosine- triphosphate enzyme) activity in the nerve endings (synapsis) of insects. The poisons mostly affect the sensory or motor nerve fibres as well as motor cortex, thus results nerve paralysis and ultimately the insect pests die.
Some pesticides are respiratory poisons. They enter through spiracles. They cause nausea, vomiting and nervousness leading to death due to respiratory arrest.
Herbicides attack the photolysis of water (photosynthesis II) and evolution of O2 in the process, regulates growth and also affects the translocation of organic solutes.
Effects of pesticides:
a. Polluting the environment: Although pesticides save about 10% of the world food supply from the pests but these cause serious environmental and health threats to the various organisms. The organochlorides have longer life-time, therefore, persist and accumulate in environment. The pesticides are generally non-biodegradables, therefore, get incorporated into the food chain and ultimately deposited in the fatty tissue of different animals including man.
b. Kills board spectrum of organisms: The use of pesticides in the farms, gardens may also kill the useful organisms such as natural enemies (predators, parasites and pathogens) of the pests as well as earthworms and detrivores in the soil. Nitrogen fixing bacteria are also destroyed by the use of pesticides.
