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In intensive systems of agriculture and horticulture the competitive effects of weeds, the rotting and nutrient depletion of crops by pathogens and destructive attacks by invertebrates species have been dramatically reduced.

Cultivation practices, choice of cultivar and techniques of bio-control have helped to reduce the incidence of weeds, pests and diseases, but the principal agents used for control of these organisms since the middle of the twentieth century have been chemicals, more commonly known as pesticides.

Pesticides include insecticides, fungicides and herbicides, which are primarily used to kill insects, fungi and weeds respectively.


What are Pesticides?

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"Pesticides are chemical substances designed to kill or inhibit the growth of unwanted organisms, usually in a selective manner."
Under the control of pesticides regulation in the UK (1.5). pesticides are considered as any substance, preparation or micro-organism prepared or used for any of the following purposes.
  1. Protecting plants or wood or other products from the harmful organisms
  2. Regulating the growth of plants
  3. Giving protection against harmful creatures
  4. Rendering such creatures harmless
  5. Controlling organisms with harmful or unwanted effects on water systems. buildings or other structures, or on manufactured products.
  6. Protecting animals against ectoparasites.

Types of Pesticides

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Pesticides are viewed basically according to their chemical structure, it should be remembered that their mode of action may be equally important and on this basis they could be grouped as follows.

1. Repellants

These work by keeping the insects away from the host plants by the use of repellents odors. In olden days the laying of kerosene-soaked sacks between crop rows had this effect, it also works by masking the typical olfactory signals from the host plant.

2. Antifeedants

Insects will land on the crop plants but these chemicals on the foliage inhibit feeding behavior. Products from the neem tree are proving to be very effective.

3. Fumigants

These are mostly volatile substances that vaporize under ambident conditions; it includes gases of various types. The gases are mostly used in food stores and other enclosed spaces; the volatile liquids likewise and also for soil fumigation against nematodesand other pests.

4. Contact poisons

These are absorbed directly through the insect cuticle.
  • Ephemeral - short lived and usually applied to the foliage
  • Residual - persistence (remain active for a long period of time). May be applied to foliage but more often to soil for long term activity.

Natural Pesticides

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Natural pesticides may be defined as naturally occurring inorganic compounds and energies and animate beings, including microbes and genetically engineered organisms and plants. Many consumers may not accept the use of nuclear energy or genetically engineered plants in pest management but have no objection to synthesized, unmodified natural compounds.

1. Microbial pesticides

Live virus bacteria and fungi are the active ingredients in microbial pesticides. The polyhedrosis virus and granulosis virus are much more effective pest control agents, then the cytoplasmic polyhedrosis virus. marine and terrestrial fungi have not shown much promise as sources of pesticides.

More than 60 bio-active compounds from fungi have been chemically characterized but most of them have high mammalian toxicity and are considered unsafe for use in environment.

2. invertebrate pesticides

Nereistoxin a metabolite in the shellfish Lambriconereis hetropoda provided the lead for the synthesis of cartap a popular pesticide. Several bio-active compounds isolated from marine sponges, corals and annelids are not commercially visible.

3. Botanical pesticides

During the past three decades over 2600 plants belonging to about 200 families have been found to possess pesticidally active compounds. Voluminous data has now been generated since the 1970s on chemistry, modes of action and fields trials of compounds from A.indica. Neem extract does not have multiple mode of action against a wide range of pests.

Organic Pesticides

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The term "organic" is widely used in reference to various gardening practices, including the use of certain pesticides. Because this term has various scientific, popular and legal definitions however its meaning has become muddled.

However, the use of the term "organic" in reference to garden pesticides more often refers to the pesticide source or history of use. Materials derived from natural sources, such as the botanical insecticides pyrethrum, sabadilla and rotenone as well as certain mineral based pesticides, such as sulfur and Bordeaux mixture, are among the pesticides commonly considered to be acceptable for organic gardening.
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