Kamis, 28 Juni 2007

Introduction of Chemical Control

The history of insecticide use dates back many centuries, certainly to before 1000 BC, when it is mentioned by Homer , but the real landmark in terms of modern ageiculture is spread of Colorado beetle(Leptinotarsa decemlineata) across the US in second half of nineteenth century. Food production and the national economy were both threatened by this potato pest and after much argument, it was finally decided to take the unprecedented step of spraying the potato crops with human poison(arsenic in the form of Paris Green). The mass human mortality predicted by the prophets of gloom did not occur and there is doubt that control of the Colorado beetle with Paris Green opened the way to a widespread use of biocides(destroyer of life in general) on crops destined for human consumption.
These were mainly of three kinds. Among the earliewst insecticides were toxic extract of plants long used by primitive tribes to tip theirhunting arrow or to bring fish to the surface of rivers and lakes. Best known of these subtances are pyrethrum(from a chrysanthemum-like plant), rotenone( a root extract of the derrist plant) and nicotine(from tobacco). These plant extract work in a variety of ways . poisoning either the nervous of respiratory system. They penetrate the cuticle of insect (contact insecticides) and are very short-lived (hours or days). The insect therefore has to be contacted by drops of spray(ephemeral contact insecticides) or , in the case of nicotine when burnt, a toxic smoke (fumigant insecticide) is inhaled by insect. The short life of these compound was initially seen as a disadvantage, but today this’ disadvantage’ gives them a special role when crops need treating close to harvest. Many plant contain toxic chemical(several are known to be very toxic to man,e.g. hemlock and atropine) and so-called’ natural insecticides ’derived from plant can be every bit as deadly as chemicals synthesised by man. However , the word ‘natural’ is enough to endear plant –derived insecticides to many who are worried about using other insecticides, and the short life of these chemicals after spraying certainly imparts safety to the environment. There is considerable interest shared by industry in discovering new insecticides in plant e.g. azadirachtin from the tropical neen three has been researched as an insecticides since early 1960s in many countries.
The second group of early insecticides were oils which , owing to their deleterious effect on plant, were mainly used only on dormant leafless plant such as apple trees over winter. Oils cover and suffocate insect and mites, including their eggs. Oils are still used in desperation today when mites, particularly, show tolerance to other pesticides, and then they may even have to be used on leafy annuals in spite of the inevitable to the plkant.
The thirds groups(including Paris Green) were stomach poisons. These were toxic radicals (e.g. arsenic of fluorosilicate) formulated as salts of metals (e.g lead or sodium). Such salts were relatively stable and plants could be sprayed without damage from poisons. Which have very general activity against life by precipitating protein. The salt must be ingested by a leaf –feeding insect before the free toxin(e.g. arsenic) is released in the gut following hydrolysis of the salt. A stomach poison has major advantage over contact poison because it is ‘addressed’ only to pest consuming the leaves and predators can move safely over the deposit. However, the stomach poisons are also rather persistent and therefore there risk of ingestion by man. Also, the metals on which the salts were based are undesirable long term soil contaminant.