Insecticides

Insecticides for vector control include the following:

1. Poisons (e.g. Paris Green, which was used extensively as a larvicide). Anopheles gambiae was eradicated from Upper Egypt by this preparation. In view of the resistance to insecticides that has developed, it could be reconsidered.

2. Fumigants (e.g. hydrogen cyanide, methyl bromide and ethyl formate) can

Fig. 3.10. Contour drains in a (A) narrow ravine and (B) wide ravine. (From Davey,

T.H. and Lightbody, W.P.H. (1987) The Control of Diseases in the Tropics.)

Fig. 3.10. Contour drains in a (A) narrow ravine and (B) wide ravine. (From Davey,

T.H. and Lightbody, W.P.H. (1987) The Control of Diseases in the Tropics.)

Fig. 3.11. A locally constructed dam for the control of Anopheles fluviatilis in Nepal. Every 3 days, the bung is removed and the head of water rushing down the stream is sufficient to dislodge developing mosquitoes.

be used on grain or clothing to destroy infestations.

3. Knock-down (e.g. pyrethrum, bioresme-thrin and bioallethrin).

4. Residual, which are sub-divided into organophosphates, carbamates and pyreth-roids. (Organochlorines, 4,4'-dichlorodiphe-nyl-1,1,1-trichloroethane (DDT), benzene hexachloride (BHC) and dieldrin, which were widely used originally, are no longer available due to their toxic and long-lasting effects on the environment.)

Organophosphates Organophosphates, such as malathion and fenthion, are volatile substances that require frequent application. They act by inhibiting cholinesterase at the nerve junctions and, therefore, can produce temporary paralysis (and respiratory failure) in humans as well as insects. They do not have a long residual action or persist in the environment. Chlorpyrifos (Dursban) and temephos (Abate) are low-toxic compounds widely used as larvicides.

Carbamates Carbamates act in a similar manner to the organophosphates except that they compete with acetylcholinesterase rather than combining with it, making their effects more easily reversed and thereby conferring an advantage to humans. Examples are propoxur and carbaryl.

Pyrethroids Pyrethrum is a naturally occurring insecticide, obtained from a species of chrysanthemum, that has been synthesized to produce a range of more active forms with good residual ability. These are stable substances with low mammalian toxicity and are widely used both for agricultural and medical control. Examples are permethrin, deltamethrin and lambda-cyhalothrin, which are particularly valuable for treating mosquito nets.

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