TYPES OF INSECTICIDES (Stomach and contact poisons)


Insects such as weevils and beetles which feed on leaves and other plant tissues by biting can be controlled by covering either the insect or the material with either a poisonous spray or dust. This type of chemical is generally referred to as a STOMACH POISON. Insects such as aphids which feed by sucking the liquid from plant tissues can best be controlled by CONTACT INSECTICIDES which are applied either as a preventative or control measure. Many of the newer types of contact insecticide paralyse the insect by entering the nervous system and preventing their respiratory processes. To be most effective, contact insecticides should have some degree of persistence so that they remain active for some time after application.

Some sprays are particularly effective for killing the eggs of insects before they hatch and are used on trees during a resting or dormant period. Many of these sprays are oils of various types but are not widely used in West Africa. Other sprays are used for covering insects such as scale insects with an emulsion which excludes air and leads to suffocation. Systemic insecticides have been used for some time, particularly on plantation crops, for the control of insects which are difficult to kill by other means. These chemicals enter into the tissues of the plant, usually through the roots although they may be also absorbed by leaves and become dispersed throughout the whole plant system. Insects which feed on treated crops are poisoned, this type of control is particularly effective against sucking insects. This type of insecticide should not normally be used on food crops owing to the risk of contamination by persistent chemicals. The comments made on page 103 regarding chemical residues in plants and the allowance of at least 14 days between the application of insecticides and harvesting are particularly important since some of the newer formulations of chemicals may have a longer period of persistence than some of the more traditional ones.

The following chemicals are in common use in many parts of West Africa; many of them are offered under trade names and care should always be taken to ensure that the required active ingredient is contained in the formulations offered. Most of thesc chemicals are highly poisonous and  extreme care should be observedwhen both mixing and applying sprays.

Also Read:

Pests and Parasites, And It Examples


1. LEAD ARSENATE Highly poisonous, not now widely used. Useful for controlling caterpillars.

2. PARIS GREEN Also posionous, used mixed with bran as a poison bait.


3. NICOTINE SULPHATE SOLUTION   Normally used with the addition of soft soap or a wetting or sticking agent. Effective against aphids, caterpillars and a wide range of sucking insects. Very toxic to mammals, extreme care is required in using this chemical.

4. DERRIS   One of the safest insecticides. Used as a dust or wettable powder. Effective in controlling aphids, caterpillars and both biting and sucking insects. Toxic to fish. No residual effect on crops.

5. PYRETHRUM   Also non-toxic to mammals and a very safe insecticide. Particularly useful in controlling caterpillars, aphids and leaf-eating beetles. No residual effect on crops. New forms of this insecticide, which are more effective, are now being developed.

Also Read:

Pests Affecting The farm

6. KEROSENE EMULSION   Usually used with a wetting or sticking agent, is used to control scale insects and mealy bug on tree crops. Not suitable for most annual crops.

7. DDT  And organo-chlorine insecticide which is not now popular, owing to the chances of residues accumulating in the human body. Is effective against a wide range of insects but its use is gradually being discouraged. Used in the form of a wettable powder or emulsion.

8. BHC   Many forms of this organo-chlorine insecticide have been in use for some years, but the use ofvthis chemical, particularly in the form of gammexane, is also being discouraged due to its residual effect. It is still used in stores in the form of lindane and for controlling specific insects but is gradually being replaced by non-residual chemicals. Effective on a wide range of farm and garden pests, some forms are effective against grasshoppers, as either a bait or spray. It is more persistent than DDT.

9. ALDRIN AND DIELDRIN These are very effective against soil insects, particularly termites and many types of ant but dieldrin in particular is very persistent and the use of both of these chemicals is being questioned due to this property. Both can be used as contact sprays against beetles, capsids and borers.

10. MALATHION  This is an organo-phosphate chemical and is considered to have a lower level of mammalian toxicity and is far less persistent than DDT. Is effective against aphids, red spider, leaf hoppers and many other types of insect and is widely used as a general insecticide.

11. KARATHANE  An effective chemical used mainly as an acaricide for controlling mites and red spider.

12. FUMIGANTS For stored products, sulphur dioxide, carbon disulphide, ethylene dichloride and methyl bromide are all used. These are all very poisonous chemicals and must be handled with extreme caution.


This term is given to the group of chemicals which are used for controlling nematodes or elworms. These organisms occur mainly in the soil and the chemicals used for their control are usually volatile i.e. they produce a vapour which penetrates the soil atmosphere. Many are applied in liquid form, using an injection process but a more recent development is a granular form of nematicide which is effective but fairly easy to apply.


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