The effective control of injurious insects demands a knowledge of the habit of the insects themselves, so that they may be attacked at the most vulnerable points.
Insects differ greatly in their habits and life history, that is why methods of control may differ greatly not only with regard to individual insects but with regard to crops that are attacked by them.
Insects that live mostly in the ground and injure the roots of plants must be controlled in a different way from those that feed on foliage.
Insects attacking the cereal crops cannot be treated in the same way as those attacking the fruit and leaves of garden or orchard crops.
Injuries to vegetable and other crops may be prevented by various cultural preventive measures, such as clean farming and crop rotation. Cultural practice can be relied on in the case of some crops but not of all of them. With most crops artificial or remedial methods must be practised, that is, the application of materials that either poison or kill insects by contact with their tissues.
These substances are known to be insecticides. Artificial methods may be classified as follows: 1) the application of poisons against biting insects; 2) the application of contact substances against sucking insects; 3) the use of poison gases.
When applying insecticides three factors must be taken into consideration: effectiveness against insects, cheapness of the material, and its application and harmlessness to insect hosts. Insecticides may be used in the form of sprays and dusts. Application of the insecticides in the form of a spray requires thoroughness of spraying. Dusting is often practised too. In some cases the results of the dust methods are very satisfactory and they give the possibility to protect orchards and fields at critical times. A large orchard can be treated in about one fifth of the time required by liquid spraying.
Insecticides are usually divided into three classes: internal poisons and those that kill by contact and fumigants.
Internal poisons are used against chewing insects and kill by being taken into the digestive tract.
The contact insecticides are not eaten but applied directly to the insect body and produce death in various ways: either by suffocation, by corrosive action, or by fumes that penetrate the breathing pores of the insects. Until recently the modes of action of insecticides was not well understood. It was supposed that contact substances kill insects by stopping the breathing pores or plugging the tracheae, producing death by suffocation. Recent investigations show that insects are not readily suffocated. The volatile portions of kerosene, carbon, bisulphides, creolin, etc. are effective long before the liquid are absorbed by their tissues.
With the penetration of the volatile substances the nervous system is seriously affected, results resembling narcosis are produced and a disturbance of the respiratory activity occurs. Insect tissues soon become saturated when exposed to the vapours of the substances and death results through the inability of the tissue to absorb oxygen in the presence of these vapours.
The most effective poisons for the destruction of biting insects are: Paris green, arsenite of lime, sodium fluoride.
The main substances used as contact insecticides against sucking insects are: lime sulphur wash, soap, kerosene emulsion, tobacco decoction, wiscible oils, pyretrum, lime dust, carbolic acid, emulsion DDT, benzine gexachloride and others.
The main poisonous gases are: carbon bisulphide, hydrocyanic acid gas, sulphur dioxide fumes, tobacco, formalin, carbon tetrachlorid and others.