76 



INSECTICIDES AND SPEAYlNG. 



arsenic. If there is any value in the use of insecticides in India, their 

 general adoption will be a matter of slow growth that must first be 

 worked out on the experiment farms. 



Insecticides are insect poisons and act in two ways. There are those 

 poisons which are placed upon the food of the insect and which act upon 

 its stomach, just as medicines and poison do upon human beings. These 

 are called stomach poisons and are meant only for internal application. 



FIG. 93. 



Sand Sprayer. 



If caterpillars are destroying the leaves of a valuable plant and we can 

 put poison on these leaves, the caterpillar eats the poison with the leaves 

 and dies. It is only necessary to put such poison on the leaves of crops, 

 and they are safe from all caterpillars, grasshoppers and other insects 

 which eat the leaves. There are also many insects which do not eat the 

 leaf, but which suck out the juice ; these feed upon the sap of the plant, 

 not on the leaf, and any poison on the leaf never reaches their stomachs. 

 For these we cannot use a stomach poison as we cannot poison the sap 

 of the plant. In such cases we must use poisons which kill when 

 the insects are wetted with them. These poisons are known as contact 

 poisons, since they work only when in contact with the skin of the insect. 

 If a colony of plant lice is sucking the juice of a cotton-plant, we cannot 

 poison the juice, so we throw contact poison on the insects ; all are killed, 

 and if the contact poison is properly made the plant is uninjured. Both 

 kinds of poisons have their uses ; we can poison any insect, even a locust, 

 with contact poison if we use it strong enough, but it is better always to 

 use a stomach poison for a biting insect, such as a locust ; whereas for 

 sucking insects we can never use a stomach poison and must always use a 



