CHAP. XI.] THE CONTROL OF INSECT PESTS Ol 97 



as the insect, whilst too weak a dose may have no effect on 

 either. What is required is a quantity strong enough to kill the 

 insect but not strong enough to harm the plant, and this must vary 

 according to the kind and age of both plant and insect. If a 

 particular strength of insecticide is found to fulfil these conditions, 

 there is no object in using a stronger dose; such will not kill the 

 insect any " deader," whilst the extra quantity used will represent 

 a waste of money and may harm the plant. But .1 super-lethal dose 

 is usually preferable on the whole to a sub-lethal one, as the extra 

 cost of labour in repeating the latter is generally greater than the 

 difference between the two; in other words, it does not pay to 

 apply an ineffective dosage, as both insecticide and labour are 

 wasted without result. In the following pages are given general 

 average quantities in which the various insecticides referred to 

 should be used and the minimum quantity stated should be tried .it 

 first on a small scale and only increased if found inefficacious. It 

 must be remembered also that different makes of insecticides may 

 varj considerably in purity. 



Secondly, an appropriate insecticide must be chosen with 

 regard not only to the insect but to the plant also. It would, for 

 example, be criminal folly to apply an arsenical poison to culinary 

 vegetables or fodder cholam just before these were intended 1 

 cut for food of man or beast. 



Thirdly, due regard must be paid to weather conditions. It is 

 useless, as a rule, to apply insecticides during periods of heavy 

 rain, as they are rapidly washed off by showers before any good is 

 accomplished. Nor is it usually a good thing to apply liquid 

 insecticides to leaves under the hot noonday sun ; such aie best 

 applied iii the earlj morning, which is also the besl time to apply 

 powders, whilst the dew is still on the leaves. Poisons tor night- 

 feeding inserts should be applied in the evening. Many insecti- 

 cides are decomposed by the action of sunlight and weather and 

 the use of some, such as sulphur, is based on this fact ; the applica- 

 tion of an insecticide cannot, therefore, be expected to protect .1 

 plant for more' than a limited period probably two or three weeks 

 at the most during dry weather and considerably less if rain 

 occurs. 



Stomach Poisons. 



Of this class of insecticides the arsenical preparations known 



Paris Green" and "London Purple 1 ' were first tried in India 



some twenty-five years ago, but their use has now been practically 



abandoned. I.e. id Arsenate was introduced ten and 



Lead Chromate of quite recent years and both are used extensively, 



7 



