6 FRUIT TREES AND THEIR ENEMIES 



caustic substances, such as caustic soda or caustic 

 lime, but the destructive action may often be en- 

 hanced by the addition of some other substances, 

 notably paraffin oil. 



For summer treatment, a much weaker insecticide 

 must be used, or the foliage of the tree will be injured. 

 With moths — the damage being done by the insect 

 when in the caterpillar or larval stage — the method of 

 attack is generally based on poisoning its food, rather 

 than on destroying its body by corrosive substances. 

 This applies only, however, to caterpillars and false 

 caterpillars which feed by biting the leaves ; when we 

 have to deal with sucking insects, such as the aphis 

 (green-fly or black-fly), poisons are ineffective, for the 

 insect punctures the leaves and extracts the juices, 

 and these juices cannot be poisoned. In that case the 

 insecticide must be one which acts externally on the 

 body of the insect, either by corroding it, or by 

 stopping up the breathing apertures in its body. 



It will readily be seen from this how essential it is 

 to have a knowledge of the life-history of the insect. 

 But more than this is required ; we ought to know the 

 exact action which the various insecticides have on it 

 at various times in its life-history, and in many, it may 

 be said in most cases, our knowledge on this point is 

 very deficient, and can only be increased gradually 

 by very extended and laborious investigations. Every 

 individual case has its own complications ; thus the 

 larval stage of an insect is generally divided into 

 various sub-stages, the grub moulting (often five times) 

 and changing its nature, and in some of these stages 

 it may be more sensitive to poisons, or less protected 



