INSECTS AFFECTIXG PARK AND WOODLAND TREES 35 



purple or other arsenical poisons. The aim of such treatment is to cover 

 the plant so thoroughly with the substances used that it will be practically 

 impossible for the pest to feed without also consuming the deadly insecti- 

 cide. Experiments have shown more than once that caterpillars will not 

 feed on foliage sprayed with jjoison till forced to do so by hunger, while 

 those placed on untreated leaves, with all other conditions the same, mani- 

 fest no such hesitanc)-. Thus it is pretty safe to assume that insects will 

 not eat poisoned leaves unless obliged to do so or go hungry, and that only 

 the most thorough spraying will produce satisfactory results. Measures of 

 value against leaf devourers may not have the slightest effect on those spe- 

 cies which obtain their nourishment by sucking, through a slender beak, the 

 fluids from the underlying plant tissues. Particles of paris green or other 

 such poison lying on the surface of the plant will not affect them and as it 

 is practically impossible to so change the vegetable fluids that an insect will 

 not or can not feed upon theni and yet not damage the plant, we must 

 depend to a great extent on contact insecticides in fighting these pests. 

 The most satisfactory method of killing such insects as aphids or plant lice, 

 scale insects, etc. is by spra)-ing them with kerosene emulsion, whale oil 

 soap solution or similar substances, which operate either by closing or 

 choking the breathing pores or spiracles along the sides of the insect or 

 they may exert a paralyzing influence through these orifices. The applica- 

 tions must be made directly to the insects themselves and the effectiveness 

 of the treatment will be proportional to the number actually hit. In short, 

 when fighting leaf-devouring insects, aim to cover the portions of the plant 

 liable to attack with some arsenical poison, while in controlling sucking 

 forms, it is essential to throw a substance, which will kill by contact, on 

 the insects themselves. 



.The foregoing are general directions subject to many modifications. 



Some pests are very resistant to poisons and require large doses before suc- 



. cumbing, init as a rule the trouble is more apt to be lack of thoroughness in 



the treatment than weakness of the insecticide. Many biting insects, like 



leaf miners, twig, bark and wood borers operate in places where it is imprac- 



