INSECT PESTS OF OHIO SHADE AND FOREST TREES 197 



small number one is able to select a material suited to meet almost 

 any situation likely to arise. In the present publication, however, 

 those relating to insect control only will be considered. 



The manner in which an insect feeds determines to a large 

 extent the nature of the material used in controlling it. The biting 

 insects, those which chew and devour parts of the host, particularly 

 the surface portions, are best controlled with poison ; but where the 

 pest pierces through the surface of the host and takes its food in 

 the form of sap, the poisons are not effective and the contact sprays 

 must be employed ; sprays which in one way or another kill by rea- 

 son of contact with the animal body. Many pests upon which we 

 ordinarily use poisons might be controlled with fair success with 

 contact insecticidese if need be, but it is always advisable to use 

 the poisons whenever possible for the reason that when poisons 

 are used the pest may not be present when the work is done, but 

 coming later may partake of the sprayed part of the host and be 

 killed ; whereas if the pest is not present when the contact sprays 

 are used, it of course in coming later would not be affected by the 

 treatment. 



In the discussion concerning the various insecticides which fol- 

 lows, no attempt will be made to present an exhaustive discourse 

 on all materials which can be used for the purpose, but rather to 

 confine the remarks to a fairly complete treatment of the important 

 and essential few. 



THE POISON SPRATS 



Many poisons in many forms have been used in years past as 

 insecticides, the more popular being Paris green, London purple and 

 the arsenites of calcium and soda, but all on account of their varia- 

 bility and tendency to injure tender plants have lost caste for gen- 

 eral usage, and we now depend almost exclusively in shade tree work 

 upon a comparative newcomer, namely, arsenate of lead. Arsenate 

 of lead possesses four attributes which particularly qualify it for 

 use as a poison insecticide. These are its chemical stability, power 

 of remaining in suspension when dissolved, tendency to adhere to 

 foliage even after one or two rains, and lack of tendency to burn 

 even the most tender foliage. The only serious criticism to be 

 made against the material is that it is a slow acting poison. 



Arsenate of lead was a homemade product at first and was 

 prepared by combining chemically carbonate of soda and acetate of 

 lead, but within a short time it appeared on the market as a com- 

 mercial product in paste form and at present we have not only the 

 paste but a powdered form as well. 



