INSECT PESTS OF OHIO SHADE AND FOREST TREES 199 



respects, particularly in the property of remaining in suspension, 

 he would make no discrimination. In buying in quantity, however, 

 it is especially desirable to buy the powdered form by sample with 

 a guarantee that the order equal the quality of the sample lot. 



THE CONTACT INSECTICIDES 



The present-day contact insecticides are much more diversified 

 than the poison sprays, due no doubt in part to the fact that the 

 missions they fulfill are more varied. For example, the poisons are 

 few in number, are rarely used except when the plants are in foliage 

 and, generally speaking, the strength of the solution does not vary 

 greatly. But the contact insecticides necessary to fill the require- 

 ments are in greater number ; are used when the plant is both in and 

 out of foliage ; and in point of strength of mixture vary from those 

 suited for application to the most tender foliage to those for the 

 treatment of the most resistant scale insects on dormant hardy 

 trees. 



The most useful of these contact insecticides may be divided 

 into five groups, viz: the soap sprays, kerosene emulsion, nicotine 

 sulphate, sulphur sprays and soluble oils. Each group possesses a 

 distinct field of usefulness in city spraying; and while the subject 

 cannot be treated exhaustively within the limits of the present 

 publication, the matter will be discussed with sufficient detail to 

 meet the practical requirements of the work. 



Soap solutions. Solutions of soap are among the oldest of the 

 modern contact sprays. Their use is declining very rapidly at pres- 

 ent, due in part to the fact that other materials are more effective 

 and in part to the difficulty with which some soaps go into solution. 

 With most soaps the application of heat is necessary to dissolve 

 them, and this is frequently inconvenient and impracticable. How- 

 ever, because soap in some form or other is usually available, in 

 special cases, where the necessity is urgent and when other ma- 

 terials are not to be had, it may be used to good effect. 



Special insecticidal soaps are manufactured and may be fur- 

 nished by most dealers in spraying materials. For the most part 

 they have a fish-oil or whale-oil base and are quite commonly known 

 as whale-oil soaps. There are other forms of insecticidal soaps, 

 however, such as tobacco soap. If soap is to be used, these special 

 insecticidal soaps are to be preferred to ordinary hand or laundry 

 soaps, but the latter may be employed in an emergency. 



The amount of soap to be dissolved for each gallon of spray 

 depends upon the strength of the soap and upon the nature of the 



