INSECT ENEJIIES OF WESTERN FORESTS 



175 



CONTROL OF DEFOLIATING INSECTS 



Defoliating insects can be controlled through the use of chemical 

 poisons. Chemical control has been in use for many years, but until 

 recently the control of tree defoliators by such methods has been lim- 

 ited to orchard, park, and shade trees. However, the application of 

 these methods is gradually being extended to forest trees. In general, 

 two types of poisons are used in this work, (1) the stomach insecti- 

 cides and (2) contact insecticides. These may be applied in the form 

 of either dusts or liquid sprays (fig. 8-1). 



Figure 84. — High-pressure spraying outfit used along higliways of the Shoshone National 



Forest, Wyo. 



The stomach poisons are used against insects that chew their food, 

 such as the leaf -eating caterpillars, sawflies, and beetles, and when 

 applied directly to the foliage become effective against the insect 

 through its mouth and digestive tract. The most useful poisons for 

 this purpose are various arsenical compounds such as lead arsenate 

 and calcium arsenate. Lead arsenate and calcium arsenate are not 

 only toxic to the insects but contain comparatively little free arsenic 

 and hence are not harmful to the foliage of many trees wdien used 

 with discretion. Calcium arsenate is the cheaper of the two, but 

 lead arsenate is somewhat more effective and less injurious. A 

 spreader is usually added to the liquid sprays in order to make them 

 spread evenly over the surface of the foliage and to adhere well. 

 Fish-oil soap, linseed oil, glucose, resin stickers, and similar sub- 

 stances are frequently used for this purpose. 



The stomach-poison spray most frequently used for the control of 

 leaf -chewing defoliators consists of the following : 



Lead arsenate, paste (or lead arsenate, powder, 3 to 5 pounds) 5 to 6 pounds. 



Water to make 100 gallons. 



Fish-oil soap I14 toli^ pints. 



