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INSECT ENEMIES 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. 84). 
FIGURE 84.—High-pressure spraying outfit used along highways 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 when 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 hquid 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. 
WAY GN EGTE’ (EQ. Taian en ag RR POP SN a ce ea 100 gallons. 
JENISIE Gil Oe Oe Ses gy te a 14%, to1% pints. 
