INSECT ENEMIES OF WESTERN FORESTS 221 



have introduced beneficial insects been able to control destructive 

 epidemics completely without other assistance. 



Since the application of biological control measures often 

 presents almost insuperable difficulties, a thorough knowledge of 

 all associated insects is of vital importance, in order that the 

 more direct control methods may be applied so as to take full 

 advantage of any aid from parasites and predators. 



ARTIFICIAL OR REMEDIAL CONTROL 



When nature fails to keep an injurious insect in check, and 

 valuable forest crops are threatened with destruction or serious 

 injury, direct remedial or artificial control measures are called 

 for. These consist in such mechanical methods as the application 

 of insecticides, as in spraying, dusting, fumigating, or baiting; 

 felling or burning infested trees ; or the use of trap trees or solar 

 heat. In brief, these artificial control measures are employed to 

 destroy the harmful insects directly in one way or another. 



The control measures applicable to the control of cone and seed 

 insects, root-feeding nursery pests, terminal or twig insects, and 

 insects injurious to forest products have already been considered 

 in connection with the discussion of these special groups. The 

 control of defoliating insects and bark beetles affecting mature 

 forest trees has been given special attention in western forests, 

 and, since specialized methods have been developed, these will 

 be given detailed consideration. 



CONTROL OF DEFOLIATING INSECTS 



Defoliating insects can be controlled with chemical poisons. 

 Application of insecticides to forest trees over extensive areas 

 is often impractical with ground equipment. It did not become 

 feasible until airplane dusting and spraying were developed. 

 This method of application, together with DDT and the other 

 new potent insecticides developed during World War II, has so 

 revolutionized the control of forest defoliators that only brief 

 mention can be made of some of the older methods. 



MATERIALS 



Insecticides commonly have been divided into two groups de- 

 pending on how they reached the insects: (1) Stomach poisons, 

 and (2) contact poisons. To these now must be added a third 

 group which act both as stomach poisons and contact insecticides. 



The most useful stomach poisons are various arsenical com- 

 pounds 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 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 substances are frequently 

 used for this purpose (lsl). 



