INSECT ENEMIES OF WESTERN FORESTS 193 



They are completely limbed and the brush cleared away so that the 

 logs will receive direct sunlight. After a few day's exposure with 

 air temperatures of 80° F. or more all of the bark beetles on the 

 top half of the logs will have been killed. Then the men return and 

 with peavies turn the logs completely over so that the other side* 

 will be exposed. 



This method has been very effectively used for several years in 

 control of the mountain pine beetle in Crater Lake National Park. 

 It has the advantage of being much cheaper than either the peeling 

 or burning methods; and in crowded stands it avoids the scorching 

 of adjacent trees, and thus does not set up influences attractive to the 

 beetles w^hich w^ould favor reinfestation, as so often happens when 

 the logs are burned. The disadvantages are that considerable slash 

 is left in the woods, and the method cannot be used in the shade 

 of dense stands, on cold north slopes, or in localities where air tem- 

 peratures during the control season are less than 80° F. 



Submerging the infested logs 



Many years ago A. D. Hopkins advocated the submerging of in- 

 fested logs as a means of destroying bark beetles where the infested 

 trees could be cut and placed in a mill pond. E-ecent experiments 

 have shown that infested logs must be submerged for at least 6 

 weeks in order to destroy the broods of the w^estern pine beetle. In 

 any shorter period than this the beetle's development is simply 

 retarded. Also, the beetles in the portion of the logs not covered by 

 water are unhampered in their development and are free to emerge 

 and escape. So far this method has been tried only in an experi- 

 mental way, but it has possibilities, where applicable, and some- 

 time may be of value in connection with a control project. 



The trap-tree method 



A method of bark-beetle control that has been used in Europe 

 with apparent success consists in felling injured, weakened, or sup- 

 pressed, noncommercial trees in accessible locations as attractive 

 baits for bark-beetle broods, and then destroying them after the 

 beetles have entered the bark. The method has been tried on numer- 

 ous projects and on rather an extensive scale in California and 

 southern Oregon in the control of the western pine beetle, but with 

 little success. Although beetles are attracted to the traps, these 

 fail to protect the standing trees in the vicinity, and frequently 

 the trap tree acts as a source of attraction to bring in bark beetles 

 that kill groups of adjacent standing trees. Moreover, the trap 

 trees have always failed to absorb any large proportion of the beetles 

 in their area, and hence the method has lost favor as an effective or 

 economical control measure. It may, however, prove of value in the 

 control of other species, particularly in cases where the trap logs 

 can be removed and utilized. 



Barh-heetle control hy logging 



Where the infestations are in accessible tracts of valuable timber, 

 the cutting and salvage of the infested trees through logging opera- 

 tions is an effective and economical method of control. This method 



130643° — 39 13 



