INSECT ENEMIES OF WESTERN FORESTS 241 



attractive to the beetles, which would 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 temperatures during the control 

 season are less than 80° F. 



Submerging the Infested Logs 



Many years ago A. D. Hopkins advocated submerging infested 

 logs as a means of destroying bark beetles where the infested 

 trees could be cut and placed in a mill pond. Experiments have 

 shown that infested logs must be submerged at least 6 weeks, to 

 destroy the broods of the western pine beetle. In any shorter 

 period the beetle's development is simply retarded. Also, the 

 beetles in the portion of the logs not covered by water are free 

 to emerge and escape. This method has been tried only in an 

 experimental way, but it has possibilities where applicable. 



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 

 suppressed noncommercial trees in accessible locations as attrac- 

 tive bait for bark-beetle broods, and then destroying them after 

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

 numerous 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, 

 they fail to protect the standing trees in the vicinity, and fre- 

 quently 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 where 

 the trap logs can be removed and utilized. 



Penetrating-Oil Sprays. 



Experiments started in California in 1932 have led to the 

 development of a method of control using petroleum oils carrying 

 a toxic agent that, when sprayed on the trunk of infested trees, 

 will penetrate the bark and kill the beetle broods beneath {HO). 

 Oils of the distillate grade carrying as much naphthalene as could 

 be taken into solution were first used. Then a 6-to-l mixture of oil 

 and orthodichlorobenzene was found even more penetrating and 

 effective. Since World War II DDT, benzene hexachloride, and 

 chlordane have been substituted as toxic agents with good results. 

 These materials are applied at the rate of about 4 fluid ounces per 

 square foot of bark surface, or until the spray begins to drip. 

 They are most effective when bark is dry and when air tempera- 



