INSECT ENEMIES OF WESTERN FORESTS 191 
Penetrating oils 
Recent experiments in California have developed some petroleum 
oils that, when sprayed on. the trunk of pine trees, will penetrate 
the bark and kill the pine beetle broods beneath. The most effective 
oils developed to date are those of the distillate grade carrying as 
much naphthalene as can be taken in solution. These when applied 
to infested ponderosa pine bark at the rate of about 4 ounces per 
square foot will give mortalities ranging from 50 to 90 percent. Oils 
of this type, however, are effective only when temperatures are above 
60° F. and cannot be used at all when the bark is wet. This method 
of control is still in the experimental stages, with the possibility that 
it may be useful in treating infested trees during the summer months 
on recreational areas or late in the spring and early in the fall when 
the fire hazard is too great to permit burning. 
The peeling method 
The peeling method can be used in the control of those bark beetles 
that, in the immature stages, work between the bark and the wood 
and die of exposure when the bark is removed, and is especially ap- 
plicable to moderately thick-barked trees that are easily peeled. It 
has been used extensively in the control of the Black Hills beetle in 
ponderosa pine of the Rocky Mountain region and in the control of 
the mountain pine beetle in western white pine. It has the impor- 
tant advantage of involving no immediate fire risk and is cheaper 
than the burning method for the treatment of isolated trees less than 
30 inches in diameter. If the bark tends to adhere to the wood, 
however, peeling is a very slow, tedious process and in the spring 
will not destroy overwintering adults, new adults, or pupae in the 
last stages of development. In general, it is more expensive than the 
burning method, especially for the treatment of trees in groups. 
Moreover, it leaves a mass of slash and crisscrossed logs in the woods 
that seriously increases the fire hazard. 
In carrying out this method, the infested trees are felled across logs 
or other felled trees to hold them off the ground, and then all of the 
infested bark is peeled with an ax or barking spud and allowed to 
drop to the ground where ants, rodents, and exposure dispose of the 
immature bark beetles (fig. 92). In some rare cases, where all of 
the infestation is within 20 feet of the ground, the barking has been 
done with long-handled barking spuds without felling the trees. In 
such cases, of course, the work can be done more cheaply than where 
felling the trees becomes necessary. 
Peeling and spreading bark 
A modification of the peeling method, involving the spreading 
of the bark where it will receive the direct rays of the sun, has been 
used with a fair degree of success in the control of the western pine 
beetle and is applicable to the treatment of ponderosa pines infested 
with broods of this species late in the spring or in the summer in 
places where burning would be dangerous. 
In this method the tree is felled across a log so as to keep a large 
part of the trunk off the ground, and the bark is peeled and spread 
