INSECT ENEMIES OF WESTERN FORESTS 195 
beetle control, reductions of more than 50 percent are rarely obtained 
in one season, and the work has to be repeated for several seasons 
until the resistance of the stand has improved through removal of 
beetle-susceptible trees or until the trees are better able to resist 
attacks. In fact, during long periods of drought and lowered tree 
resistance, almost continuous work may be necessary to hold the 
beetle population down to reasonable limits. 
In this work special consideration must be given to the natural- 
control factors and an effort made to favor their effectiveness while 
reducing the population of the destructive species. The avoidance 
of burning around stumps where predators congregate, the saving 
of certain infested trees to permit multiplication of the beneficial 
parasitic insects present, and the improvement of stand resistance 
are some of the ways in which natural control may be encouraged 
(pp. 169, 172, 173). 
WHEN CONTROL MEASURES ARE ADVISABLE 
When the natural balance in a forest is disturbed and an outbreak 
of bark beetles threatens to destroy a large number of valuable trees, 
the application of direct control measures (27) is advisable provided 
effective methods are available and the value of the timber that can 
be protected will justify the expense of the work. Control measures 
are expensive and unless the timber is valuable enough from the 
commercial, watershed-protection, or aesthetic standpoints to war- 
rant the cost of control measures, it is best to allow Nature to bring 
the epidemic under control in her own way. 
To reach a decision certain data must be obtained. In the first 
place, the primary agency responsible for the death of the trees 
should be determined. If trees are dying because of drought, fire 
injury, flooding, or other causes, there is obviously little use to dis- 
pose of the insects, which may be only the final cause of their death. 
Nor is it wise to attempt to exterminate native bark beetles present on 
an area under normal conditions. Only when the natural-control 
factors have been disturbed and an outbreak threatens should arti- 
ficial measures be taken. 
In the second place, it must be determined whether effective and 
economical control measures are available. In some cases, because 
of the habits of the beetles, no satisfactory methods of control have 
been devised. For instance, the control of the white fir engraver 
beetles through burning the bark of dying trees is of little value 
since a large number of these insects may continue to breed in 
perfectly green trees, causing only local damage. Moreover, the 
control measures must not be so expensive as to exceed the value of 
timber that might be saved. 3 
And lastly, it 1s most important that cooperation be secured from 
all owners in the infested area so that the control campaign can 
cover all of the contiguous infested territory in a single season. 
Small tracts cannot be successfully cleaned up if neighboring or 
intermingled tracts are left untreated. 
ConTROL COSTS AND PROBABLE RESULTS 
The cost of control work varies with the size and type of timber, 
the method of treatment, the intensity of the infestation, the rough- 
