INSECT ENEMIES OF WESTERN FORESTS 231 



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 

 dispose 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 artificial 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. 



And lastly, it is most important that cooperation be had from 

 all owners in the infested area so that the control campaign can 

 cover all the contiguous infested territory in a single season. 

 Small tracts cannot be successfully cleaned up if neighboring or 

 intermingled tracts are left untreated. 



BARK-BEETLE CONTROL PROJECTS 



The objective of bark-beetle control is to destroy such a high 

 percentage of the destructive beetles that the aggressive character 

 of an outbreak will be broken and the remaining infestation will 

 be held in check by natural control factors. This involves the 

 treatment of as nearly all of the infested trees within the natural 

 boundaries of an affected area as is feasible within physical 

 limitations; and if migrations threaten from neighboring areas 

 these areas also should be included in the control campaign. 

 Beetle outbreaks, like forest fires, if not promptly taken in 

 hand, are soon apt to increase beyond the practical limitations 

 of artificial control measures. 



The Control Unit 



The feasibility of a control project will frequently depend 

 on the possibility of limiting the area to a unit that can be 

 covered in a single season with the available manpower. First 

 consideration must be given to the flight habits of the beetles, 

 and the control area should be made large enough to 

 reduce to a minimum the possibility of any large number of 

 beetles flying in from neighboring infested tracts. As far as 

 possible, control units should be bounded by natural barriers, 

 such as high ridges, open valleys, or broad strips of timber of a 

 different type. If these are lacking, then the control area must 

 include all infestation within the flight range of the beetles. This 

 range for practical purposes depends on the intensity of the 

 beetle population in any neighboring area. For instance, in cases 

 where a treated unit is surrounded by scattered infestations of 



