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 Adyisabole 



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 (:^7) 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 w^hether 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 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- 



