34 THE SCOLYTID BEETLES. 



fully attracted to trap trees and that it actually prefers to attack 

 healthy living trees. 



The Mountain pine beetle is attracted to a greater or less extent 

 to felled and fire-scorched trees, but will at the same time attack 

 near-by living ones, as will all of the other species. It is therefore 

 under exceptionally favorable conditions only that this method would 

 be sufficiently successful to warrant its adoption as a means of con- 

 trolling this class of beetles. These exceptions would be as a rule 

 on a very limited scale, as is referred to under the special discussion 

 of some of the species. 



One of the objections to the trap-tree method of combating the 

 Dendroctonus beetles is in the fact that a few living trees deadened 

 or felled in the midst of a healthy forest where the destructive 

 species are present may, as has often been demonstrated, not only 

 attract the beetles to the trap trees but to the near-by healthy trees, 

 thus inducing instead of preventing a destructive outbreak. 



INTRODUCTION AND PROTECTION OF NATURAL ENEMIES. 



The introduction and protection of natural enemies of these bark- 

 beetles is a subject of special interest and one that should receive 

 attention in the future, especially in the fine of investigations and 

 experiments to determine facts on which to base reliable conclusions. 

 In the case of a destructive insect which has been introduced from 

 another country it is plain that if its natural enemies did not come 

 with it they should be introduced; but with native insects it is quite 

 a different proposition and will require much detailed investigation 

 before the results from transfers and introductions can be predicted 

 with any degree of certainty. The protection of natural enemies 

 already present is, however, worthy of special consideration. If, for 

 example, certain parasites and predatory insects are abundant in the 

 bark of the infested trees and certain methods of procedure are 

 adopted for combating the destructive beetles which will at the same 

 time allow the beneficial insects to escape, it will naturally operate 

 against the enemy. But if, on the other hand, the beneficial insects 

 are destroyed along with the destructive ones it may have the oppo- 

 site effect. 



The parasites usually attack the broods beneath the thinner bark, 

 like that toward the top and on the larger branches of the large trees 

 or the trunks of the small ones. Therefore, whenever the parasites 

 are common, it will be best simply to remove the infested thicker 

 bark and leave the thinner bark for the parasites. 



Burning the infested bark on the trees or immediately after it is 

 removed will destroy the beneficial insects with the injurious ones, 

 but if the bark be removed in the early fall or early spring and left 

 for several days before burning (if burning is necessary), many of 



