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 aU 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 line 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 
