INSECT DEPREDATIONS IN NORTH AMERICAN FORESTS. 87 



service, however, where but few trees are being killed or injured, 

 because their concentrated work on such trees may contribute toward 

 the prevention of an abnormal increase of the insects. They also 

 render some service as allies of the other beneficial factors which assist 

 in artificial control. It is evident, however, that where many hun- 

 dreds or thousands of trees are being killed the comparatively lim- 

 ited number of birds in any forest under the most favorable condi- 

 tions could have little or no beneficial effect. Therefore, while the 

 birds should be classed among the valuable friends of the forest, and 

 should be protected, it is plain that they can not, even with the utmost 

 protection, always be relied upon to protect the forest against destruc- 

 tive ravages of insects. 



We must remember, in this connection, that there are complicated 

 interrelations between birds, injurious insects, and beneficial insects 

 which do not necessarily always operate to the benefit of the forest. 

 In fact, it may sometimes be quite the reverse. Therefore, in order 

 to derive the greatest benefit from the conflict between the birds, the 

 insect enemies of the trees, and the insect friends of the trees, we 

 must utilize our knowledge of the factors which are contributing 

 toward the preservation of a balance, so that whenever the enemies 

 of the forest threaten to get beyond natural control we may enter 

 the field through artificial means and endeavor to force them back 

 to their normal defensive position. 



Beneficial Climatic Conditions. 



The benefits to be derived from climatic conditions which are det- 

 rimental or destructive to insect enemies of the forest, while some- 

 times very great, are necessarily unreliable, and thus can not be 

 depended upon to assist in artificial control. In fact, the very con- 

 dition which may contribute to the destruction of one depredator 

 may favor the multiplication of another. 



UTILIZATION or WASTE CAUSED BY INSECTS. 



When we come to consider the vast amount of standing timber in 

 the forests of the country which has been injured or killed by insects, 

 and will go to waste if it is not utilized within a limited period, we 

 realize that there are great possibilities in its utilization as a means 

 of preventing the reduction of future supplies of living healthy tim- 

 ber. It is all the more important that the insect-infested timber 

 should be utilized, because in so doing we can contribute more per- 

 haps than in any other way to the reduction of the insects to or below 

 their normal numbers, and thus provide against serious injury in the 

 future, as well as to the maintenance of control. 



