THE SPRUCE INVESTIGATION. 269 



many individuals are thus destroyed. The insects which have 

 died trom disease are usually covered or surrounded by a 

 snow white or yellowish mass which is the external growth 

 and spores of the fungus which caused their death. 



BIRDS AS FRIENDS OF THE SPRUCE. 



Woodpeckers and other birds doubtless destroy vast numbers 

 of insect enemies of the spruce, but at the same time they nec- 

 essarily destroy many of the predatory and parasitic insects, 

 yet I would recommend the encouragement and protection of 

 all insectivorous birds, since they are undoubtedly of very 

 great service in preserving a balance among the contending 

 species of plant and animal life, and at times may render most 

 valuable service in reducing the numbers of dangerous insect 

 enemies. 



OTHER BENEFICIAL ELEMENTS. 



Certain climatic conditions has much to do with the destruc- 

 tion of insect enemies of the spruce, far more indeed than is 

 generally supposed. But since it has possibly quite as much 

 influence in contributing to the multiplication and spread of 

 these enemies, and in bringing about devastating invasions of 

 the most destructive species, it would be difficult to say whether 

 more good or harm results from this source. Unless we assume 

 that without the favorable conditions offered for the devastat- 

 ing invasions, there would be no need of opposite conditions to 

 destroy them, when we must conclude that the harm resulting 

 from such influences is far greater than the benefits. After all, 

 such influence for good or harm is beyond the control of human 

 efforts and need only be considered in this connection to ex- 

 plain certain remarkable phenomena in the rapid multiplica- 

 tion and spread of the destructive pine bark beetle over thous- 

 ands of square miles and its sudden and complete extermin- 

 ation over the entire invaded area ; which subject is fully dis- 

 cussed under this head in the report on the pine investigation. 



To what extent the other insect enemies of the spruce are in- 

 fluenced by a series of warm winters and wet or dry summers, 



