THE PINE INVESTIGATION. 387 



Robbers. Under this head, there are a number of large and 

 small bark and wood boring grubs, which in extending their 

 broad and winding mines through the bark and wood rob or 

 deprive the Scolytids of their food supply and thus destroy 

 enormous numbers. 



-Parasitic diseases. There are doubtless numerous fungus 

 and bacterial diseases which attack the adults and young of 

 bark beetles, but only two or three have been identified. lf> as 

 was thought probable, the destructive bark beetle was destroy- 

 ed by diseases, we have in this class of enemies the most effi- 

 cient destroyer of Scolytids. 



Birds. Great numbers of bark beetles are destroyed by wood- 

 peckers; but so far as 1 have observed, very little ultimate good is 

 accomplished by them. During the great invasion of the de- 

 structiue bark beetle (D. frontalis), 1 found that while the wood- 

 peckers fed upon the adults, pupae and larvae of this species, 

 they usually confined their attention to a single imfested tree, 

 among hundreds of other trees that were likewise infested, and 

 while they would often strip the outer bark from the entire 

 trunk and larger branches, and destroy many thousands of the 

 insects, all that were thus destroyed would not be apparently 

 in the proportion of more than one to many thousands that 

 emerged from the other trees. 



Unfavorable climatic and other conditions. In this class 

 of enemies, or opposing forces, Scolytids are often de- 

 stroyed in great numbers, or are prevented from breeding and 

 increasing. 



With the predaceous and parasitic insects, diseases, birds, 

 and other enemies and unfavorable conditions all combined 

 against the Scolytids, it would appear that the destructive 

 species would be kept in complete subjugation. Yet, as has 

 been frequently shown, great outbreaks and invasions occur 

 from time to time. 



Insect and other friends of scolytids. Scolytids have 

 friends as well as enemies in insects, birds, and in man, to say 

 nothing of many favorable climatic or other conditions which 



