THE GENUS DENDROCTONUS. 27 



and mines, and kill their victims by inserting their beaks into their 

 bodies. 



The so-called robbers (see fig. 30) consist of large bark-boring 

 grubs or larvae of long-horned beetles, which sometimes rob the 

 barkbeetle larvae of their food supply or kill them outright, by 

 destrovins the inner bark before the broods of barkbeetles have 

 completed their development. These, however, do not occur so 

 commonly with the more destructive barkbeetles as with those 

 which, like the bark-boring grubs, are in the bark as the result, and 

 not the cause, of the dying condition of the tree. 



While some of the Dendroctonus beetles have numerous insect 

 enemies, others have comparatively few. Some of the smaller species, 

 like the southern pine beetle, which often occupy the thin bark on the 

 upper portion of the trunk and branches of the larger trees, and 

 sometimes on young trees, have many parasitic enemies, while others 

 of the small species, as 1, 2, and 5, and the larger species, such as the 

 Black Hills beetle and the Douglas fir beetle, which usually occupy 

 the thick bark, have none at all, or very few. 



So far as determined, the southern pine beetle has 11 parasitic and 

 about an equal number of predatory enemies; the eastern spruce 

 beetle has 5 parasitic and 4 predatory enemies, and the eastern larch 

 beetle 6 parasitic and 2 predatory enemies. Of the western species 

 the mountain pine beetle is the only one on which a parasite has been 

 found, but there are four or five predators common to all, which evi- 

 dently exert quite an important influence in protecting the forests of 

 some sections. With a little assistance on the part of the owner of 

 the forest, this class of beneficial insects will exert a much more power- 

 ful influence in preserving a desirable balance among the contending 

 forces, and thus prevent destructive outbreaks of the beetles. This 

 balanced condition appears to prevail at the present time within the 

 range of the southern pine beetle, and with proper attention to local 

 outbreaks of the beetles it could be maintained. However, this whole 

 subject of parasites and predatory enemies of forest insects and their 

 economic relations is one which has not as yet received the attention 

 it deserves. Mr. Fiske gave the matter considerable attention during 

 his field work in forest insect investigations, but his detail to another 

 branch of the Bureau prevented him from continuing it. 



BIRDS. 



Wherever the Dendroctonus beetles have been found in standing 

 timber the work of woodpeckers has been more or less common, and in 

 some trees quite a large percentage of the beetle broods has been 

 destroyed by the birds. The evidence gathered in Maine a few years 



o Family Cerambycidse. 



