INSECT ENEMIES OF WESTERN FORESTS 185 



les in that a primary tunnel penetrates the sapwood, and at inter- 

 vals along this tunnel secondary tunnels branch horizontally, the 

 branches more or less following the annual rings. The tunnels are 

 of the compound type, in that larval cradles are constructed at 

 regular intervals, both above and below the primary and secondary 

 galleries. The species so far recorded in the West are as follows : 



Species of Gnathotrichus Hosts and distribution 



aciculatus Blkm Pines, Douglas-fir, and white fir. Rocky Moun- 

 tains and Southwest. 



alni Blkm Alder and poplar. Western Oregon and Wash- 

 ington. 



denticulatus Blkm Pines and white fir. Southwest. 



retusus Lee Pines, Douglas-fir, and hemlocks. Rarely in firs 



and spruce. Pacific coast and northern Rocky 

 Mountains. 



sulcatus Lee. (fig. 83) Spruce, hemlock, Douglas-fir, and true firs; 



sometimes also pines, redwood, cedar, and 

 other conifers. Western States. 



The ambrosia beetles of the genus Trypodendron are small, 

 stubby, dark-colored beetles, with a roundish prothorax and a 

 smooth, more or less shining body, often with lighter colored longi- 

 tudinal stripes. They attack the wood of many species of conifer- 

 ous and broadleaved trees and are distributed throughout the 

 greater part of the United States and Canada. Their galleries (fig. 

 84) are of the compound type with larval cradles arranged in 

 series both above and below the main tunnels, which branch in a 

 horizontal plane and cut across the grain of the wood. 



/ 



— l //ii(i4|f 



ImMaj 



Figure 84, — Typical tunnels and larval cradles made by the ambrosia beetle 

 Trypodendron borealis Sw. in the sapwood of western white pine. 



