18 



SOME INSECTS INJURIOUS TO FORESTS. 



depredations })y insects in adjoining forests, and the relation of time 

 of fellino; tind)er in re<2;ular loi2:^inp; operations to attack by Dendroc- 

 tonus and other bark and wood borin^; insects. 



Accordingly, investigations were begun by the writer on May 17, 

 1905, with headcpiarters at Centerville, Idaho, and continued until 

 October 10, 1905. 



DEATH OF THE PINE CAUSED BY THE WESTERN PINE- 

 DESTROYING BARKBEETLE. 



Observations by the writer served to confirm the conclusion of Mr. 

 Burke that the primary enemy was a barkbeetle identified by Doctor 

 Hopkins as the western pine-destroying barkbeetle (Dendroctonus 

 hrevicomis Lee). 



CHARACTER OF THE INSECT AND ITS WORK. 



The adult insect is a stout, brownish-winged beetle (fig. 7) from 

 one-eighth to three-sixteenths inch in length, which attacks the Jiving 



trees in swarms, and 

 burrows into the living 

 bark, through the inner 

 layer of which each 

 female excavates wind- 

 ing galleries (fig. 8 and 

 Pfs. II, III) in which to 

 deposit eggs. These 

 galleries serve to cut off 

 the natural movement 

 of the sap and com- 

 pletely girdle and kill 

 the tree. In the vicinity 

 of Centerville, Idaho, 

 the eggs, deposited dur- 

 ing June, July, or Au- 

 gust, in little niches in the sides of the galleries, hatch within 4 or 

 5 days into small whitish larva* (fig. 9), which mine at right angles 

 from tht^ primary gallery through the outer layers of the inner bark 

 until they have completed their growth, which requires from about 

 20 to 30 days. They then bore into the outer cork}^ bark (fig. 12, a) 

 where they excavate little cells in whicli to transform, first to the pupa 

 (fig. 10) and later to the adult. When the broods of the first genera- 

 tion have thus developed — in about 60 or 70 days — they bore out 

 through the l^ark and fly to other trees to repeat the process and con- 

 tinue their depredations. 



The presence of this destructive insect in a forest is indicated (1) })y 

 dead and dying trees scattered about or in clum])S or large patches. 

 (The dying ones, with fading yellowish and reddish foliage, are called 



Fig. 7. — The western pine-destroying barkbeetle {Dendroctonus 

 brevico7nis): o, adult female; f<, e, </, details of punetuation; e, 

 adult iiink'. Natural size at left (original). 



