320 INSECTS AFFECTING VEGETATION 



CONTROL OF BARKBEETLES WHICH KILL TREES. 



The barkbeetles which kill trees attack the bark on the trunk 

 and destroy the life of the tree by extending their burrows or galler- 

 ies in all directions through the inner living bark. The broods of 

 young grubs or larvaB develop within the inner bark, on which they 

 feed. Those of some species develop to the adult stage within the 

 inner bark and are exposed when the bark is removed, while those of 

 other species transform to the adults in the outer corky bark arid the 

 larvas are not exposed when the bark is removed. Some species have 

 two or more generations in a season or annually, while others have 

 but one, and in a few species it requires two years for a single gener- 

 ation to develop. 



The barkbeetles of the genus Dendroctonus represent the most 

 destructive enemies of the principal coniferous tree species of Amer- 

 ican forests, and at the same time are among the easiest to control. 

 The general requisites for success are embodied in the following 

 rules: 



(a) Give prompt attention to the first evidence of a destructive 

 outbreak, as indicated by an abnormal percentage of yellow or red 

 topped dying trees, and especially when such trees occur in groups of 

 ten or more or cover large areas; (6) secure authentic determination 

 of the particular species of insect responsible for the trouble; and (c) 

 take prompt action toward its control according to specific expert ad- 

 vice, published or otherwise, on the best method for the destruction 

 of the necessary 75 per cent or more of the insects in the infested 

 trees. Some of the methods to be adopted to meet the requirements 

 of various local conditions are as follows: 



(1) Utilize the infested timber and burn the slabs during the 

 period in which the broods of the destructive beetles are in the im- 

 mature stages or before the developed broods emerge from the 

 bark ; or 



(2) Fell the infested trees and remove the bark from the main 

 trunk and burn the bark if necessary;* or 



(3) Remove the infested bark from the standing timber and 

 burn the bark when necessary ;* or 



(4) Immerse the unbarked logs in ponds, lakes, or streams, 

 where the bark will remain soaked long enough to kill the insects; or 



(5) Remove the unbarked logs or products to a locality where 

 there are no trees liable to attack within a radius of 20 miles or more. 



MAINTAINING CONTROL OF BARKBEETLES. 



Future trouble of a serious nature from barkbeetles which kill 

 trees can be prevented within a given forest or area of greater or less 

 extent if an insect-control policy is adopted in connection with, or in- 

 dependent of, a fire-control policy by which groups of dying trees 

 will receive similar prompt attention as that required for the preven- 

 tion or control of forest fires. 



If the broods develop to adults in the outer bark, it must be burned ; if they 

 develop in the inner bark and are exposed when the bark is removed, burning is not 

 necessary. As a rule the burning of the tops to destroy the insects is not 

 necessary. 



