HORTICULTURAL DIVISION 



699 



to lead not only to a weakened tree or group of trees and to insect attack, 

 but not infrequently to confusion in diagnosing the cause of the trouble 

 and loss of money in attempts at remedying the situation. 



There are several different types of insects that are injurious to park 

 trees, among which are bark beetles, borers, aphids, scale insects and 

 defoliators. 



BARK BEETLES 



PINE, SPRUCE AND FIR BARK BEETLES (Dendroctonus and Ips). 

 Throughout the United States immense numbers of coniferous trees are 

 killed annually by bark beetles. The insects 

 themselves are small, black to brown, cylin- 

 drical beetles, and their destructive work con- 

 sists of mines or galleries between the bark 

 and the wood that girdle the trees and kill 

 them. The young grubs or larvae of the bark 

 beetles live and grow on the inner bark. Evi- 

 dence of attack consists of (i) fading foliage, 

 (2) pitch tubes or resin tubes on the trunk of 

 the tree, and (3) reddish boring dust on the 

 bark and at the base of the tree. The newly 

 grown adult beetles emerge from infested trees 

 and fly to attack others. Evidence of their 

 eaving is found in great numbers of "shot 

 holes" in the bark of the trunk and limbs. 



THE HICKORY BARK BEETLE (Scolytus 

 quadrispinosus}. Hickories are frequently in- 

 fested and killed by the hickory bark beetle. 

 This insect is a small dark brown to blackish beetle about the size 

 of a rice grain. Its work is similar in general to that of the foregoing 

 species. 



THE TWO-LINED CHESTNUT AND OAK BORER, AND THE BRONZE BIRCH 

 BORER (Agrilus spp.}. Chestnut, oak, birch and other trees are often 

 attacked and sometimes killed by flatheaded bark borers. Flat winding 

 mines between the bark and the wood of the trees are the principal positive 

 evidence of attack. 



Control. Bark beetle control is a complex problem and a number of 

 factors enter into it. 



i. Many bark beetles prefer weakened trees, so that the maintenance 

 or restoration of vigor is frequently of prime importance. Proper pruning, 

 thinning, transplanting, care of wounds, fertilizing, maintaining water 

 supply and protection of ground conditions should be carefully planned for 



PLATE No. 255 



THE MOUNTAIN PINE BEETLE 



Photograph by United States Bureau of 



Entomology. 



