120 THE SCOLYTID BEETLES. 



When the eggs hatch, the minute white grubs or larvae eat their 

 way into the soft inner bark, which by this time has commenced to 

 die and is in the best condition for. their food supply. When the eggs 

 are deposited in separate cavities, each larva makes a separate mine, 

 but when they are massed along the sides, or placed close together, as 

 they usually are, the larvae work side by side and consume all of the 

 inner layer of bark until they have progressed some distance, when 

 they begin to separate and each larva makes an independent mine. 

 While the individual burrow may cross and recross its neighbor, it 

 preserves a course of its own and increases in width as its occupant 

 increases in size, until the larva ceases to feed. The latter then 

 excavates a cavity, either in the bark next to the wood or next to the 

 outer dry bark, where in due time it changes to the pupa. Here it 

 remains in a semidormant condition until the legs, wing covers, and 

 other parts develop. It then sheds its outer skin and becomes an 

 adult winged beetle, soft and yellow at first, but gradually hardening 

 and becoming darker. 



When the individuals of a brood are fully matured in the spring, 

 they bore through the inner bark between the transformation cells 

 and congregate in the larger common chamber thus formed until it 

 is time for them to emerge. They then bore their way out to the 

 surface and emerge to fly in search of suitable trees in which to exca- 

 vate galleries and deposit eggs. Scarcely anything is known of the 

 flight habits, but the beetles probably fly during the evening or at 

 night. 



ECONOMIC FEATURES. 



CHARACTER OF ATTACK AND INJURY. 



So far as known, this beetle confines its attack to the spruce in the 

 region north of latitude 43°. Toward its southern limit it evidently 

 does not occur at altitudes below 2,000 or 2,500 feet, while at latitude 

 47° in Maine and in extreme northern Michigan it would be found below 

 an altitude of 600 feet. It is rarely found in trees under 10 inches 

 in diameter breast high, but prefers those 18 inches or more in 

 diameter. Whenever it occurs in sufficient numbers to attack and 

 kill large numbers of trees the flying beetles within a given locality 

 usually concentrate on groups or patches of timber of greater or less 

 extent. The trees die from the attack, and when the new broods 

 develop they emerge from the dying trees and settle on the living 

 timber in another locality, and so on, until all of the matured timber 

 is killed. They usually settle on a tree and enter the bark in such 

 numbers that there is little chance for its recovery. Their numerous 

 egg galleries are extended through the most vital part of the tree 

 (the cambium), where the new growth of wood and bark takes place. 

 Thus the injury produces a weakened condition of the tree. This is 



