344 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



fully hardened and mature the young beetle crawls into the mine 

 leading away from the pupal cell and completes this mine to the sur- 

 face of the tree or log. It then flies away. Mating and egg laying 

 soon follow to provide for another generation. 



SEASONAL HISTORY. 



Probably in the great majority of cases the larva does not change 

 to the pupa until the spring following the season in which the egg 

 ia laid, passing the winter either in the larval mine or in the pupal 

 cell. However, pupation may take place in the fall and the winter 

 be passed in this stage, or the adult stage may be reached in the fall 

 and the winter be passed in this form within the pupal cell. The 

 following spring the larva? which have wintered over transform to 

 pupae. The pupa? soon transform to adults and the adults emerge 

 and take flight. Likewise, the pupae which have wintered over trans- 

 form to adults and emerge. The first to emerge, however, are those 

 individuals which have wintered over as adults. Sometimes a spe- 

 cies may have two generations a year, or a partial second genera- 

 tion. In these cases development takes place rapidly after the eggs 

 are laid in the spring, the adult insects of the first generation emerg- 

 ing in late summer or fall, and laying eggs for the second generation. 

 The second generation passes the winter as outlined above. In still 

 other and more rare cases two or more years may be necessary for the 

 complete development of certain species. 



THE WESTERN LARCH BARK-BORER. 



(Tetropium velutinum Lee.) 



At the present time the western larch bark-borer is quite a serious 

 pest in the Glacier National Park in Montana. In the vicinity of 

 Lake McDonald about 10 per cent of the stand of western larch or 

 tamarack is being killed annually by this bark-borer. Besides larch 

 it attacks fir, Douglas fir, western hemlock, and pine, in the Rocky 

 Mountain and Pacific coast regions. 



The eggs are deposited in clusters under overlapping scales of bark 

 (fig. 19, a) and the minute larva? hatching therefrom proceed to the 

 inner bark, where they immediately commence their mines (fig. 19, &). 



The work of this borer in larch is confined to the bark, though in 

 some of the other host trees mentioned above it sometimes enters the 

 sapwood. The larval mine is irregular and winding in the inner 

 bark. The number of mines is so great as to completely girdle the 

 tree and cut off the sap, thereby causing the death of the tree. Often 

 almost the entire inner layer of bark, or cambium, is destroyed for 

 quite a considerable space upon the trunk (fig. 20, a). 



