EVERGREENS OF COLORADO 65 



PRUNING. 



It is seldom necessary to prune carefully dug nursery grown ever- 

 greens when they are transplanted, unless it is desired to change their 

 shape and the same is true of those taken from the mountains when a 

 ball of earth has been taken with the roots. A partial thinning of the 

 branches of forest grown trees may sometimes be desirable to reduce the 

 loss of moisture from the leaves and compensate in part for the reduced 

 root system. The terminal bud, which gives rise to the leader or main 

 stem, sometimes becomes injured or destroyed, and in such a case two 

 or three of the lower buds may push up equally strong shoots. In this 

 case all but one shoot should be removed and this one may be tied erect 

 to the stub of the original leader so as to eventually take its place. 



INSECT PESTS AND DISEASES OP EVERGREEN TREES. 

 The trees of the pine family are subject to numerous disorders due 

 principally to insects and to parasitic plants. Insects of certain species 

 often cause enormous destruction of living timber and the trees thus 

 killed are apt to be consumed by fires when dry. Probably the most de- 

 structive of the insects which attack the evergreen forest trees in Colo- 

 rado are those known as the pine bark beetles, which belong to the genus 

 Dendroctonus. The most common of these beetles is one which attacks 

 principally the yellow pine, although the lodgepole pine, limber pine and 

 Englemann spruce are also subject to it. This insect deposits its eggs 

 in burrows which it makes in the bark of the host trees in such a way 

 that the larvae which hatch from them are enabled to live upon the inner 

 bark. In this stage of its life history, the insect forms burrows through 

 the living bark, often in such numbers and extent as to entirely girdle 

 the tree and thus cut off the circulation of food material in the vital part 

 of the trunk. Trees which are badly infested usually show signs of the 

 trouble by the dying of the tops, while exudations of resin often appear 

 from the bark where the insects are at work. In case of slight injury, 

 which is sometimes limited to any small part of the trunk, the tree may 

 recover from the attack, but will eventually lose the bark over the af- 

 fected part of the tree. When the foliage of a tree turns yellowish or 

 brown in color, however, it is an indication that the attack is too severe 

 for the tree to recover. The most effective remedy on a large scale has 

 been found to consist in the cutting and burning of infested trees. Some- 

 times merely the bark is removed, which exposes the larvae of the in- 

 sect so that birds can get at them, or the bark is burned and thus all of 

 the insects are destroyed. 



During the latter half of summer, the blue spruces often show dead 

 tips to the twigs which, upon close examination, appear in the form of 

 galls or swellings thickly covered with short needles. These dead tips, 

 which are sometimes mistaken for the real cones of the tree, consist of 

 the season's growth which has lengthened but little, while the needles 

 which they bear have become greatly enlarged at the base and each con- 

 tains a small cavity. Earlier in the season these swollen leaf bases are 



