Effects of the Balsam Woolly Aphid upon Wood Anatomy of Some Western True Firs

Balsam woolly aphid ( Chermes piceae ) infestations on the main stem of North American Abies cause the trees to produce annual rings with abnormally large amounts of dark, reddish wood. Anatomical investigations of the xylem tissue of Abies grandis , A. lasiocarpa , and A. amabilis revealed that in abnormal wood the cells were circular rather than rectangular in cross section; secondary cell walls were abundantly marked with checks; the proportion of thick-walled, summerwood-like tissue was greatly increased; and there was an unusual proliferation of traumatic resin canals. Other distinguishing characteristics of the abnormal wood were: Cell walls in the springwood were about 50 percent thicker than normal; tracheids were some 40 percent shorter; fibril angle was two to three times greater; and the number of rays per unit area was nearly doubled. These effects of woolly aphid attack occurred in all three tree species and in about equal proportions.