The balsam woolly aphid, Adelges piceae, is the most destructive insect on fir in the coastal half of Maine. It is most injurious on wet water-logged areas of soil. Areas most favorable for attack are indicated by Sphagnum mosses, water grasses, gold-thread, and swamp dewberry in the depressions with mosses, such ferns as Dryopteris spinulosum and Osmunda, Maianthemum, Cornus canadensis and Lycopodium on the hummocks. Injury is of 2 types. The "twig phase" produces swollen and gouty tips and nodes, commonly arresting growth and killing the top or whole tree. The "trunk phase" attacks the stems and kills especially trees of merchantable size. Attack causes the production of 2-3 thick abnormal rings of ligniferous wood of greatly reduced commercial value, but the average growth of trees attacked was reduced from 26 to 49% before death. Two complete generations develop. Numbered trees on 16 quarter acre plots showed that when the infestation passes the medium stage, physiol. disturbances occur in the host and, unless checked, death occurred in an average of 3.15 yrs. Temps. below[long dash]15[degree]F may kill all above the snow line and permit formation of a thick hard bark which largely prohibits future attack. No parasites were found. A few syrphid fly larvae and larvae of the lycaenid butterfly Feniseca tarquinius were found for the first time attacking the host. Recommended sprays are dormant lime sulfur 1 to 19, nicotine sulfate 1 to 400 and suitable miscible oils.