Stressors of Spruce-Fir Forests
Balsam Woolly Adelgid
Introduced into Brunswick, Maine, in 1908, the balsam woolly adelgid first appeared in the southern Appalachian Mountains in the 1950's. The pest has also found its way into the Pacific Northwest. In the eastern United States, the adelgid feeds on balsam and Fraser firs.
Vast stands of Fraser and balsam fir have been killed throughout much of these species' range in the East. Since fir primarily is found in high-altitude scenic areas of the South, most of the impact by the pest is on aesthetics and recreation.
The complex and interesting life history of the balsam woolly adelgid is simplified somewhat in the following description. Full-grown adelgids are microscopic, but their white, waxy, threadlike covering makes them appear as dots of "wool."
In the late spring and early summer, a group of yellow-brown eggs is deposited and attached to the bark behind the female's body by a minute thread. These eggs hatch into mobile crawlers that actively search out suitable feeding places. Because crawlers are so small, dispersal is further aided by wind, although only a small percentage of them find their way to other trees.
Once a crawler finds a suitable feeding site, it inserts a stylet through which it extracts juices from its host. The stylet is several times longer than the crawler's body. Soon after beginning to feed, the insects excrete waxy "ribbons" from their backs and sides. Eventually, they are completely engulfed in this protective woolly mass. By this time, the adelgids are immobile, a condition which lasts for life.
Interestingly, there are no known male balsam woolly adelgids in North America. All reproduction is parthenogenic, that is, achieved without fertilization by males. Adelgids produce two, and occasionally three, generations annually in the South.
Although not a direct cause of tree mortality, feeding by the balsam woolly adelgid causes gout, a syndrome characterized by enlarged nodes and buds. When feeding begins, the adelgid injects a saliva into the tree to facilitate uptake of materials. This substance is toxic to cell tissues. A form of wood resembling compression wood forms at the attack sites. Called "rotholtz," this type of wood forms greatly thickened cell walls that interfere with translocation of water and nutrients, thus starving the tree.
The BWA attacks Fraser fir on any wood surface. Twigs, branches, and the bole are all suitable feeding sites. Trees of all ages are attacked, but damage is usually minimal until the tree reaches maturity in about 30 years. Decline after about three decades is far short of the tree's normal life span of about 150 years. Fir usually have an opportunity to produce one or two seed crops before succumbing to severe decline and ultimately mortality. This reproduction ensures the maintenance of the fir component in the forest type but greatly reduces the age structure.
Typically, trees with advanced infestation exhibit top kill. Eventually, needles turn red and fall off. Stands killed by the balsam woolly adelgid are characterized by trees in various stages of decline, including snags and spiked-top individuals adjacent to more recently infested red-needled trees. Figure 23 shows snag stems of firs killed by the balsam woolly adelgid standing among healthy trees.
As the balsam woolly adelgid continues to spread throughout southern spruce-fir stands, older trees will be increasingly replaced by younger individuals. Red spruce, a species immune to attack from the BWA, will make up a greater part of the spruce-fir type.
In the past four decades, the balsam woolly adelgid has spread to every fir stand in the southern Appalachian Mountains. A reasonably accurate distribution map of the insect can be produced by simply plotting the location of its host. Figure 24 was produced from Forest Inventory and Analysis data on spruce-fir distribution.