Balsam woolly adelgid

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Balsam woolly adelgids are small wingless insects that infest and kill firs, especially Balsam Fir and Fraser Fir. They are an invasive species from Europe introduced to the United States around 1900.

Because they are not native, the Fraser fir has not evolved any type of defense against this predator. These insects typically lay about one hundred eggs and have three generations per year. The adelgid attacks the tree by feeding in fissures within the bark of trees larger than about four centimeters in diameter at breast height. As it feeds, it releases toxins contained within its saliva. These toxins reduce the sapwood conductance, which causes water stress and kills the trees.

The Great Smoky Mountains National Park, in Tennessee contains about 75% of all spruce-fir ecosystems. These ecosystems covered vast portions of the Southeast during the last ice age, when the climate was cool and moist. Since the retreat of the glaciers, this ecosystem has been confined to the higher elevations in the mountains of the Southeastern United States. The spruce-fir forest has evolved as an island, away from any other ecosystems of its type. This has presented valuable research and a unique habitat. These forests have a very dense canopy and a moist understory.

"Ghost" Fraser Firs killed by the Balsam woolly adelgid on Clingmans Dome, Great Smoky Mountain National Park

Since the invasion of the balsam woolly adelgid, discovered in 1957, Fraser fir mortality rates have been 90-99%. Although some areas are being regenerated by young firs, there is much change in understory composition, including invasion by both woody and herbaceous species. Red Spruce, the spruce component of the spruce fir ecosystem, has also been suffering declines. Some researchers attribute these declines to damage from wind, which is usually blocked by the firs. Balsam woolly adelgids have destroyed about 95% of the Fraser firs in the Great Smoky Mountains National Park, creating "ghost forests". One concern is that acid rain and the deterioration of the ozone, among other pollutants are contributing to a weak immune system for the Fraser firs and making them more susceptible to the balsam wooly adelgids.

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