Naked Hemlock Trees
A few weeks ago I stopped at a roadside rest in Virginia near the Blue Ridge Mountains. I was admiring the trees when I noticed an Eastern Hemlock (Tsuga canadensis) tree that was nearly naked. On close inspection I could see that it was heavily infested with Hemlock Woolly Adelgid (Adelges tsugae). You can barely see the insect itself, but the white, waxy wool-like coat is clearly visible on the stems of the tree at the base of the needles.
This aphid-like insect was introduced to eastern North America from Japan in the early 1950s. It feeds on the sap stealing nutrients from the needles causing them to turn color and eventually fall off. The tree is slowly weakened until it is dead after four to six years. Over 80 percent of the Eastern Hemlock trees in the Shenandoah National Park in Virginia are now dead. These trees can normally live for up to 800 years.
The Hemlock Woolly Adelgid is parthenogenetic, which means that all individuals are female and reproduce asexually. They lay up to 75 eggs in the spring and 300 in the fall in sticky sacs called ovisacs. Dispersal and movement of hemlock woolly adelgid occurs mainly from the wind and by birds, deer, and other forest-dwelling mammals that come in contact with the sticky ovisacs and young larvae. Isolated and long-distance infestations are usually caused by people transporting infested nursery stock.
Right now it is found from coastal Maine to the southern borders of New Hampshire and Vermont and south to northern Georgia and as far west as eastern Kentucky and Tennessee. It has been slow to spread northward. In Massachusetts some hemlock forests have been infested for over a decade without killing the trees. Apparently severe winters limit the spread and abundance of the adelgid. Researchers have found that they will die if exposed to a mean winter temperature of 23 degrees F.
Nearly half of the northeastern United States had a mean winter temperature under 23o F, including upper-state New York and most of Vermont, New Hampshire and Maine. Climatologists predict that over the next century the 23oF mean winter temperature will move farther and farther northward and higher and higher in elevation. Under the current CO2 emmission scenario, by mid-century half of the area currently too cold for Hemlock Woolly Adelgid could become a suitable home for them. Under the lower emissions scenario, only 20% of the current CO2 emissions, isolated areas of northern New York, Vermont, New Hampshire and the northern half of Maine would remain cold enough to keep the Hemlock Woolly Adelgid at bay and the Hemlocks healthy.
Source: Annie Paradis, Joe Elkinton, Katharine Hayhoe and John Buonaccorsi. 2008. Role of winter temperature and climate change on the survival and future range expansion of the hemlock woolly adelgid (Adelges tsugae) in eastern North America. Mitigation and Adaptation Strategies for Global Change 13: 541-554.