Unwelcome Christmas Guests

At first glance, this looks like a Christmas tree with an unusual holiday decoration. But this eastern hemlock does not make a good holiday tree since it tends to lose its needles soon after it is cut. And the fluffy white balls are not ornaments but the hemlock woolly adelgid, an invasive species quickly leading to the decline of hemlocks throughout the eastern US and especially here in the southern Appalachians.

The hemlock woolly adelgid belongs to an insect family closely related to aphids. They can be easily recognized by the waxy coating they secrete, appearing like cotton balls. They insert their straw-like mouthparts into the base of the hemlock needles, feeding on starches stored there. This ultimately starves the tree, leading to its death from 3 to 15 years after the initial attack depending on the health of the trees.

These pests were unintentionally introduced from their native Japan and here in Tennessee they are attacking both eastern and Carolina hemlocks. They have spread into around 90 percent of their geographic range and in many areas 80 percent or more of the hemlocks have died. They have been especially devastating in the nearby Smoky Mountains, which has more old growth hemlock than any other national park.

For reasons that aren’t always clear, some trees seem to coexist with the adelgids. When I was growing up in New York my dad and I took almost daily walks in a park next door to my school, and I continued to join him on his walks when visiting home for many decades. On one more remote part of our hike we found an eastern hemlock that though infected seemed to be holding its own. But there were never any young trees below it, suggesting once it died, there would be no more to replace it.

A number of techniques are being used to help save these trees, including chemical and biological controls. This has been effective for saving small populations but is more difficult to apply in saving entire forests. Of course, it is important to be very cautious with any of these interventions since they could have unintended consequences affecting forest ecology in broader and undesirable ways.

Hemlock forests provide habitats for species that prefer cool and moist environments and are expected to especially affect native amphibians such as salamanders, brook trout that spawn along hemlock shaded streams, and some birds that preferentially nest in them. During winter, they provide shelter for birds and deer. Their loss will affect local hydrology and impact the carbon cycle. Many scientists suggest their extinction might have an even greater impact than the loss of the American chestnut.

This eastern hemlock growing along my driveway has been limping along for the close to 12 years I’ve been here, and longer since it was already 15 feet tall when I arrived. It is clearly heavily infested as seen in this photo, but I assume because of cold weather they are regularly knocked back. The hemlock woolly adelgid is unusual among insects in being dormant in summer and feeding in fall through winter, but this makes them vulnerable to cold snaps. At temperatures of 4 or 5 degrees below zero, ice crystals form in their bodies, and they die. Though the current extreme cold snap is not fun for me, it will definitely provide a white Christmas. And as my present this year I am hoping the sub-zero weather will finally eliminate the adelgids, giving a new lease on life to this beautiful tree.