Hemlock Woolly Adelgid on My Hemlock!
I like my hemlock. It lords over my backyard, sentinel to the hemlock forest beyond. My hemlock is the most majestic Eastern Hemlock (Tsuga canadensis) I have ever seen. I cannot quite touch my fingertips together when hugging my tree. The lower limbs emanate downward from the trunk then take an elegant swoop upward. Each limb bears needle-laden branches that hang down like fringe. With increasing height the limbs get shorter each reaching skyward giving my tree a stately, conical shape. In winter, the lower branches, laden with snow, lay right down on the ground, demonstrating resiliency in a way only a snowbelt tree can. When I come by to sweep away the snow, the limber branch springs back, relieved of its burden, none the worse for wear.
I’ve seen robins, hummingbirds, nuthatches, woodpeckers, and many other birds visit or roost in my hemlock, not to mention a barred owl and a screech owl. I really like my tree, but now I fear for its life.
HWA infested branches of hemlock
About six years ago I spotted the tell-tale white woolly exude from a nasty hemlock predator, the hemlock woolly adelgid, on a tree by our driveway. My husband quickly took it down to eliminate any spread. We monitored neighboring trees and found no further infestation. Crisis averted. Even years later, all our backyard hemlocks looked fine, until this year.
Every single hemlock I’ve checked on my property has white cottony tufts at the base of the needles. Even my majestic guard of the woods. As the summer has progressed, I’ve seen my tree defoliate, looking more and more barren and less and less robust. It has been a trying summer with very little rain putting additional stress on my hemlock. Will my tree make it?
HWA first instar nymph in dormancy
Hemlock woolly adelgid (Adelges tsugae) is an aphid-like insect. It is well known in Asia, but was first discovered in the US as early as 1907 in Washington State. It didn’t show up in the East until around 1951 near Richmond, Virginia. The origin of this eastern strain has been traced to a lineage directly from Japan, and is distinct from our western strain. The adelgid has spread fairly unabated since 1951, covering more than half the eastern hemlock’s native range from Maine to Georgia. The predator also affects Carolina hemlock, which is found only in the southern Appalachians.
Although the western lineage has predators to keep it in check, this eastern type does not. Having not evolved with one another, the tree has no defenses against this insect. Although hemlocks have developed defenses against chewing insects, the adelgid pierces through the plant tissues and sucks out stored nutrients, to which the tree has no resistance. It also induces reactions in the tree, affecting the tree’s ability to transport water.
The most tell-tale sign of the pest is the cottony white exude that can be seen usually on the underside of the foliage at the base of needles. This is a substance the adelgid produces to protect themselves and their eggs from drying out. The adelgid itself, depending on the stage in its life cycle, can be yellowish-tan to black and only 1/16-inch (1.5-mm) long.
HWA nymph eating
For treatment, there are insecticides that can be injected directly into the trunk or roots of a tree. Also, insecticidal soaps can be sprayed directly on a tree, but it has to be thorough and cover each insect to be effective. Obviously these measures can’t save a forest. The best hope lies in natural predators released into infested areas. Several beetles are good contenders, one from Japan, one from China, and one from the western US. A good candidate is one that eats only Hemlock woolly adelgids and nothing else.
I went for a walk in a park recently at a lower in elevation than my home. I walked along a stream and examined hemlocks there. I didn’t see nearly the infestation I saw up on my mountain abode. A couple tufts here and one there (I picked them off and ground them into the dirt) but most trees were clean. Hemlocks are, among conifers, the most prone to drought, and this summer has been a dry one here. I’m thinking the park’s riverside trees have not been stressed to the extent my mountaintop trees have. My trees are weaker and more vulnerable allowing the adelgid to gain a foothold. Cold winters have in past years, kept the adelgid at bay, but this past winter was not much of a deterrent. Scientists have found that over time, the adelgid is able to withstand colder temperatures, allowing it to creep ever northward.
HWA dormant sistens first instar nymphs
I like my hemlock. But what will become of it? Maybe hemlocks will develop some chemical resistance. Maybe a native beetle or other predator will develop a taste for adelgids. Someday, this predator/prey dance will come into balance when this rampant pest can be deterred. But will it be in time for my tree? Not likely. In the meantime, hopefully my tree will demonstrate some of its winter resilience and find a way to spring back, when relieved of its burden, none the worse for wear.
Tags: Adelges tsugae, adelgid, Eastern hemlock, exotic, hemlock, Hemlock woolly adelgid, insecticidal soap, insecticide, instar, introduced, nymph, pest, Tsuga canadensis, woolly