What is it?
Hemlock woolly adelgid, often abbreviated as HWA, is an invasive, aphid-like insect that uses a long, piercing mouth to feed on starches and other stored nutrients in hemlocks. They are very small, no more than 1.5 millimeters in length, and vary from dark reddish-brown to purplish-black in color. Most often, they are identifiable by the “woolly” wax filaments that they cover themselves with and that provide their namesake.
Where did it come from?
Hemlock woolly adelgid is native to Asia, in particular Japan, and was likely transmitted to North America through nursery and ornamental plants. It’s first discovery in the United States was in Oregon in the early 1900s, and in 1951 it was detected in a private collection in Virginia. HWA was not considered to be a significant pest threat for a few decades because it did not cause significant damage to hemlocks in Asia or western North America due to natural predatory dynamics, relationships that were not understood at the time. Initial spread in the eastern US was slow, but HWA can now be found all along the east coast from Georgia to Maine. It’s first discovery in Maine came in the early 2000s, and it was found in the Kennebec Estuary in the 2010s.
Life cycle
Hemlock woolly adelgid goes through two generations each year. Understanding how these patterns work is key to both identification and management of the pest. Unlike most insects in the Northeastern US, HWA is dormant during growing seasons and active in the winter.
Immature adelgids will be present on trees in July and remain dormant until October, when they will begin feeding and development towards adulthood. They fully mature in early March, at which point they will produce eggs that are surrounded with the white “woolly” material that gives them their name. This is also when HWA is most easily identifiable. These eggs hatch, feed, and then ultimately mature from late May to early June. They then feed until their dormancy in July.
While some adults will develop wings and fly away, they are seeking a spruce host that does not exist in North America and will die when they do not find it.
What does it do?
HWA has a long, piercing mouthpart that it inserts into the tree, where it reaches the tree’s store of starches and carbohydrates. As the adelgid feeds, the loss of these nutrients weakens the tree and makes it more vulnerable to other environmental stressors like drought or disease. Due to the damage, the tree will have more trouble moving water and any early buds may die. Many hemlocks with HWA also experience early needle drop, which is a sign of poor health and possibly an infestation. Ultimately, infestation leads to tree death. In healthy trees, this can take ten years or more, while trees that are already unhealthy or suffering from drought or disease may die in 3-5 years or less.
Why is it so bad?
Black-throated Green Warbler
Tree death is, of course, not a good thing, but the loss of hemlocks in our forests is especially damaging. Eastern Hemlock, the species affected by hemlock woolly adelgid in Maine, is the most shade-tolerant tree in the eastern US and has been found to support more than 120 vertebrate species in forests where mature stands are present. This includes birds like the Black-throated Green Warbler and Acadian Flycatcher that are strongly associated with hemlock forests, as well as White-tailed deer and other mammals that use them for foraging and thermal cover.
Hemlocks are also very important for stream health in forests, often supporting a wider variety of aquatic species than are present in streams sheltered by hardwood trees. This includes supporting trout populations and macro-invertebrates that benefit significantly from temperature regulation and flow control provided by hemlocks. While hemlocks would eventually be replaced by other tree species, they will not provide the same type or level of habitat benefits.
Beyond their obvious environmental value and natural beauty, hemlocks are also an important source of lumber, accounting for about ten percent of lumber harvests in New England. As hemlocks are weakened and killed by Hemlock Woolly Adelgid, all of these benefits are lost in our forests.