Field Notes

Hemlock-loving invasive insects are moving north

3 min read / December 14, 2023

Is climate change affecting hemlock trees in Vermont?

Hemlocks are long-lived evergreens that bring cool air and moisture to the forest floor and help to support many types of animals. We love them but are worried about their future here in Vermont.

Climate change is bringing warmer temperatures and wetter weather to Vermont. Hemlock trees would probably be fine if those were the only things our changing climate brings. The real threat is coming from two invasive insects: hemlock woolly adelgid and elongate hemlock scale.

Read on to learn how to identify these two pests and what to do if you find them.

How to identify hemlock woolly adelgid

If left unchecked, hemlock woolly adelgid can wipe out an entire towering hemlock stand.

When we have winters where the temperature doesn’t get much over zero for days, their populations can fall by 90% or more. With our warming winters, we are getting fewer of these cold snaps, especially in southern Vermont, so these insects are expanding their range.

If you live in southern Vermont, your trees are at risk. Tracking the spread of this insect is important for researchers and foresters. This is where you can help.

You don’t need to do a rigorous scientific survey, just flip over some branches as you walk through your favorite hemlock stand. The best time to do this is in the winter.

Hemlock woolly adelgid is always found on the underside of the branches where the needles attach to the twig. Look for fuzzy, white balls. The above photo shows an example of an infested twig.

Be aware of some look-alikes

If you see a white mass part way up the needle, or bunch of needles drawn together in a white mesh, it’s not hemlock woolly adelgid. See the two photos below for examples of look-alikes, both of which are probably from spiders.

What can you do if you find hemlock woolly adelgid?

Because the insect has been in the southern Appalachian region for decades, a lot of research has been done. It’s been shown that bringing more sunlight to hemlock trees can help. This can be done by cutting some trees down (called thinning).

Thinning around hemlocks increases sunlight on the trees, which makes them more vigorous and better able to withstand the insects. Please note that hemlock doesn’t respond well to heavy thinning, so go easy.

We have a better chance of finding a more permanent solution, if we can keep hemlock in our woods a little longer.

If you own forestland, we encourage you to talk to your forester about managing your woods for the insect.

When to report a sighting

Researchers are aware that this insect is in Windham, Bennington, Windsor counties. However, if you find it outside of these areas, please report your findings to Vermont Invasives by clicking the “Report It!” button.

How to identify elongate hemlock scale

While you’re out looking for hemlock woolly adelgid, keep an eye out for another exotic insect that is harming Vermont hemlocks, elongate hemlock scale.

While this insect doesn’t usually kill trees, it does stress them. A stressed tree is more susceptible to things like hemlock woolly adelgid, drought, hemlock borers, and a fungus called armillaria root rot.

Elongate hemlock scale is also found on the underside of hemlock twigs, these insects array themselves on the needle, not at its base.

The females are brownish, flat, and longer than they are wide (about 1/16th of an inch long). The males are white. See if you can find the males and females on the needles in the above photo.

These insects are especially problematic when found with hemlock woolly adelgid. Double trouble, you might say.

What can you do if you find elongate hemlock scale?

Elongate hemlock scale is known to be in Windham and Chittenden counties. However, if you find it outside of these areas, please report your findings to Vermont Invasives by clicking the “Report It!” button. It’s helpful for researchers to understand where the insect is should a solution be found.

And certainly, if you own forestland, speak with your forester about your discovery.