How to Treat Hemlock Trees for Hemlock Woolly Adelgid

Hemlock woolly adelgid, or HWA, is a tiny, white fuzzy insect. Though small, it is destroying hemlocks from Maine to Georgia. HWA infests large hemlock forests as well as hemlock trees in your yard. Hemlock woolly adelgid poses a serious threat to eastern hemlocks, and by extension to the health of our forests and rivers. The good news is there is effective treatment for HWA. So, if you have hemlocks on your property read on to learn how to prevent and treat HWA.

Step 1: Identifying Hemlock Trees

Eastern hemlock, also called Canadian hemlock (Tsuga canadensis) is a native North American conifer. Great trees for landscapes, the lacy evergreen foliage offers privacy and beauty all year round. Moreover, unlike many evergreens, hemlocks are shade tolerant. Here’s how to identify a hemlock tree.

Hemlock needles are very different from pine or spruce needles. Unlike most evergreens, hemlock needles are short and flat. Moreover, they are only about a half-inch long. This is much shorter than most pine needles. You can test a needle to see if it is hemlock. First place a needle between your fingers. Then try to roll it. Pine and spruce needles will roll, but not flat hemlock needles. Also, check the underside of needles. Those of the hemlock have pale white stripes, unlike pines and spruces. Caution though, the undersides of Douglas fir and balsam fir needles may also have pale white stripes. So, be sure to look at the shape of the needles as well when identifying a hemlock.

Step 2: Check for Hemlock Woolly Adelgid

HWA is a sap-sucking insect accidentally introduced into the United States from Asia. While predators in Asia and the Western US naturally control HWA, no such predators are present in the range of eastern and Carolina hemlocks. Though organizations such as the New York State Hemlock Initiative are working on biological control solutions, right now HWA is spreading faster than it can be controlled.

This is bad news for homeowners with hemlocks in their landscape. However, all is not lost. Effective treatment is available! Most importantly, watch your hemlock closely. The sooner you catch HWA, the better your tree’s chances.

HWA is easiest to spot in spring and early summer. This is because, it targets new growth. Placing themselves where the needles meet the stem, hatching insects feed on the sap at the base of needles. In the end this causes the needles and the whole branch to die. Overtime, the infestation starves the tree. While HWA only attacks hemlock trees, there are a few other insects that can be confused with hemlock woolly adelgid. The easiest way to identify hemlock woolly adelgid is to check for the white “wool” created by the adelgid. These ‘balls of wool’ are wax secreted by an adelgid during feeding. Once you’ve found HWA, move quickly to treat your tree.

Step 3: Treatment Options

Cooperative extensions and the DEC are developing biological controls, including releasing silver fly at Durand Eastman Park and Gosnell Big Woods. However, these efforts are still in their infancy. It will take time to see if silver fly can sustainably control HWA in eastern hemlocks. Meanwhile, homeowners have four main options for treating HWA.

Soil Drench

Soil drenching is considered the most effective DIY method of treating HWA. To do so, first look for an insecticide with the active ingredient imidacloprid. You can find this under a variety of names, such as Bayer Advanced Tree & Shrub Insect Control and Ortho Max Tree & Shrub Insect Control. Choose bottles of concentrate, rather than the ready-to-use spray bottle. Next, dig a trench around the tree about one foot from the trunk and about 3 inches deep. Then, mix the concentrate according to package instructions, and pour it into the trench.

Note that soil drenching can take several months to take effect. If your trees are heavily infested, consider pairing with one of the options below or contact a professional arborist.

Finally, imidacloprid is an neonicotinoid. Known to harm bee populations, use this class of pesticide with caution. Though hemlocks rely on wind for pollination, rather than insects, do not apply imidacloprid near streams and rivers.

Spraying

Have smaller hemlocks? Then you can use a foliar spray to soak the needles and stems with imidacloprid. Again, follow the manufacturer’s instructions and avoid applying near water sources.

Insecticidal Soap

Thirdly, you can use sprays of dormant oil or insecticidal soap. While these pesticides are less toxic than other options, still avoid using around water sources. Follow the manufacturer’s instructions. Understand too, that you’ll need to repeat this treatment every year. Also, treating hemlocks with oil or soap goes well with soil drenching. This is because the soap or oil will provide immediate protection as you wait for the soil drench to take effect.

Commercial Treatments

Finally, a tree professional might be a better bet, if you have larger trees or many hemlocks in your landscape. Licensed pros can apply a basal bark spray. More potent and readily absorbed by the tree’s cells, basal bark spray is great for trees on a slope or rocky ground where soil drenching isn’t an option.

Need Help with Your Hemlocks?

Looking for arborists in Webster? Let Arbor Tree Experts help. Established in Webster in 1985 we are a fully licensed and insured tree service in Webster. We offer tree removal, tree pruning as well as stump grinding. Likewise, we can help you choose the best trees for your unique landscape. Contact us for a quote.