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Eco Talk: The threat of the hemlock woolly adelgid

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Hemlock wooly adelgid

The hemlock wooly adelgid is an insect that attaches to hemlock tree needles.

Hemlock woolly adelgid (HWA) is a tiny insect originating in East Asia, thus marking it as an invasive pest. An invasive species is an introduced species that tends to spread unchecked and often outcompetes native species because they did not have natural predators or diseases in their new location.

According to the state Department of Environmental Conservation website, HWAs are an aphid-like insect first found on the East Coast of the U.S. in 1951 in Virginia. It was confirmed in New York in 1985 in the lower Hudson Valley and on Long Island, and has since continued to spread. In 2012, the DEC confirmed its presence in southern Cayuga County.

The HWA attacks eastern hemlock trees in forests and ornamental hemlock trees found in our residential landscaping. It feeds on the hemlock trees' young twigs by sucking the sap, causing their buds to die and needles to dry out and drop from the plant prematurely. The continuous feeding causes the infected hemlock tree stress, with the tree usually dying within four to 10 years following the first HWAs invading it.

As of 2015, 90 percent of the geographic range of eastern hemlock in North America has been affected by HWA, according to the DEC. So, why should we care about this tiny insect pest? First, let’s learn a little more about the hemlock.

The eastern hemlock is a key species in our landscape. It grows well in shade and is very long-lived. Some of the oldest trees in New York are estimated to be over 700 years old. These iconic trees line the fabulous gorges and lakes around the Finger Lakes, helping to make this region a tourist destination and providing environmental benefits as well.

The eastern hemlock excels at growing in areas other trees have trouble growing in. It provides shade for the waterways it lines for animals that live on the land and in the water. The cool, dark and sheltered environments of the hemlocks provide food, protection and ideal conditions for salamanders, migrating birds and unique plant communities in our area. The hemlock trees help protect against erosion and sediment runoff from the landscape, thus providing an additional layer of defense for water quality in our lakes. Their shade also helps cool warm water temperatures to maintain conditions suitable for freshwater fish such as brook trout.

How can I tell if there is an infestation of HWAs? You need to look for white woolly masses smaller than a cotton swab on the underside of the branches at the base of the needles. This is the HWA feeding, which interrupts the flow of nutrients and stresses the tree, causing dieback and needle loss. More on the HWA’s description and life cycle can be found on Penn State Extension’s website at

Cornell University and the DEC, through funding from the state’s Environmental Protection Fund, created a new biological control lab to work on this pressing issue with a ribbon-cutting ceremony in November. Biological control is the use of natural enemies to manage a population of a pest. According to an article in the Cornell Chronicle and November 21, 2017 press release, the purpose of the lab is to “research and rear biological controls to slow the spread of hemlock woolly adelgids.”

Research has found a beetle and a silver fly that prey on the HWA in the Pacific Northwest; these two predators are an effective biological control. The Cornell-based lab will be researching how effective these predators will be on the invasive HWAs populations on the East Coast and work to increase the number of both species so they can continue to be released. It is critical to build the number of predators in the target area to protect the surviving hemlock trees. Preventing the spread of HWA with biological controls is considered to be the most effective way to stop the damage they are causing.

If you are interested in managing your hemlock trees or notice a new infestation, contact CCE Cayuga at (315) 255-1183 or send an email to so we may help you access proper resources.

The eastern hemlock is an important tree, particularly in gorges and steep hillside streams in our region. Imagine what our local landscapes might look like without these stunning trees and the potentially dramatic changes to the current ecosystem they are protecting.

Judy Wright is a senior resource educator with Cornell Cooperative Extension of Cayuga County. For more information, visit or call (315) 255-1183.


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