Symptoms of Hemlock Woolly Adelgid

Recognizing Hemlock Woolly Adelgid early and following up with proper and consistent treatments is the key to managing and controlling an outbreak. Healthy hemlocks naturally have a shiny, dark green color. If you notice that yours begin to have a grayish-green appearance or if the foliage yellows, needles are dropping (especially on the interior of the branches) or you notice dieback of some limbs and a lack of vigor in the tree these symptoms probably mean that your Hemlocks are under attack.

Without treatment, Hemlocks growing in areas with compacted soils, poor drainage or drought prone areas may decline and die in approximately 4 years from this pest’s infestation. Hemlocks that are healthy and growing in more ideal conditions may persist longer in a weakened state before dying. Unfortunately, once this pest is in an area, it will always pose a threat to the hemlocks, even those that have been treated. All hemlocks should be checked several times a year and treatments applied when Woolly Adelgid is spotted.

Tree Treatments & Spraying for Hemlock Woolly Adelgid

Once a certified arborist has diagnosed your trees, treatment can be started. A successful tree treatment option to control an infestation of woolly adelgid is to spray trees with horticultural oil. The Horticultural oil is an insecticide consisting of super refined petroleum oil. The adelgids breath through their skin so when this oil is sprayed on them and dries it smothers them. We recommend two treatments per year for infested trees. The first spray treatment for tree is done in the spring, beginning around the end of March and throughout April. And the second treatment starts in mid September thorough October. When it looks like the adelgids are under control sometimes a spring spray to maintain control is all your hemlocks will need. This tree treatment provides protection. But don’t be fooled if they look like they are gone and you decide to forgo the treatments they will eventually return and re-infest your trees. Sometimes the cottony masses produced by mature HWAs may persist for months, even after the insect is dead.

It’s important to have a certified arborist come evaluate the health of your trees and let you know what would be most the most beneficial treatment options are for your hemlocks. Spraying trees may eliminate the adelgids, but other treatments or remedies may be needed. Tree Tech offers diagnosis and treatment by certified staff arborists. Contact us today for a site visit to evaluate your trees.

Hemlock Woolley Adelgid History

Hemlock Woolly Adelgid (HWA) pronounced a⋅del⋅gid [uh-del-jid], is native to Asia and in the 1920s was accidentally introduced into the Pacific Northwest. It eventually made it to the Northeast by the 1950s. It is an aphid-like insect less than 1/16th of an inch long that feeds on the Hemlock’s starch reserves (sap) that are critical to the trees growth and survival. It is a constant threat to the welfare of the Eastern Hemlock. The HWA is most visible and recognizable as it matures and produces a covering of a white, wool-like wax filaments to protect itself and its eggs from natural enemies. You can see these white masses from late fall to early summer on the underside of the branch tips of your hemlock trees.

HWA Lifecycle

The Hemlock Woolley Adelgid is not typical of most insect species here in the northeast. It is inactive for most of the growing season and active during the winter. HWA overwinters as adult females that lay eggs in the late winter (All HWA are females). These eggs begin to hatch in April. The newly hatched mobile larvae (crawlers or nymphs) search for feeding sites on the young branches and settle at the base of the needles to feed on the sap and at the same time may also inject toxins as they feed which may help to cause premature needle drop. These nymphs complete four instars (stage of an insect between periods when the skin is shed) before hatching. In June the females begin to secrete the white, waxy covering in which they lay their eggs. These eggs hatch in July and the second generation nymphs attach themselves to the base of the needles and become temporarily dormant until early fall when they begin to feed and become adults that overwinter on the Hemlocks.

How is Hemlock Woolley Adelgid Spread?

Dispersal and movement of the hemlock woolly adelgid occur primarily during the first crawler stage by wind , birds, deer and other forest dwelling mammals as well as humans that come in contact with the sticky ovisacs and crawlers. So it is best not to place bird feeders in Hemlock trees birds can pick up eggs or crawlers in their feathers and transport them to other trees.

The Future of the Hemlock and HWA

The Hemlock Woolley Adelgid not only has devastating effects on our landscape trees but Hemlocks are also important trees in the forests and they too are being destroyed by this pest. In the forests hemlocks create their own distinctive microclimates and provide habitats for a variety of wildlife. For instance Hemlock lined streams keep the water temperatures cool enough for fish such as brook trout and help to keep the streams from drying up during the summer droughts. What’s being done? A certain species of Asian hemlocks are resistant to HWA and nurseries are beginning to grow and sell those in the United States. The only native species that seems to be highly resistant to the HWA is the Western Hemlock but unfortunately does not grow well at all outside of its range of the northwestern USA.

There is also ongoing research on biological control of the adelgid through natural predators.

For more information, consult a certified Tree Tech arborist to assist with diagnosis and tree treatment. To request a site visit click here. Tree Tech has been providing expert tree care since 1986.