Frederick County Master Gardener Program
Hemlock woolly adelgid (Adelges tsugae (Annand)) is a serious exotic insect pest of Canadian hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana). It has no natural
enemies within this country. Hemlock woolly adelgid (HWA) was introduced here about 40 years ago from its native country, Japan, where it is not considered a pest. The adelgid is found in the Eastern United States from North Carolina to Southern New England. It feeds by
sucking sap from the branches. If controls are not applied to pest infested trees most will die in 3 - 4 years. The insect hitches a ride on birds, animals, humans and the wind to spread out to new host trees. Working as a landscape contractor on Long Island, N.Y. I watched
hundreds of hemlock trees, most used for privacy hedges, succumb to the slow death associated with this pest.
Identification- Look for small white cottony puffs at the base of the hemlock needles, particularly on the younger growth. They prefer maturing trees on stressful sites, and often attack lower branches first. The pest is named for the "woolly'
appearance it has due to the fluffy wax coating covering its body for most of its life. I have received numerous phone calls from people who mistakenly think the white cottony balls are some type of fungal disease. They often spray chemical fungicides (disease control)
which will not control this or any insect. Please read pesticide labels before you apply them. If you cannot identify the pest, do not spray, call our helpline number provided at the end of this article for help.
Life cycle- Adult adelgids lay eggs in March and April. There are two generations per year. In Maryland, eggs are present from mid-April to mid-June. Newly hatched young are called crawlers because they move around the plant as they look for a place
to feed. This stage does not last long, crawlers mature quickly into nymphs which remain stationary attaching themselves to twigs to feed. Nymphs develop their wax overcoat slowly as they mature into the final adult stage. The adults survive the winter to lay the next
generation's eggs the following spring and the cycle continues.
Control- The secret to effective control is coverage. Sprays must be applied so that the entire tree is thoroughly soaked, you should see the spray dripping off the trees. Both the upper and lower sides of the needles and twigs must be covered. Do
not spray between mid-April to mid-June because the eggs have not all hatched until after that.
Non-chemical controls are harmless to humans, pets, wildlife and beneficial insects like ladybird beetles, these products are not poisons. Horticultural oil and insecticidial soap are both non-chemical and will control HWA if thorough coverage is
achieved. Use horticultural oil as a dormant spray from November through early March. This will kill the adults before they have a chance to lay eggs. The oil kills by encapsulating the insect and suffocating it, you can now understand why thorough spray coverage is
Immature adelgids should be controlled after they have all hatched in mid-June. Horticultural oil when used at half rate is called summer oil, if used during humid weather conditions it will burn foliage. Insecticidial soap is a better choice for
summer sprays because there is less chance to burn foliage. Soap kills by desiccation (drying), it draws out the insect's body fluids. Again the insect needs to be immersed in soap to be effective. Both of these control products have no residual effect and will only kill
what they contact when applied properly. A ladybird beetle landing on a tree that was sprayed with oil or soap in the morning would not be harmed later in the day when the spray is dry.
There are chemical controls available. Unfortunately these are professional products that require a license to apply and are unavailable to homeowners. If you cannot reach the top of your infested trees with your spray equipment or consistently get
poor control consider contacting a professional for the job. The best insecticide for controlling HWA is Imidacloprid, sold as 'Merit'. Merit is a systemic insecticide, which means it is absorbed by the tree and then moves internally to other parts of the tree. It has a
very long residual effect providing season long control. Merit can be applied using either of three methods, foliar spray, injection, or as a soil drench.
For most situations I prefer soil drenching because it is less expensive than injection and more effective than sprays. This is why, Merit is absorbed slowly and then moves up from the point where it was applied. Drenching tree roots with Merit will
ensure the entire tree is protected from bottom to top. Soil drenches should be applied underneath the ends of the branches (drip line) and then watered in to move the product down to the root zone (about 8 - 12").
It takes 30 - 60 days (more for tall trees) for Merit to reach all parts of the tree. Soil drenches should be applied in April for season long protection. Foliar sprays of Merit are applied in mid-June after all eggs have hatched. Spray coverage is
not as critical as with oil and soap because the product will be absorbed. Re-application of Merit is usually not required that season.
The use of Merit is thought to stimulate high populations of another insect pest of hemlock, the spruce spider mite. It is thought that Merit triggers a reproductive hormone in the mite or possibly kills a predator of the mite which sometimes feeds
by sucking plant juices if prey are absent, researchers are currently investigating these theories. If Merit is applied monitor trees for mite damage.
Future controls may include nature itself. A three-year biological control study has just ended which tested a natural predator of HWA. This beneficial insect is a coccinellid beetle, another native of Japan. Results were promising and now other
predators of HWA are being evaluated. It has also been found that hemlock trees planted in shade with north or northeast exposure and protection from high winds are more resistant to attack. Before you install any plant find out what environment it prefers. Plants out of
place are stressed and attract pests. Finally do not fertilize trees infested with Hemlock woolly adelgid because that further compounds the pest problem by increasing the amount of soft new tissue available to feed on.
Read other articles about tree care
Read other articles written by Robert Bishop