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Hemlock Wooly Adelgid: What it is, what to do about it.From Ann Gibbs, State Horticulturist, in an email on July 30, 2008: "In early July, hemlock wooly adelgid (HWA) was detected by an observant park employee at Ferry Beach State Park in Saco, ME. To date the entire beach, along with adjacent properties, has been surveyed, and HWA has been found at low levels scattered throughout the park at about a 1% level. A subsequent front page story on July 18 in the Portland Press Herald yielded many calls and HWA was found by homeowners on two properties in Kennebunkport and South Portland." What is the Hemlock Wooly Adelgid?*
Adelges
tsugae, the hemlock woolly adelgid,
is a fluid-feeding insect that feeds on hemlock trees throughout
eastern North America. The egg sacs of these
insects look like the tips of cotton swabs clinging to the undersides
of hemlock branches.
Hemlock woolly adelgid was introduced from Asia into the Pacific Northwest in 1924. It was probably introduced into the northeastern US in the 1950's, and it was first discovered in Maine in 1999. This insect has been damaging hemlock ever since, and it is spreading. Eastern hemlock are more susceptible to hemlock woolly adelgid damage than Asian and western hemlock trees due to feeding tolerance and predators that protect the latter species. Hemlock woolly adelgid sucks fluid from the base of hemlock needles. It may also inject toxins into the tree as it feeds, accelerating needle drop and branch dieback. Although some trees die within four years, trees often persist in a weakened state for many years. Hemlocks that have been affected by hemlock woolly adelgid often have a grayish-green appearance (hemlocks naturally have a shiny, dark green color). Other factors can influence the impact of the hemlock woolly adelgid. Other insects, such as elongate hemlock scale, hemlock borer, and spittlebugs, which are also found on hemlock, can compound the impact of hemlock woolly adelgid. Drought and fungi, such as Fabrella or Korfia tsugae can weaken hemlock and cause it to become more susceptible to insect damage. Low winter temperatures, cold snaps (episodes of freezing and thawing), and heavy thunderstorms can reduce populations of the hemlock woolly adelgid. Particularly in the mountains, it is not uncommon to find hemlocks where the insect has been killed on the top third of the trees, where it's colder and windier, but survive on the bottom two-thirds. On the other hand, mild winters can result in sharp increases in hemlock woolly adelgid populations. What can we do about it?**Scouting is one of the most important things we can do to stop the spread of this pest. Early detection is key in slowing the artificial spread of HWA. While natural spread of the insect, caused when HWA is carried by birds, wildlife and wind, is inevitable, it is slow. When HWA is artificially exported from infested areas, on recreational vehicles, clothing, live hemlocks, or pets, there is great potential for a much more rapid distribution of the insects. If you live in or near an infested area, help prevent the artificial spread of HWA by pruning hemlock branches that delivery vehicles, people and pets may brush up against. Because birds can move HWA on their feet and feathers, try to place feeders away from any hemlock trees and consider removing them between March and August when adelgid eggs and mobile young (crawlers) are present. The safest insecticides for controling HWA are foliar sprays that contain Horticultural Oils or Insecticidal Soaps. Care must be taken to completely cover the undersides of the foliage for maximum effectiveness. These products are relatively non-toxic, as they kill by smothering. They can, however, burn foliage if applied during the growing season or in direct sunlight. Click here to download an informational poster on the Hemlock Wooly Adelgid from the Maine Forest Service. *Information and photographs from Pennsylvania
Department of Conservation and Natural Resources |