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Devastating invasive pest threatens hemlock trees in region

About a month ago, while clearing some trees that posed a hazard above Cascadilla Gorge, Cornell natural-areas staff spotted small fluffy white sacs along the base of the needle on an eastern hemlock: telltale signs that a devastating pest had invaded Cornell's hemlocks for the first time.

The culprit: woolly adelgids, aphid-like insects that have decimated hemlocks along the coastal eastern United States and southern Appalachian Mountains with a nearly 100 percent mortality rate.

Hemlock woolly adelgids, an invasive species first discovered in the central Finger Lakes area last summer, have since been identified in 19 Finger Lakes sites, and now include Cornell Plantations' Cascadilla Gorge and Beebe Lake natural areas. They are native to Japan.

Aside from aesthetic qualities, hemlocks play important roles in the ecology of local gorges. They moderate stream temperatures in the summer creating a habitat that is favorable to native brook trout as well as wildlife and plants that rely on cooler microclimates. They also play a role in maintaining water quality by stabilizing steep gorge slopes and buffering sediment runoff.

"We are facing a threat to hemlocks that adorn our gorges," said Todd Bittner, director of Cornell's Natural Areas. Unless the insect's spread is contained, "it's going to change our gorges."

"People don't realize how important hemlocks are in the Finger Lakes area both socially and ecologically," added Mark Whitmore, a Cornell entomologist in natural resources.

In a few weeks, wintering female adelgids, fixed at the base of the infested hemlock needles, will each lay some 200 eggs. Woolly adelgids complete two generations each year on hemlocks in North America and are hard to spot until they establish themselves on branches.

Even though adelgids are sensitive to cold, they have survived one of Ithaca's coldest winters in years.

"We're at the northern front of the adelgids' movement," said Bittner.

Whitmore speculated that the local adelgids may be adapting to colder temperatures or able to survive in milder microclimates created by the Finger Lakes. Long-term climate warming, he said, may eventually play a role in their spread.

Natural areas staff members are extensively surveying each hemlock in Cascadilla Gorge and around Beebe Lake using global positioning systems and geo-spatial mapping software to gauge the severity of the infestation. Treatments of individual trees will depend on environmental factors, the severity of the infestation and Plantations resources. Hemlock forests cover 800 of the 4,300 acres of natural areas the Plantations protects. Working with several local entities, staff members are also leading three volunteer training sessions to raise public awareness to help assess the extent of the local infestation.

"The immediate strategy is to slow down the spread and buy ourselves some time to develop more effective control strategies," said Bittner. The long-term strategy, and perhaps the only effective one for larger areas, is to release such well-researched predators as the cold tolerant Laricobius nigrinus, a predatory beetle from the western United States. Such biocontrol tools are still being developed and will take time to implement.

Members of the public who spot fluffy white sacs on a hemlock should enter a report on the Cornell Natural Areas Web site: The site includes maps and such information as how to sign up for a volunteer training session and to identify the pest.

Media Contact

Nicola Pytell