PITTSBURGH — Invasive insects have been found in a third landmark Pennsylvania forest, and scientists are considering a possible link to global warming.
The U.S. Forest Service reported this week that the hemlock wooly adelgid has been discovered in the Tionesta forest near the New York border. Experts said the insect is often deadly to hemlocks, and the loss of those trees can lead to a cascade of environmental changes for some wildlife, fish, and plants that need the cool, deep shade that the old growth trees provide.
Forest Service silviculturist Andrea Hille said the Tionesta area of the Allegheny National Forest is one of the last remaining areas of uncut hemlock-beech forest in the Allegheny Plateau region, which stretches into New York state.
Hille and other scientists say they can’t conclusively link the adelgid problem to just climate change, but it is a concern.
“Climatic conditions are changing and generally warmer trending,” Hille said, adding that the global warming impact on forests “is something that we’re thinking about, and insects are a part of that.”
This year, officials also announced adelgid infestations at Cook Forest State Park, a National Natural Landmark, and at the Flight 93 National Memorial.
The insects appeared in Virginia in the 1950s and in southeastern Pennsylvania in 1969 and have since spread to the warmer parts of many states. But now they’re increasingly moving to areas that were once known for bitterly cold winters, including Tionesta.
“At 4,000 acres, it’s too large to expect to reasonably save all the hemlocks there,” Hille said, adding that there is real concern about how it will affect the forest and research projects into the old-growth ecosystem that have been underway for decades.
Treating the infestation is expensive and slow, so the Forest Service will likely only be able to focus on some trees.
Wooly adelgids are difficult to eradicate once they’re established because they have only one natural predator in eastern forests — extremely cold winters. Native to Asia, they lay eggs on the underside of hemlock branches, and the young insects feed on the sap of the trees, causing them to lose needles and die within five to 10 years.
Pennsylvania’s official report on the likely impacts of climate change makes the connection, too.
“For some species, hemlock for example, higher mortality rates associated with higher winter temperatures were likely linked to greater winter survival of insects and pathogens,” the 2013 Climate Impacts Assessment notes, adding that a warming climate could allow the adelgid “to expand its range within the state.”
Hille said the infestation is only confirmed in a few Tionesta trees, and more surveys are planned to determine the extent. Ultimately, she said, hemlocks that die off could be replaced by adelgid-resistant hardwoods such as red maple or cherry trees, changing the forest ecosystem.