Hemlock Woolly Adelgid
Climate Change Invites Invasive Insect North
As average temperatures climb, populations of hemlock woolly adelgid (pdf), an invasive insect that destroys Eastern hemlock, are creeping up the East Coast. Intolerance of cold weather has checked its spread north of Massachusetts, but as temperatures rise, hemlock stands in northern New England are becoming vulnerable to this non-native species introduced from Japan.
Named for its wool-like filament, hemlock woolly adelgid first appeared on the East Coast in the early 1950s and quickly became established in the dense forests of Virginia.
Adelgid infestations can kill a hemlock in just a few years, sucking it dry of the nutrients it needs to survive. The insect inserts its piercing mouthpart into the base of a hemlock needle and then sucks out the sap, killing the tree’s nutrient supply. With two generations produced each year, adelgids can quickly spread to new territory.
Eastern hemlocks are integral to the ecosystems they inhabit. They provide dense shade necessary to keep forests cool. According to a recent study, brook trout are three times more likely to be found in streams surrounded by hemlock because they help provide cooler water temperatures.
Ken Sturm, wildlife biologist at Canaan Valley National Wildlife Refuge in West Virginia said that while the high altitude refuge hasn’t been affected yet, the threat is near. Woolly adelgids now occur in forests surrounding the refuge.
“If it’s not on the refuge already, it will be here soon,” said Sturm. “If you drive anywhere in West Virginia other than Canaan, there are hardly any living hemlocks around. It’s tragic.”
Cathedral State Park in West Virginia boasts a forest historically dominated by Eastern hemlock and more recently, woolly adelgid.
“It’s likely that we’ll see hemlock wink out slowly if the adelgid hits us hard. I’m worried about the effect it will have on shading and the loss of botanical diversity to our forests,” said Sturm. “The park has big, beautiful 200 to 300 year old trees that are being destroyed in a few years time. It’s so sad.”
With warming temperatures, woolly adelgid has the potential to do similar damage to Northern forests. A study done at the University of Massachusetts (pdf) predicts that under the worse circumstances, the entire Northeast could be infested by this destructive pest by century’s end.
Among other variables, the study reports that adelgid are unlikely to spread if the mean winter temperature is negative 5 ºC or lower.
“That’s the temperature where the population seems to be stabilizing, so there’s not an increase from year to year,” said Annie Paradis, lead author of the study.
Traditionally this has meant good news for New England. But based on one of the study’s models, virtually all of the Northeast will experience average winter temperatures above negative 5º C by the end of the century. By this measure, the woolly adelgid will become tolerant of temperatures throughout the region. Hemlock is much more dominant in northern New England than it is down south, so the impacts are likely to be much greater.
The spread is already happening. Isolated adelgid populations have been found in southern Vermont and New Hampshire.
If temperatures continue to rise, said Bob Childs, entomologist at the University of Massachusetts, the Bay State could soon experience a significant adelgid infestation.
“Up until now, Massachusetts has been the zone where it can establish but then every four, five, six years we have a winter that knocks the number down again,” said Childs.
But this defense may soon be a thing of the past.
“Just last year, adelgid started rebounding,” said Childs. “We had a pretty mild winter, so hemlock woolly adelgid is on the upswing again.”
Woolly adelgids are unique in that they remain inactive during the growing season, only feeding and developing during the winter months. With no natural predators in the U.S., the trees they infest grow without any biological defense, said Childs.
One of the pressing concerns is the difficulty of stopping the spread on a wide scale. While pesticides and cutting practices provide temporary protection in smaller landscapes, it’s extremely difficult to attack the pest once it hits the open forests.
Laricobius nigrinus, a predatory beetle from British Columbia has been released on the East Coast in an attempt to introduce something that will feed on the adelgid.
“Laricobius nigrinus really likes hemlock woolly adelgid and their life cycles are in sync,” said Childs. “The hope is that the beetle will establish itself.”
The woolly adelgid is present on the west coast, but doesn’t cause the same extent of damage. Populations build up on western hemlocks, according to Childs, but don’t normally hurt a tree unless it was previously stressed.
“Now the thinking is that this little beetle may be the controlling factor,” said Childs.
As the adelgid moves north, the introduction of this beetle native to the West may be the best hope for the eastern hemlock.
Story by: U.S. Fish and Wildlife Service intern Michael Gardner.
Photo on homepage: Hemlock branch infested with woolly adelgid. Credit: Connecticut Agricultural Experiment Station, bugwood.org
Science Seminar Presentation
To view the archived Science Seminar presentation given on April 8, 2010 at the U.S. Fish & Wildlife Service Northeast Regional Office entitled "Hemlock Wooly Adelgid Impacts and Management Implications for Hemlock Forests" presented by Dr. David A. Orwig from the Harvard Forest:
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