Pest Threatens Old-Growth Hemlock Trees

  • A member of the hemlock rescue team uses a magnifying glass to observe Hemlock Woolly Adelgid eggs beneath a wool-like covering that gives the insect its name. Photo: Jessica Paholsky

July 20, 2013

The trees of Cook Forest State Park survived the clear-cutting of Pennsylvania more than a hundred years ago. Now the virgin old-growth hemlocks in the park are threatened by a pest that has plagued trees from New England to Georgia.

The “pest” in question is the Hemlock Woolly Adelgid. It’s common in much of Pennsylvania. But this is the first time it’s been found in this park, which has some of the oldest and largest hemlocks in the state.

Rachel Wagoner is part of a rescue team for the trees. Most members work at Cook Forest, but some are from other state parks offices, and a couple, like Wagoner, work for Pennsylvania’s Department of Conservation and Natural Resources in Harrisburg.

A rescue mission ealier this spring drew a lot of participants because they were testing out a modified treatment method. Instead of injecting the insecticide into the ground around the trees, they sprayed it onto the trunks. That’s because the soil in Cook Forest is rocky. Wagoner helped custom-build the spray-packs used by the team. 

They treated some hemlocks in the first area where the adelgid was identified at this park.  It's about half a mile into an area of woods that has never been cut for timber.  A Seneca Hemlock in this area is 300 years old, and is the tallest known hemlock in the Northeast.  At over 147 feet tall, the Seneca is also the third tallest hemlock north of the Smoky Mountains. And the hemlocks at Cook Forest hold another, more dubious,  distinction. They mark the first time Hemlock Woolly Adelgid has been found on the Allegheny Front--a slope that divides two sections of the Appalachian Mountain chain. The bug has been found in 56 of Pennsylvania’s 67 counties.

Wagoner had hoped the higher altitude on the Front might stymy the adelgids, but here they are. On this expedition she holds out a small branch with bits of white fuzz to prove it.

“So, this is where it gets its name from.  This is a hemlock and that’s kind of woolly looking and inside there’s an adelgid. Hemlock Woolly Adelgid,” she explains.

Now that warm weather is here they’re crawling out of their woolly houses to feed the trees.  An infested tree can turn greyish-green, lose its needles, and eventually die.  Hemlock Woolly Adelgid can kill a hemlock in Pennsylvania in less than ten years.  Trees in the southern Appalachians are being killed off even faster. A study by the U.S. Forest Service said the pest has the potential to kill off most of that region's hemlock trees within the next decade.

Don Eggen is in charge of forest pest management for the Bureau of Forestry. He says we need to save the hemlocks because they’re environmentally important as a “foundation species.”

“If these hemlocks were not here this habitat would be completely different. Every single thing you see here, the undergrowth, the soil, the birds, the invertebrates, everything else including the stream.  Trout fishermen love hemlock because that chemistry of that stream will change if you don’t have hemlocks,” Eggen says.

Eggen says the Bureau of Forestry is developing a hemlock conservation plan. It will include information about hemlocks and the pests that attack them, and strategies for treatment. In addition to insecticides, Eggen says they release beetles that eat Hemlock Woolly Adelgid. But cold winters often kill the beetles, so they’re testing hardier species in the lab.  They’re also studying hemlocks that seem to have a natural resistance.

“As a matter of fact they have found, we call it the ‘bullet-proof’ stand. All the hemlock around it are dead. So we want to look at the genetics. Between looking at host resistance and maybe breeding, along with preditors, that’s the long-term solution," Eggen says.

And a long-term solution would be nice. Right now, the insecticides have to be applied every three years. And just the treatments in Cook Forest State Park will cost 40 to 50-thousand dollars this year.

Assistant park manager Sean Benson says these iconic trees bring more than 300-thousand visitors to the park every year.

“Really, Cook Forest is known for two things. One is the Clarion River, which is a wild and scenic river. And then, of course, the old growth. So we’re faced with an uphill
battle to protect something that is very, very important to us," Benson says.

The hemlocks they’ve treated so far should be safe from the Hemlock Woolly Adelgid, but Rachel Wagoner and the others are already looking at which trees might need treatment next year.