New Hope For Hemlocks

Scientists trying to save the hemlock tree from the woolly adelgid, an invasive insect, hope integrated pest management (combining pesticides, the release of predator beetles, and the development of hybrid trees resistant to the insect) may be the answer.

ASHEVILLE - In a forest clearing in a park near Asheville, you’ll find what may be one of the last hopes for restoring an icon to the forests of the eastern United States.

The three-acre area was clear-cut years ago; the trees harvested for lumber and other wood products. However, rows and rows of two-foot tall trees now fill the clearing. The seedlings are American hemlocks and researchers have big plans for the tiny trees.

“They all look good,” Albert Mayfield, research entomologist with the United States Forest Service, tells me as he walks from seedling to seedling and checks their condition. “If all goes well, this will be the start of a new hemlock forest. Hemlocks used to cover this entire area.”

The Forest Service hopes this new stand of hemlocks will begin to replace the hemlocks that were wiped out by the woolly adelgid, an invasive insect that has devastated hemlock trees throughout the south.

“Number one, we are establishing a new hemlock stand because we’ve had a lot of hemlock mortality in this area due to the woolly adelgid,” said Mayfield. “Also, we’re establishing a stand where we hope to establish predators that will feed on the adelgid.”

The area where we are standing used to be lush with majestic hemlocks until the woolly adelgid invaded the area. The insects were accidentally introduced in Virginia through the importation of Japanese hemlock trees. Adelgids were spotted at the North Carolina–Virginia border in 1995. It took just 10 years for the insect to devastate hemlock stands throughout the state. The adelgids settle at the point where the needle attaches to the twig, sucking the sap from tender shoots.

“And then the cycle which eventually kills the tree begins,” explains Mayfield. “As the tree is depleted of carbohydrates, it stops making new growth, so it won’t make a flush of foliage when it has been heavily infested. And as the tree stops making new growth, that actually stops the adelgid population because the adelgid doesn’t have any new foliage to settle on.”

The tree gets a break and it begins to produce new growth again. However, because the adelgids are still in the stand, the invasion returns to the branches with the new growth. The tree can only go through the cycle so many times before it weakens and dies. Trees usually die within five-to-10 years.

The challenge for foresters is that the adelgid is only about one millimeter long and essentially undetectable except for its white egg casings. For the most part, the insects stay inside the white, waxy material on the branches, feeding and laying eggs. The trouble is, by the time the casings are visible, the tree has already been infested.

So the hope for the hemlock is found in another insect, the predator beetle. That’s because predator beetles eat woolly adelgids. There have been promising results from tests in which hemlock trees have been saved when predator beetles were released into their branches. 

“The beetles are pretty amazing in how they attack the adelgids,” said Andrew Tait, Research Technician with the US Forest Service. “Once they are released in the branches of a hemlock tree, the beetles will start walking around the branches, searching for those woolly adelgid egg sacs. They will eat the hemlock woolly adelgids and lay their eggs in the egg sacs. Ideally, the predator larvae will hatch in the adelgid egg sacs and begin to eat the eggs and begin protecting the tree before the adelgids hatch.”

The question remains whether enough beetles could be released in a forest to counter the adelgid onslaught. It’s a question researchers in the US Forest Service Research Station in Asheville hope to answer. Scientists are isolating hemlock branches collected in the forest, in hopes of determining how well the beetles controlled the adelgid population. Groups of branches, containing both beetles and adelgid egg sacs, are placed in bottles. The samples are checked weekly to see how many adelgid larvae are hatched.

“We want to see if the predator beetle is within the ova sac of the adelgid, and the best way to do that is to monitor these containers for predator larvae, feeding on the adelgid and then dropping into the container,” Katie Reidy explains as she inserts another group of branches into a clear container and then places the branch tips into a moisture pack that will keep the branches alive. “That gives us a sense of how many predator beetles are out there and how well are they established in the stand.”

As the research continues, more and more scientists believe an Integrated Pest Management plan is the best way to save the hemlock. The strategy involves using chemical pesticides to kill the adelgids in select areas. It’s not practical in a large forest area. The chemicals would buy the trees some time, during which predator beetles, which eat the adelgids, are released into the branches and hopefully save the tree. Over time, it’s hoped hybrid hemlock trees, which can protect themselves from the woolly adelgid, could be developed to provide a long-term solution and save the hemlock.

“I believe the integrated plan provides us the best solution and that’s what we hope to attempt in our new three-acre hemlock forest,” adds Mayfield. “Over time, trees that have been treated with chemicals and given a chance to develop without the adelgids would be healthier and have a better crown and a better chance to survive.”

That’s because the adelgid population grows rapidly and it takes the predator beetles several years to build up their population. Trees that have been infested die so rapidly there isn’t enough time for the predator beetle to catch up. In theory, the integrated strategy should prolong the life of the hemlock long enough to give the beetle a chance.

While scientists are still determining the best way to save the hemlock, there is general agreement that the tree is worth saving. That’s because the hemlock is a foundation species, which means it helps determine what lives in the forest. There are some species of wildlife that only live in hemlocks. The shade from the trees cools the streams, and the chemistry surrounding the tree sets the soil. If the hemlock tree disappears, the forest is changed forever.

“If we do nothing, the hemlock is very likely to go extinct in the southern states,” adds Mayfield.


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