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Hope for hemlocks: New tactics found to fight deadly pest

A combination of selective breeding, crossbreeding and insect biocontrols could save one of Appalachia’s most important tree species.

Several new scientific discoveries give hope that eastern hemlocks will not go the way of chestnut, elm and ash trees and largely disappear from forests in the Chesapeake Bay watershed.

The eastern hemlock, Tsuga canadensis, is one of the most prevalent, longest-living, beautiful and ecologically vital trees in Appalachian forests. Sometimes called the redwood of the East, it can take up to 300 years to mature and can live more than 800 years.

It also has a long history with humans. Native Americans used hemlocks for medicines. Settlers used the tips of branches for tea and as a dye for wool and cotton. Towns sprung up in the mountains of Pennsylvania around tanning operations that used tannin from hemlock bark to turn animal skins into leather. The trees’ intense shade cools streams and supports fish habitat. And many homeowners still want hemlocks in their landscaping tableaux.

But for 40 years, the old denizens of the forest have been under relentless attack from woolly adelgids — rice-sized, aphid-like insects native to Asia. Here, there are no natural enemies to keep them in check. They continue their advance west and north by about 8 miles a year and have infected about half of the eastern hemlock’s range.

So far, the tiny invaders have killed millions of hemlocks from Georgia to southern New England. Some of the hardest-hit areas are south of the Chesapeake region, but Pennsylvania, Maryland, Virginia and West Virginia have also seen heavy mortality.

Hemlocks once dominated the forests of Pennsylvania, where the hemlock is one of the three state trees, along with American chestnuts and eastern white pines. Now, adelgid infestations have been found in 64 of 67 counties. The state’s latest Eastern Hemlock Conservation Plan doesn’t mince words in making the case for saving the species: “Without intervention, most trees in natural settings will die.”

There still are an estimated 124 million hemlock trees greater than 5 inches in diameter alive in Pennsylvania. But that is some 13 million fewer than estimated in 2004, and the rate at which trees are dying has increased fourfold since 1989. In the state’s Tuscarora State Forest, the Hemlock Natural Area, a 120-acre stand of virgin hemlocks “untouched by man” is now simply gone, touched by insects.

In Virginia, the Limberlost grove of hemlocks, oldest in Shenandoah National Park, is now a graveyard of felled giants. In Maryland, the hemlocks in Cunningham Falls State Park have been virtually wiped out.

But now, forest managers have some reason for hope. Among them: the discovery of healthy, apparently adelgid-resistant hemlocks in New Jersey, a successful crossbreeding project to produce more adelgid-resistant trees, and the introduction of two new insects that feast on adelgids.

“This is one pest we’re having success at [defeating],” said Craig Kuhn, who heads the forest pest management section of the Maryland Department of Agriculture.

Holding the line

It’s been nearly 70 years since the tiny, voracious, mass-producing woolly adelgids were first found on hemlocks imported from Japan to beautify the Gilded Age Maymont estate outside of Richmond. The pest may also have been introduced to three other estates in the East and South between 1910–15. The insects were first found in Pennsylvania in 1967 and in Maryland during the 1980s. By then, adelgids had begun marching up and down steep ravines and mountainsides and into people’s yards.

Adelgids kill by inserting their mouth parts into twig tissues near the base of needles. This feeding consumes plant sugars needed by the tree. The telltale sign of infestation is the white wax-like substance the insects secrete around their bodies.

Mass feeding by adelgids typically kills a hemlock in 4 to 15 years. Others keep living but in a moribund state. “They’ll just start looking like lollipops with just tufts of needles on the top and the rest dead,” Kuhn said.

Forest and park managers have bought time by injecting insecticides into the tree or surrounding soil. That protects the most visible, historic and popular hemlock trees along trails, in parks and along streams where the trees create a unique “microclimate” that dozens of animals, birds and insects depend on.

Native brook trout especially need water cooled by shade from overarching hemlocks, and their disappearance threatens efforts to save the wild trout. Trout are three times more likely to occur and four times more abundant in streams draining hemlock forests than in those draining hardwood forests. For this reason, Pennsylvania state forestry crews treat some hemlocks along at-risk streams.

But because of cost and accessibility, chemical protection in Bay states is mostly reserved for the hemlocks visible to the general public. A chemical effort in Pennsylvania, for instance, has saved hundreds of majestic hemlocks in the Cook Forest State Park. Called the Forest Cathedral, the stand of ancient trees draws tens of thousands of visitors each year. Designated a National Natural Landmark, it is the oldest hemlock grove in the East, with some trees almost 400 years old. In all, 146 stands of hemlocks are chemically treated in Pennsylvania.

In Western Maryland, at Swallow Falls State Park, home to the state’s most popular hemlock stand, as many as 3,000 trees are chemically treated to keep them alive and robust.

New tactics

Meanwhile, a cooperative effort among scientists, land managers and other advocates has been pushing for more solutions. Along with the federal government, they formed the Hemlock Woolly Adelgid Initiative in 2003. The coalition now includes four federal agencies, 20 state agencies, 24 universities, 7 institutions in China and 9 private industries in Japan.

The first defensive effort involved capturing and releasing several predatory beetles that have a taste for adelgids. One species was from the Pacific Northwest. Another species, a pinhead-size type of ladybug, came from Japan.

From 1999 to 2011, more than 2.5 million Japanese ladybugs were released in 15 eastern states. But the insects tended to fly away from targeted hemlock stands.

Two other species of beetles have fared better since releases began in 2005. Established colonies are killing 30% to 40% of adelgid eggs sacs, said Scott Salom, a professor of entomology at Virginia Tech, who has studied the biological control of adelgids since the late 1990s.

Unfortunately, adelgids have two distinct life stages per year, one of them producing a new generation of adults in late spring. And because the beetles do not feed in late spring, the adelgid population has a chance to bounce back.

Enter two species of silver flies that are found on hemlocks in the western U.S. and do consume adelgids in the spring. Since 2015, the silver flies have been released in controlled settings throughout the East.

Though still being studied to make sure they don’t negatively impact native insects, there is hope that the flies may be the missing link in the biological control of adelgids — and the long-term strategy for saving existing mature hemlocks.

“It looks promising because it nails the part of the life cycle [that beetles] don’t get to,” said Donald Eggen, forest health supervisor for the Pennsylvania Bureau of Forestry.

“The great hope,” said David Mausel, a regional entomologist with the U.S. Forest Service, “is that the silver flies paired with [the beetles] is the one-two punch.”

Optimism also is buoyed by experiments with healthy hemlocks found in New Jersey in an otherwise devastated hemlock grove that had been infected for more than 30 years.

In 2015, trees grown from cuttings of those healthy trees, as well as from presumably susceptible hemlocks, were planted in the wild next to infested forests in seven states, including Pennsylvania, Maryland, New York and West Virginia. After four years of observation, scientists reported that 96% of the “bulletproof” trees survived, compared with only 48% of susceptible trees. The resistant trees also grew faster and retained more foliage.

Though some scientists want to grow more test trees and perform more monitoring before declaring victory, many in the fight to save the species would like to start planting the adelgid-resistant trees right away — the sooner the better to restore stricken hemlock stands and the ecosystems they provide. Similar tests are under way in Virginia’s James River State Park.

Another recent development is the successful production of a hybrid hemlock by crossing adelgid-resistant hemlocks from China with the native Carolina hemlocks (Tsuga caroliniana), which grow in the southern Appalachians, including parts of Virginia.

Greenhouse trials of the hybrid occurred at the U.S. Department of Agriculture’s Beltsville Agricultural Research Center in Maryland. Now, research is focused on determining if the hybrids will grow in the range of the eastern hemlock. Attempts to cross Chinese hemlocks with eastern hemlocks were not successful.

Scientists warn that the main use of these hybrids may be on residential land. Most nurseries don’t sell eastern hemlocks anymore because of the risk of infection.

And because the hybrids grow slowly and must be reproduced from cuttings, they may not play a major role in restoring hemlock forests. And in those wild settings, Salom said, “You want the eastern hemlock. It’s just not the same tree.”

A loss that hits hard

Aesthetically, the loss of stately eastern hemlocks would be missed. But the biggest impact would be on the environment. Their deep shade, cool temperatures, water-cleansing ability and slowly decomposing trunks create unique microclimates.

It’s a critical habitat for many terrestrial and aquatic species. Nearly 96 bird species and 47 mammal species are tied to hemlock forests, not to mention insects and amphibians like salamanders. There are unique lichen and plant communities that dwell among the hemlocks.

And wild trout. Because hemlocks store and slowly release water, scientists warn that marginally coldwater streams may dry up if they lose their hemlock buffer.

Attempts to replace dead hemlocks with fir, spruce and pine trees, or rhododendrons, have proven that they are inadequate substitutes. “It’s got a specialized niche in our forest that is irreplaceable,” Salom said.

If being attacked by insects isn’t enough, warming global temperatures are another threat to hemlocks, because cold weather helps to kill off adelgids.

Those involved in fighting for hemlocks’ survival often harbor a soft spot for the tree.

“It’s way more than a timber species,” stressed Mausel of the U.S. Forest Service, who considers hemlocks important for everything from holding tree stands for hunting to writing poetry. “The U.S. Forest Service remains committed to the sustainability of hemlocks, and the next 20 years are as important as the last 20 years.”