An Invasion intervention An insight by William Langhorne

On a July afternoon in 1936, President Franklin Roosevelt stood backed by the Blue Ridge Mountains of Shenandoah National Park and delivered a promise. “Through all [national parks] we are preserving the beauty and the wealth of the hills. … We seek to pass on to our children a richer land.” Less than 90 years later, this promise is under serious threat by, of all things, an insect slightly larger than a grain of salt.

Hemlock woolly adelgid, a species of insect native to Japan, first invaded the United States in the 1920s. It feeds on the sap of the eastern hemlock, an ancient and ecologically vital species, and can kill a tree in four to 10 years. Unchecked by natural predators and spread by birds, wind and infected vegetation, there has been little to stop its rapid march across New England and Mid-Atlantic States. In the late 1980s, infections broke out in Shenandoah National Park and the forests of Connecticut. By 2002, it had reached Great Smoky Mountains National Park. In 2015, the National Park Service estimated that the infestation had killed up to 80 percent of Shenandoah National Park’s hemlock population.

David Orwig, a senior ecologist at Harvard Forest, has been monitoring the impact of hemlock woolly adelgid in woodlots near Petersham, Massachusetts. These forests contain eastern hemlocks ranging from 70 to 200 years of age.

“The vast majority of these hemlock trees are already infested with woolly adelgid,” Orwig says in a video posted on the Harvard Forest site. “We anticipate tremendous change over the next decade or two.”

Orwig considers the adgelid, which has already killed hundreds of thousands of trees in 18 states within the past thirty years, a serious threat to Northeastern forests. The dynamic of the Harvard Forest has already started changing. Deciduous trees such as the black birch have begun replacing the towering evergreens.

As the hemlocks die off, their deep canopies will disappear, changing the feel of these ancient woodlands. The open forest floors, covered in spongy carpets of evergreen needles, will give way to shrubby undergrowth. The cool, moist environments created by the shade of their branches will vanish. Streams flowing through these woods will warm, putting stress on trout and other cold-water fish. Slowly but surely, this invisible species is dismantling the region’s visual majesty.

According to a U.S. Forest Service study, the eastern hemlock also plays an important role in regulating forest water cycles. The large surface area of its evergreen canopy transpires, or releases water, in large quantities during the winter. The hemlock’s replacements — deciduous trees, which transpire in the spring, and shrubby evergreens, which have a smaller canopy area — will not be able to maintain this winter water release. This change in transpiration levels will likely cause a rise in stream flow in the summer and a decrease in the winter, further altering the dynamics of old hemlock forests.

But hope remains. Some think the threat of the adelgid can still be stopped. Since Carole Cheah was hired as a postdoctoral researcher in 1994, she has been working at the Connecticut Agricultural Experiment Station to regulate hemlock woolly adelgid. Her research focuses on using Sasajiscymnus tsugae, a Japanese lady beetle, as a method of biological control. Cheah thinks this beetle is especially promising since it has the longest annual predatory impact on the adelgid populations and can be easily mass reared in labs.

Directly monitoring the impact of the beetles is done completely by hand and requires careful inspection of tree branches. “I’m a one-woman show,” said Cheah. When she started, her team was larger, but it has since been reduced due to insufficient funding. This has led her to focus on gathering tree health data in areas where the beetle has been released rather than monitoring the beetles by hand. However, the Maine Forest Service, with its larger workforce, has conducted extensive monitoring and found that despite cold winters, the beetles have established colonies along the coast.

Nonprofessionals can now take part in this form of biological control thanks to the commercialization of the Japanese lady beetles. Tree Savers, a company based in Pennsylvania, has been mass rearing and selling the adelgid predators.

Photo by Carole Cheah

“Their lab is top notch and they really care about the quality of the beetles,” Cheah said. These sesame seed–sized insects are sold for $2 apiece, which Cheah said is reasonable when considering the large impact a small number of beetles can have on controlling adelgid spread. Beetles released by Tree Savers have caused significant reductions of adelgid numbers in southeastern Pennsylvania. Last year the company donated 2,000 beetles to Cheah, allowing her to expand her research into new state forests.

Overall, Cheah is very optimistic about the decline of adelgid populations. In addition to the impact of the Japanese lady beetles, the data she has gathered on adelgid winter mortality since 2000 have shown that New England winter weather can have a drastic effect on controlling the pest.

Cheah has also noticed that climate change has had a positive impact on reducing the adelgid populations. Northern hemisphere polar vortexes, weather phenomena in which cold air descends south from the North Pole, have been increasing in strength and frequency in recent years. As this cold air reaches New England, the effects of the winter kills are magnified. In 2016, an unusually warm winter, a polar vortex caused a cold snap that lasted only a few hours but brought temperatures as low as minus 22 degrees Fahrenheit in some parts of Connecticut. Normally, adelgid mortality rates can rise above 90 percent in typical winter temperatures. This sharp burst of cold killed 98 percent of adelgids across the state.

While these results are very promising, adelgid populations are always able to bounce back during the spring. As an all-female species, these insects reproduce parthenogenetically, meaning they do not have to mate to produce offspring.

“Any survivors can just explode. One female is capable of producing over 250 eggs. I once counted up to 500,” Cheah said.

She believes the Japanese lady beetles will be most effective after these mass spawnings.

“It would be a very good system,” Cheah said. “They would come in and just clean off the adelgids, regulate the numbers that survive from the winter.”

With the adelgid population presently under control, Cheah has turned her attention to two other factors, which she believes are having an even bigger impact on the eastern hemlocks of Connecticut. Her recent research has shown that the past two years of drought have severely weakened the moisture-sensitive hemlocks. In combination with the resilience of elongate hemlock scale, an invasive insect that is capable of withstanding New England winters, this effect has caused more devastation than the adelgid infestation.

“I believe [the elongate scale] has been the quiet enemy, not getting all the fanfare that the adelgid does,” Cheah said.

Even in the southern Appalachians, where the adelgids are a more significant threat, other factors have made it difficult to determine the main culprit of hemlock decline. The proliferation of native insects such as hemlock borer and hemlock looper have further strained the already-weakened trees. In recent years, drought has also had a big impact in the region.

“Very few reports will tell you about these droughts,” she said. “They want you to concentrate on the insects. It’s much more exciting to say the trees are dying from the insects than to say years of droughts also helped to kill the trees.”

Several studies based on hemlock pollen records have found huge decreases of hemlock populations about 5,000 to 9,000 years ago due to drought alone.

“It’s a complicated picture out there,” said Cheah.

Despite these compounding factors, Cheah is still optimistic for the future of the eastern hemlocks. She doesn’t believe they will go extinct, at least not in the northern parts of their range, such as New England.

“There’s a lot of literature out there … that says it’s on the brink of extinction, but I’m out there all the time. I’m amazed at how resilient these hemlocks are.”

If these regions get more precipitation, Cheah is certain the health of the hemlocks will improve and that they will have a better chance of fighting off the insect infestations.

In concluding his speech on that afternoon in July 1940, President Roosevelt addressed the importance of not only the “recreation” but also the “re-creation” that Americans would find in their national parks. The future of the eastern hemlock might not be as dark as it seems.


Photos by Schirin Rangnick and Carole Cheah

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