Hunting for an insect the size of a poppy seed in a forest of trees that can soar 100 feet high is, admittedly, a tall task.
But after seeing the “gray ghosts” of dead eastern hemlock trees haunt forests farther south, officials in New York have been intent on preventing the same fate for the state’s eastern hemlocks as the aphid-like hemlock woolly adelgid (HWA) creeps north.
The effort takes collaboration—and layers of tactics. One method of detection views the forest canopy through satellite imagery; another may examine DNA from a single branch.
The nearly 50-year archive of Landsat satellite data, processed and distributed through the USGS Earth Resources Observation and Science (EROS) Center near Sioux Falls, SD, is one tool being used in that collaborative effort.
That historical record can be especially important for a threat that has emerged over several decades. HWA was spotted in Virginia by way of Japan in 1951, according to the U.S. Forest Service. It was detected in southern New York in the 1980s and has moved through Hudson Valley and the Catskills.
In 2017, a researcher spotted HWA in Adirondack Park to the north, where dense stands of hemlocks grace the park’s 6 million acres of protected public and private land. HWA was confined to a couple of trees on Prospect Mountain, and the New York State Department of Environmental Conservation (DEC) quickly eradicated it.
Then last summer, a visitor in a DEC campground on the shore of nearby Lake George reported spotting HWA. This time, the DEC discovered an infestation that had already spread over 200 acres.
‘They lost all their hemlocks. Their mountainsides are just gray’
There’s never a good time to find a tree-killing insect in a forest. But the Adirondack hemlocks stand a better chance than their southern brethren because more research, detection methods and treatments exist now for HWA. In addition, colder winters and shorter growing seasons can help slow HWA progression.
In the Appalachians, “they didn’t have the tools to jump on it back then, and they lost all their hemlocks. Their mountainsides are just gray,” said New York State Forester Rob Davies with the DEC.
“We want to try to avoid that because hemlocks are incredibly important from an ecological standpoint. Hemlock forests are very dark. Their canopy is really thick. And it keeps our streams cool, which is critically important for the fish habitat. It keeps our waters clean,” Davies said.
Residents and visitors also benefit from hemlocks, said DEC forester Bryan Ellis. “People come from all around to recreate and enjoy the hemlock resources that provide a really unique environment around our lakes, on our mountains and our hiking trails.”
The DEC isn’t alone in trying to save the hemlocks. It’s getting help from partnerships with private groups, university researchers, and federal agencies.
Exploring Ways to Detect HWA
Help is welcome because HWA detection can be difficult, depending on the time of year.
“When we do ground surveys, it’s boots on the ground flipping over branches,” Ellis said. The sap-feeding insects may infest a tree toward the bottom, but also may appear toward the top, where climbing would be the only way to find them. Their namesake wool covering is distinctive in late fall to early summer. However, the Lake George discovery coincided with the time of year when HWA is most difficult to spot, so surveyors used hand lenses to look for the tiny dark specks.
Ellis and his DEC colleagues are hopeful remote sensing can help.
“The application of remote sensing to forest health in New York has been something we’ve been trying to increase … mainly because we have too much land and too many use impacts for the size of the resources we have to deploy,” said DEC research scientist Jerry Carlson.
Seeing Change from the Sky
Assistant professor of ecology and environmental science Andrew Reinmann’s group at the City University of New York Advanced Science Research Center had been linking invasive species field work with remote sensing in the Catskills. That caught the attention of people in the Adirondacks, including the private group Fund for Lake George.
As a result, a few months ago, Reinmann’s group started using Landsat satellite data to detect forest change in an area that includes the southern Adirondacks—up to 15,000 square kilometers. Although defoliation from HWA may start low on the tree, Reinmann expects Landsat to reveal a decline in canopy greenness because of changing chlorophyll content.
For each 30-meter Landsat pixel of the area, taken from early July in 2016 through 2020, they’re quantifying a value for normalized difference vegetation index (NDVI), which tracks vegetation greenness via the red and near-infrared bands of the electromagnetic spectrum. Then they’ll look for a change in greenness over time, and the extent of its change, for indications of an invasive species.
A small, steady decline can be investigated by field crews. Reinmann welcomes the chance to put academic work into the hands of those directly working with the forest.
“Of course, we don’t want to find any adelgid, but if it is out there, we’re hoping that this process helps to increase the number of places where we can find it. And while these crews are walking around, they’re able to now collect more data about canopy condition and canopy health than what they were doing before,” and that will help improve the remote sensing portion, Reinmann said.
Looking Back Can Help Look Forward
The ability to go back in time with satellite data can add valuable perspective when interpreting a disturbance in landscape. With its unrivaled archive, Landsat is useful for learning from previous invasive species infestations elsewhere and then monitoring for new infestations, said Dr. Francis Dwomoh, a remote sensing scientist with a background in forestry at EROS.
“Forests and ecosystems like that are the life support systems for all of us. So critical monitoring to see where things are going wrong, or even where things are going right so we can replicate things, it’s very important,” Dwomoh said.
“We can design early warning systems … so that we can intervene and then conserve these areas,” he added.
Reinmann and the New York foresters also see potential for using 3D light detection and ranging (lidar) technology to provide information about the lower part of the forest canopy.
Remote sensing techniques can help prioritize forest management resources, said Brent Kinal, a DEC research scientist with a background in geographic information systems. “Having some data, almost in real time, that is showing potential stressors in a given area will help us deploy those field crews to areas that might be experiencing some sort of forest health issues rather than just having them look around for needles in a haystack.”
Another tool the DEC hopes will help in the future is environmental DNA. HWA DNA has been detected in water that washed off hemlock branches without a visible infestation.
Challenges of Treating HWA
Another challenge for foresters combating HWA is deciding how best to treat it.
Successful treatment can involve a rapid response with pesticides, particularly for a small infestation such as the 2017 discovery. Last fall, the DEC could treat only some of the trees in the infested area because of the tree density and regulations on the pesticides. The location itself, along the 32-mile-long Lake George, is also a challenge because it’s accessible only by boat.
In places where HWA is firmly established, such as the Catskills and Hudson Valley, foresters must choose which areas to prioritize for treatment, such as trails, other highly visible areas, and old-growth stands.
Biological controls are considered the long-term answer for infestations, the foresters said. The DEC is a partner, along with the U.S. Department of Agriculture and the Environmental Protection Agency, in the New York State Hemlock Initiative at Cornell University, which was established to further research and implementation of biological controls.
A beetle has been released and established, and a couple of silverfly species have been released, in certain areas in New York to prey on the HWA. “We’ll hopefully, when we see that established, get some really lasting protections for our hemlocks,” Ellis said.
HWA still won’t go away. But biological controls and an array of detection methods aim to keep the invasive population under control—and keep the vibrant, vital hemlocks from fading into gray ghosts.