In the forests of Appalachia, eastern hemlocks create a microclimate in the persistent shade cast by their branches of dark green needles. The cool, moist conditions — with a soggy layer of leaves and needles and streams filtering through the trees’ shallow, branching root systems — provide habitat for more than 400 species of arthropods and a diverse array of amphibians.
But an invasive pest called the hemlock woolly adelgid threatens the iconic tree and the species that depend on it. For years, forest officials have used a pesticide called imidacloprid to kill the adelgids, but scientists are now discovering adverse effects of the pesticide’s use. A study published last December in Global Ecology and Conservation found that the chemical treatment may have adverse effects on some species of wildlife — including salamanders.
Researchers in West Virginia captured several species of salamanders from streams near hemlock stands treated with imidacloprid, and found traces of the pesticide stored in the amphibians’ tissue. This storage, called “bioaccumulation,” can have significant impacts on the salamanders, of which two species are found only in West Virginia.
“We found a decrease in body condition, which is kind of like body mass index in humans,” said Sara Crayton, a doctoral student at West Virginia University and a lead author of the study. “Lower body condition usually means lower fitness. Effected salamanders will be less likely to survive and reproduce.”
The amphibians bioaccumulate the pesticide both through eating tiny organisms already storing it and by soaking it in directly through their skin. “Our skin serves as a barrier, but theirs has no ability to keep substances from going in and out,” Crayton said, also noting that salamanders breathe through their skin which requires access to moisture in streams and soil. Much of that moisture is lost when a hemlock forest suffers an onslaught from the woolly adelgid.
These findings are yet another setback for land managers hoping to ward off the pest, which is threatening the microclimates that hemlocks provide.
“It feels like a lose-lose situation,” Crayton said. “Lose your hemlocks or potentially cause damage to amphibians. There’s no easy solution.”
Juvenile hemlock woolly adelgids are smaller than the tip of a ballpoint pen. Even when full grown, they’re about as large as a grain of rice and difficult to spot after they’ve embedded themselves near the base of a tiny hemlock needle. What’s easier to spot are their fuzzy white egg sacs — the woolly adelgid’s namesake — that accumulate on the underside of branches.
The woolly adelgid’s fuzzy white egg sacs on the underside of a hemlock branch. The pests penetrate plant tissue with their microscopic mouthparts and disrupt the flow of water and nutrients through the tree. Photo by Connecticut Agricultural Experiment Station Archive/ © Bugwood.org.
A red salamander. The Appalachian region is home to more species of salamanders than any other region on the planet. Researchers have found traces of the pesticide stored in the tissue of salamanders living in streams near hemlock stands treated with imidacloprid. Photo by Amy Enchelmeyer.
The pests penetrate plant tissue with their microscopic mouthparts and disrupt the flow of water and nutrients through the tree. Prolonged feeding will cause needles to take on a pale, grayish hue before falling to the ground. The then-naked overstory allows sunlight to penetrate through to the forest floor. Moisture evaporates from the soil, the temperature increases, and the unique habitat on the forest floor all but vanishes. Even if the hemlocks are replaced by another species, the replacements are often deciduous trees that won’t provide the year-round shade that so many species rely on.
Imidacloprid is currently the most effective means of controlling the pest and is applied by injections into the soil or directly into a tree. Crayton’s findings and other studies, however, highlight the importance of continued research into other control methods.
Forest managers and scientists are experimenting with predator beetles as a biocontrol method, for instance. Beetles from the adelgid’s native range in Asia that eat the pest may be able to help stymy its spread. While the adelgid’s complex lifecycle make predator control difficult, efforts to breed trees for genetic resistance to the pest might provide hope for both hemlocks and salamanders
Benjamin Smith, a researcher with the Forest Restoration Alliance at North Carolina State University, has spent the last decade working on breeding adelgid-resistant hemlocks.
While some small pockets of hemlocks in North America seem able to withstand the adelgids, Smith has had to look beyond Appalachia for genetic resistance to the pest.
“It would be ideal to have a higher level of resistance than just what we see on the landscape right now,” Smith said.
In Asia — the adelgid’s home range — several species of hemlock evolved alongside adelgids and developed physical and chemical defenses that prevent mortality. By hybridizing native eastern hemlock with Asian hemlock species, Smith is trying to foster seedlings that have genes for resistance but also genes from native parents to fill the void left by mature hemlocks killed by the woolly adelgid.
This process is called backcrossing, in which an Asian species is crossed with a native species for many generations with the goal of producing a tree ready for planting in Appalachian forests. With each generation of hemlock that Smith grows in the lab, extensive testing is done to identify and weed out the non-native genes that don’t contribute to resistance. Once a hybrid is established, it can be bred once again with a native hemlock to retain local characteristics and ecosystem functions.
Though progress in hemlock hybridizing is exciting, it’s not a cure-all for the adelgid problem.
“Even if we had resistance tomorrow, that won’t help the trees that are 200 years old and 150 feet tall,” said Smith, adding that other approaches should continue to be utilized, including biocontrol and chemical treatment.
The largest eastern hemlock ever recorded was 173 feet tall, but by the time it was measured it had already died from the effects of the woolly adelgid. Surviving hemlock giants are most vulnerable in the Southeast, but the pests are moving west and north — with a warming climate, this insect is increasingly prevalent in places where cold temperatures once kept the adelgid in check.
As the hemlock woolly adelgid spreads at a rate of about 10 miles a year, Crayton, Smith and others studying hemlocks and the ecosystems they support hope to identify a solution before it’s too late for the quintessential conifer.
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