by Marisa Baldine
December 02, 2021
Eastern hemlocks are an iconic tree species that grow throughout the eastern United States. and follow the Appalachian Mountains all the way south to Georgia. The towering deep-green trees can grow in groups and reach heights of close to 200 feet tall. These trees benefit the ecosystem in a variety of ways but their future is threatened by the invasive insect, the woolly adelgid. These gigantic trees take hundreds of years to grow and are being wiped out in less than a century.
The benefits of hemlocks
These large trees are able to act as buffers that reduce the amount of stormwater runoff and absorb excess nutrients from the soil. The dense branches shade and cool the surrounding soil and streams, while the understory (the area directly under the tree), can be even cooler than that—up to 10 degrees in fact! Species sensitive to temperature changes, like brook trout and salamanders, are often found in streams near hemlock forests. In fact, brook trout are four times more abundant in streams bordered by hemlock trees than those bordered by hardwood forests.
Hemlock trees provide food and habitat for more than 120 species of wildlife. The dense canopy creates an ideal nesting habitat and protection from the elements. Deer will rest under hemlock trees during heavy snowstorms. Hemlock bark, branch tips and seeds provide food for a wide range of species including porcupines, snowshoe hares, northern flickers, black-capped chickadees and American goldfinches.
A growing threat
Aerial photographs of hemlock forests present a frightening preview of the future. The remaining bright green hemlocks are surrounded by ghost-like remnants of trees that have been killed by the hemlock woolly adelgid.
How can a tiny pest smaller than a thumbnail cause such a big problem? These insects eat the food storage cells found in the hemlock’s branches and reproduce at an alarmingly high rate. All of the woolly adelgids in eastern North America are female but they are capable of reproducing asexually twice per year Each year, a female can lay 200 eggs. And within the same year, those eggs reach sexual maturity and being reproducing. Put together, an adelgid infestation can kill a full grown hemlock tree in less than four years.
The pest originally entered North America in the mid-1900’s and quickly spread throughout forests in the eastern United States. One of the only things that limited the range of the woolly adelgid was cold weather, but as climate change has warmed our typical winter temperatures, its range has expanded farther north.
Hope for the future
Researchers are frantically working to save the remaining hemlocks. Desperate times call for desperate measures and many agencies have started using chemical agents to control the adelgid population. These chemical sprays are usually applied to the bark or soil below the tree. Chemical controls are used as a temporary technique until a long-term solution is found. New research shows that chemical controls might be negatively impacting salamanders by reducing their ability to grow, but without these chemical controls, the forests the salamanders depend on will be destroyed.
However, researchers have seen some promising results from biological controls and breeding programs. Laricobius nigrinus, a beetle that is native to the west coast of North America, has been introduced to the east coast to control the adelgid population. Although the 400,000 beetles released in the eastern United States have been effective at destroying the egg clusters they come into contact with, they are only active during one of the two adelgid breeding seasons. The eggs that survive quickly reproduce and the population rebounds. Seeing some success with this method pushed researchers to consider using other biological control methods. They soon realized that two bugs might be better than one and started releasing both Laricobius nigrinus and silver flies. Like Batman and Robin, each of these bugs picks their own fight—Laricobius nigrinus takes out the fall population of eggs, while the silver flies attack the spring populations. Together they can drastically reduce the adelgid population.
Researchers are also working on carefully breeding regional hemlock trees with those that have shown resistance to the woolly adelgid. These breeding programs will retain the unique regional genetics but with the addition of the genetics of resistant trees. Despite the havoc inflicted by the woolly adelgid, researchers have occasionally found trees that have survived an infestation. In a field of dead trees there will sometimes be a small patch of healthy, green hemlocks. They call them “bulletproof” trees that create a higher content of the insect repelling compound called terpenes. To save the eastern hemlock, researchers have been cloning these trees in hopes of rebuilding hemlock forests. The cloned “bulletproof” trees have been cloned and replanted with promising results. However, both breeding and cloning take significant time because hemlocks are slow growing and it takes at least ten years for a tree to start reproducing.
What comes next?
Hemlocks cannot be easily replaced. In mixed hardwood forests, if one tree species dies, another can often take their place. In a hemlock forest, they are often the only tree and the entire ecosystem revolves around the environment created by the hemlocks. Researchers are unsure of what will fill the void left by hemlock groves if the woolly adelgid is not controlled. The sensitive, unique ecosystem surrounding the hemlocks cannot withstand the sudden changes.
Researchers are diligently working to find the best combination of controls to beat the hemlock woolly adelgid but they need more time to see if these solutions work. Here are some things you can do to help stop the spread of the hemlock woolly adelgid:
- Monitor hemlock trees on your property so you can act quickly if they are infested.
- Consult your local cooperative extension office if you find woolly adelgids on your property. They are most commonly seen from November to April.
- Keep bird feeders far away from hemlock trees. Migrating birds can carry the tiny adelgids and help them spread.
- Inspect all nursery plants, tools and organic materials purchased from a nursery.
For more information on the plants and trees found in the Chesapeake Bay watershed, visit our field guide.