Deep in the woods of Maryland, a hidden war is being waged. On one side, the native trees that have provided food, shelter, clean water and beauty to the region for thousands of years. On the other, foreign invaders that have upset the natural balance of the state’s ecosystems and are threatening to make the forest less diverse and a less enjoyable place to visit. The winner will help determine if and how our natural systems will continue to deliver these benefits to both animals and people in the face of future threats such as climate change and development.
This battle is not new; non-native pests and pathogens have been attacking our natural heritage for more than 100 years. The chestnut blight, a fungus introduced to Eastern forests from China around the beginning of the 20th century, quickly brought the American chestnut, one of our most ecologically and culturally important trees, to functional extinction throughout the eastern United States. Additionally, since the late 1800s our state’s forests have been battered by frequent outbreaks of the gypsy moth, an invasive from Europe, which defoliates and eventually kills thousands of acres of forest annually. But this newest wave of invasives may prove fatal to other species that are some of the most ecologically valuable trees in the forest.
The first pest, the emerald ash borer, is a beetle native to Eastern Asia that was accidentally brought to the United States in the bottom of a cargo ship sometime in the 1990s. Originally discovered in Michigan in 2002, it arrived in Maryland the next year at a nursery in Prince George’s County, and despite a ban on transporting firewood has since expanded its range to other parts of the state, reaching the Eastern Shore in 2015. The emerald ash borer, or EAB, lays eggs in the bark of ash trees. When the larvae hatch, they bore into the trunk and feed on the sugar-rich sap within. This destroys the tree’s phloem, robbing it of its ability to transport water and nutrients between tissues and eventually killing it. The second invader, the hemlock woolly adelgid, or HWA, is a small aphid-like insect native to Japan. It arrived to the Eastern US in Virginia in the 1950s, reaching Maryland by the 1980s. These insects feed on Eastern and Carolina hemlocks, and can be easily identified by the white wooly sacs that protect the eggs. When the eggs hatch, their larvae crawl to the bases of the needles on the hemlock branches and feed on the sugars the tree produces via photosynthesis. This, combined with the toxins they sometimes inject in to the phloem, causes the needles to dry out and drop off the tree, leading to its death.
Both of these species have the potential to drastically alter the composition of Maryland’s forests by virtually eliminating ash and hemlock from the landscape. Aggressive infestations of both EAB and HWA can kill a target tree within four years, and studies have found that once established in a stand of ash or hemlock, tree mortality rates can reach 98 percent. This would be a major loss on several fronts. Ash trees are valued economically for their furniture-quality wood, culturally for their use in traditional Native American crafts and ceremonies, and ecologically as a source of food and protection for a variety of animal species as well as an important actor in preventing soil erosion along river banks. Similarly, hemlocks play a vital role in the riparian and cove habitats in which they are found, improving water quality by preventing soil runoff and providing shade to keep stream temperatures suitably low for species like brook trout. Additionally, both ash and hemlock are commonly used as ornamental trees in urban and suburban areas, meaning that this battle between tree and pest could be occurring in your backyard right now. Local tree loss will have regional effects too – dying ash and hemlock means more erosion and runoff into streams and rivers, which in turn means a more polluted and less healthy Chesapeake Bay.
So how can we turn the tide in this war? Bring in the reinforcements. As this year’s Chesapeake Conservation Corps member at The Nature Conservancy MD/DC chapter, I am working on a project to tilt the scales in favor of forests. The reason both EAB and HWA are such a grave threat to forest health is that neither have natural enemies in the area. Without predators to keep them in check, they have been free to run rampant. Therefore, classical biological control has been identified as the best long-term option for management. Classical biological control, or biocontrol, involves finding a natural predator that is specific to the target pest and effective in reducing its population. In EAB’s case, there are three parasitic wasp species that have been approved for release in the United States and have proven successful at establishing at release sites. For HWA, there are three beetle species, two from the Pacific Northwest and one from Japan, which have successfully been released and established in the region or are slated for release in the near future. My role in this effort is to write internal proposals for the release of these biocontrol agents on TNC lands, and then (pending approval) assess our preserves in the state to determine which would qualify as high-potential release sites. Ideally, I will be able to make some initial releases by the end of the summer, but that step may fall to my successor. Nevertheless, it is an exciting project that has the potential to help protect Maryland’s forests from degradation and protect the health of the Bay. The Nature Conservancy’s preserves encompass some of the most unique habitats in the state, and guarding them against invasion is good for both the rare species that dwell in those areas and for all of us who want to visit and enjoy them.
Nobody knows who will emerge the victor in this decades-old struggle between native trees and the invasive pests and pathogens seeking to destroy them. In an age of ever-increasing globalization, new invasives will continue to arrive at our shores, further weakening the integrity of our forests and requiring renewed searches for control options. Meanwhile, climate change will bring new stressors to bear, such as drought, wildfires, and storm damage. In order to save Maryland’s forests and the myriad services they contribute to our communities and the Chesapeake Bay watershed, we must all be aware of what is going on in our backyards and nearby woods, and support efforts to protect them. Only by working in tandem with state agencies, conservation groups, and local communities can we contain and control the invasives here now, and protect the health and intactness of our forests for years to come.
Andrew Ireland is a Chesapeake Conservation Corps member at The Nature Conservancy.