Invasive Insects: Achilles heel of the northern forest
By Allaire Diamond
A microscopic insect, just hatched, crawls out of a bird feather and sinks its mouthparts into the base of a hemlock needle.
A brilliant green beetle, smaller than a penny, nestles beneath the bark of an ash tree and chews on its nutritious cambium layer.
A large white-spotted black beetle, with curving 'horns' longer than her body, gnaws a pit in the bark of a sugar maple tree to lay her eggs.
Individually, the lives of these insects—the hemlock woolly adelgid, the emerald ash borer, and the Asian longhorned beetle—follow the basic pattern of evolution: hatch, eat, reproduce, and die. Collectively, however, they form huge invasive populations freed from the limitations of their native Asian habitats, populations that threaten to damage and forever change the forests of Vermont. Their arrival and spread reflects human movements, behaviors, economies, and knowledge in our interconnected region and world.
Like the arrow in Greek mythology that ultimately killed Achilles, these exotic invasive insects kill trees by attacking their most vulnerable parts.
Emerald ash borers and Asian longhorned beetles feed on trees' living cambium layer by excavating extensive 'galleries' under the bark, disrupting the movement of water and nutrients throughout the tree and eventually girdling and killing it. At that point, the insects or their offspring depart for greener pastures. The hemlock woolly adelgid's interaction with hemlock trees remains mysterious, but the impact of its populations is unequivocal: stands are reduced to brittle skeletons as needles yellow and drop for two to six years until the trees die.
Are these insects in Vermont?
For now, only the hemlock woolly adelgid has entered Vermont. After appearing in Rockingham in 2007, it has now been detected in 38 locations in seven towns in the southeastern part of the state.
The emerald ash borer, which feeds on all species of ash, waits uncomfortably close to our door. In 2010, the insect was detected 30 miles from Vermont in Quebec and New York. Both the emerald ash borer and hemlock woolly adelgid have spread in natural forest stands.
The Asian longhorned beetle targets many of our most valued hardwood species, and has the potential to affect diverse hardwood stands in ways that the other two insects, who only associate with one type of tree, do not. So far, its impact has remained concentrated on the urban forest of nursery stock and street trees in several cities.
How dire is the threat?
Vermont's forest professionals forecast a grim future that can be mitigated, but not avoided, especially with regard to the hemlock woolly adelgid and emerald ash borer.
Unfortunate scenarios have played out elsewhere: old growth hemlock stands of the southern Appalachians are no more; upland hardwood forests of Michigan are largely ash-free; and Worcester, MA is bare and shadeless, after the Asian longhorned beetle killed thousands of trees along city streets. Those emerald ash borer and Asian longhorned beetle outbreaks are thought to have started 10-15 years before they were detected.
Economically and ecologically, these infestations devastate. Annual contributions to the Vermont economy by the forest industry and forest-related tourism and recreation are estimated at $1.5 billion. Eastern hemlock is Vermont's seventh most common tree in forests. Its highest value is ecological as it shades streams, providing essential cold-water habitat for fish and critical winter shelter for wildlife. Without it, fish habitat would be compromised in hundreds of miles of streams and deer populations could crash during severe winters for lack of adequate cover. White ash is a valuable timber tree and green ash forms riparian forests that prevent streambank erosion. And black ash is prized in basketry.
The Asian longhorned beetle attacks over 15 families of hardwood trees, but is especially fond of maples.
A group effort
Fortunately, many government agencies are involved in detection and monitoring efforts, eradication schemes, public education, and research on the insects' biology and impact. For example, this summer we will see a massive, coordinated "purple trap" effort to catch emerald ash borers. In all counties but Essex, one of these traps will be hung every two square miles: a total of 2,200 traps that will be monitored throughout the season. If you see a down trap contact the USDA at (866) 322-4512.
Just as these insects arrived because of human actions, we are still their primary vector into new areas. While adelgids can hitch rides on bird feathers and feet, and the beetles can fly, new infestations are frequently traced to firewood and nursery stock. In response, plant pathologists check every hemlock tree that enters the state as nursery stock, and hemlock from adelgid-infested areas is barred from entering Vermont. Nursery stock from places affected by Asian longhorned beetles is similarly examined.
"Public awareness is our most powerful tool," said Emilie Inoue, State Survey Coordinator for the VT Agency of Agriculture, Food and Markets. Armed with awareness, visitors can buy firewood locally when they travel, contact a professional if they suspect they've found an insect or its sign, and serve as "eyes on the ground." Emilie explains that every detection nationwide has stemmed from a private citizen finding and reporting something suspicious. Even if a potential exotic insect turns out to be something else, a false alarm is better than an undetected invasion.
What can you look for?
Emerald ash borers and their D-shaped exit holes are distinctive, yet very small and difficult to spot on the furrowed bark of ash trees. Easier to spot are the gnawed mazelike galleries under the bark once it begins to slough off. Woodpeckers often attack infested ash trees to get at the beetles and larvae, so looking around woodpecker holes could reveal chewed areas. Emilie notes that ash in general seems to be in decline across the region, so a tree with yellowing leaves or dieback does not necessarily indicate borer presence. She recommends that people keep track of healthy trees that suddenly begin to decline, and examine those individuals closely for signs of emerald ash borer.
Asian longhorned beetle adults are ¾ to 1 ¼ inches long with distinctive white and black-banded antennae and black bodies with white spots. Entomologists at UVM recommend scanning hardwood trees (except oak) for these insects between June and September, standing about five feet away from the trunk of the tree and slowly looking up and down with binoculars.
The tiniest invader, the hemlock woolly adelgid, is difficult to spot, but infested trees often have a 'snowy' appearance from the white fluff on the adults' bodies, which are clustered around the underside of needles on new growth. Infested hemlocks also exhibit needle die-off during the growing season.
If you burn wood, keep an eye on your firewood when cutting or stacking and call one of the people listed in the box below if you see tunneling or an insect or larva that looks unusual.
Once the insects arrive, what can be done?
Infested trees should not be left alone. If landowners do not want to cut, chip, and burn infested trees from their properties on site, those trees can be treated with a variety of approaches.
Trees infested with hemlock woolly adelgid can be treated with foliar (leaf) sprays, consisting of horticultural oil or insecticidal soap, fungal insecticides, or contact insecticides. Insecticides can also be injected into tree trunks, minimizing their impact on the surrounding area. Forest resource professionals do not recommend wholesale harvesting of Vermont hemlocks in preparation for hemlock woolly adelgid, as cold winter temperatures may cause its impact to be less than in areas to the south.
The emerald ash borer's entry into Vermont seems imminent, and state and federal experts have some suggestions for landowners wishing to prepare. Landowners planning timber sales in stands with more than 20 percent ash may want to consider treatments to enhance other species. Since the emerald ash borer targets stressed ash trees, removing these may also be an option if forest management is planned. Landowners and municipalities considering ornamental or street tree plantings should consider other species besides green ash. Wholesale salvage logging in preparation of future invasions is not recommended. If and when the insect arrives, some experts suggest that girdling stressed or less valuable trees in a sensitive ash area could keep the emerald ash borer from more valuable trees.
There are no current recommendations for Asian longhorned beetle management in Vermont, other than vigilance for detection. In places where it has been found, trees are currently being cut, chipped, and burned.
VLT Stewardship Foresters Dan Kilborn and Pieter van Loon frequently speak with VLT landowners and their foresters about these insects. Through observation and knowledge-sharing, people have the potential to limit the devastation caused by invasive insects. As the owners of over half a million acres of Vermont, VLT's hundreds of farm and forest landowners form a powerful, knowledgeable, and concerned group whose "eyes on the ground" can truly make a difference.
What you can do?
- Be aware of hemlock, ash, and hardwood quarantines set up by state and federal agencies if you are buying wood products or transporting hemlock from an infested area.
- Don't move firewood more than 50 miles from where it grew; forest landowners could consider selling firewood to visitors to Vermont.
- Learn to identify these insects and their distinctive signs. Monitor trees on your land and in your community and notify your VLT foresters (Dan Kilborn or Pieter van Loon), county forester, Emilie Inoue (802-241-4091) or the Department of Forests, Parks and Recreation's Forest Biology Lab (802-241-3606) if you have any suspicions.
- For hemlock woolly adelgid, remove birdfeeders April through August.
- Visit www.dontmovefirewood.org to learn more. Spread the word! VLT members and landowners are some of the most informed advocates for Vermont's forests.