Hedgerows, Dead Trees, and the Emerald Ash Borer

by Tim McKay

PEACHAM – I grew up with hedgerows full of dead elm trees. Now, we are faced with the prospect of hedgerows full of dead ash trees.

In 2018, I wrote about the arrival of the Emerald Ash Borer (EAB) in central Vermont. Since then, EAB has been found in scattered locations all over the state. It is time to talk, practically, about what individual landowners can and should do. Looking at the many hedgerows around fields and villages we see lots and lots of ash trees. What do we do?

All landowners need to be informed and stay informed. Vermont has an excellent portal to all EAB information. Go to vtinvasives.org and you will find lots of information. The first step is to be able to recognize an ash tree so that you can walk your land and see what you have. 

The EAB has been found in several places in and around Groton State Forest, affecting all the surrounding towns. In New Hampshire, EAB was detected in Concord six years ago and infested areas are now found from the Massachusetts line to Lake Winnipesaukee, including southeastern Grafton County. To the west, EAB was first discovered in western New York in 2009 and along the Hudson River south of Albany in 2010. Infestations now occur in 30 of New York’s 62 counties. EAB is well established in the Montreal area and has been found in Grand Isle, Chittenden, Addison, and Orleans counties. Go to vtinvasives.org to find the latest map of where EAB has been confirmed. 

If they are not here already, the first EABs will alight near you within a few years. The insect may not be detected for three or four years after it arrives, while a population becomes established. By Year Five the population will begin to grow exponentially, reaching a killing crescendo in another couple of years. By Year Ten virtually all our ash trees will be dead and the EAB population will crash and reach a low endemic level. Ash seedlings will continue to grow, but will be killed as saplings by the endemic EAB population. With luck, we will be left with a few mature trees that are somehow resistant.

The EAB adults feed on leaves and cause little damage. They lay eggs in crevices in the bark in mid- to late-summer. The larvae then bore inward to the phloem (inner bark where sap is carried from the leaves to other parts of the tree) and the outer sapwood where the tree is growing. They begin to feed on those tissues until the weather gets too cold. They feed in a serpentine pattern, leaving characteristic S-shaped galleries under the bark which get larger as the larva grows. A fully mature larva is about an inch long. After over-wintering, a mature larva pupates and develops into an adult as the weather warms. In Vermont’s climate, the adults begin emerging in June. They bore out through the bark, leaving a telltale D-shaped hole about an eighth of an inch across, and immediately fly to find leaves to eat and other adults to mate with. Then the adult lays about 200 eggs and starts the next cycle of life.

The infestation is not normally noticed until the tree is severely damaged. The early action is usually high in the crown of the tree. Dead branches appear, but many things can cause dead branches in a tree. Woodpeckers move in to drill for the EAB larvae and other delicacies. If an area of trunk is already badly damaged by the insect, a woodpecker will use its beak to break off pieces of bark, revealing the EAB S-shaped galleries on the bare wood underneath.

It takes two or three years for the EAB to kill a tree. Everywhere it has been, the insect has killed virtually all the ash trees. Only about one percent of ash trees appear to be surviving, presumably because of some resistance in that tree’s genetics. The best we can hope for is to slow the spread of the insect. We also need to leave some ash in the hope of perpetuating the species. Young, pole sized trees are of little economic value but are producing seed that will germinate. They will still be fine for firewood after they die, so leave them to their fate. Ash seedlings will grow and some will survive long enough to produce seed themselves before the EAB kills them. Thus, the gene pool will be perpetuated.

Ash trees dominate some hedgerows. Hedgerows get a lot of sun and often contain shrubs as well as trees. From a wildlife perspective, they are edge habitat. Flowering can be prolific and important to pollinators. Various mammals use hedgerows as travel corridors. Peacham village is surrounded by open fields, which make hedgerow travel corridors particularly important. 

The loss of ash trees could be an opportunity for you to enhance your hedgerow as wildlife habitat. Evaluate the ash trees in your hedgerows first in terms of hazard. Trees that will threaten nearby buildings or driveways should be taken down before they start dying because cutting dead trees is more unpredictable and therefore dangerous. There are numerous folks in Caledonia County who specialize in taking down trees safely. Put a note on Front Porch Forum and you’ll get some recommendations.

Also, evaluate the trees for commercial value. Some hedgerow trees have sizeable, straight trunks that contain valuable lumber. In a hedgerow, the lowest four or five feet must be left because of the likelihood of fence wire being embedded in the wood. But if the tree has a trunk with minimal or no branching that is at least eight feet long and ten inches in diameter at the small end, you have a marketable log.

To attract a small-time logger to cut and buy these logs you will probably have to join with your neighbors to get enough to make it worth the logger’s while. Of course, these trees also yield firewood. A forester should be engaged to work with you and your neighbors to evaluate the trees and work with a logger. Ash should be cut between October and April to avoid trucking wood from which adult EABs are emerging. If you’re not familiar with logging, one thousand board feet, commonly written as MBF, represents the logs from three or four good ash trees. A standard ten-wheeler log truck holds about 3,000 BF, and a big one with a trailer holds 5,000 to 6,000, so you and your neighbors will need to accumulate 12 to 20 decent trees to get a truckload.

Think about the future hedgerow. Consider planting native trees and shrubs that will be beneficial to wildlife. For nuts, consider oak, beech, hazelnut, or chestnuts from the American Chestnut Foundation. Blueberry, nannyberry, cranberry, raspberry, blackberry and elderberry will provide berries. Apple, cherry, hawthorn and dogwood all are beneficial. Basswood is common in the area and is an excellent tree for honeybees. Do you want tall trees (beech, birch, oak, maple) to grow for the future? Look at what young trees and shrubs are already in the hedgerow that will be released when the ash is removed. 

Whether you cut ash in a hedgerow or in the woods, you will let a flood of sunshine hit the ground and seeds will germinate. Invasives will flourish, so be prepared. If you already have honeysuckle, barberry, buckthorn, or any of the other bad guys, consider control work before you cut the ash, and be ready for years of control work as seeds of invasives germinate.

In the 17 years of research since EAB arrived in Detroit, much has been learned but no effective control of the insect has emerged. Parasitic wasps, both native species and three species imported from Asia, have been tested and show some promise in slowing the EAB killing wave. So far, the wasp population cannot grow fast enough to keep up with the EAB population as it builds in an area, but the wasp has promise as a long-term check on the residual EAB population after the killing wave has passed. A fungus is being tested for its effects on EAB and early results are promising. There is some potential for widespread dispersion of the fungus in the future. 

Chemical control is possible using systemic herbicides injected into individual trees. It costs $200-$300 per tree and the treatment has to be repeated every two or three years. There is no effective foliar treatment, so widespread chemical control is not possible. Treatment should only be considered for a particularly precious tree.

Work on developing resistant ash is the subject of a lot of research. Ash demonstrates a range of susceptibility to EAB, with a few individuals lingering long after all the rest have died. Scientists are working on the genetics of Asian ash trees and hope to breed some combination that will be resistant. Genetic engineering could speed that process considerably.

EAB larvae are killed by sustained frigid weather, but because they are under the bark, it would need to be colder than 30 below zero for at least 48 hours to have much effect. We are not cold enough to affect EAB.

Woodpeckers are capable of decimating EAB populations. There is not much a homeowner can do in this realm, but research is showing that forest landowners can have significant impact with techniques to encourage woodpeckers.

For the latest updates on EAB in Vermont and links to national information, go to vtinvasives.org, where you will find the latest map of the infestation along with information on EAB and all other invasive plants and insects in Vermont. This is also where you should report possible detections of EAB in your area.

[Editor’s note: Tim McKay owns Peacham Woodworks in Barnet, and is a natural resources conservationist, addressing problems such as soil erosion, water quality, wildlife habitat and stream dynamics.]