University of Vermont
Department of Plant and Soil Science

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EMERALD ASH BORER

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

It’s hard to believe that such a beautiful insect can cause such not-so-beautiful damage.  This invasive insect pest has now spread into Vermont and surrounding areas, where experts predict it will destroy virtually all ash trees over the next decade.  This will have a major impact on our forests, forest products, and landscapes.  Whether you have ash trees in your own landscape, or your local community, it is important to know how to spot this pest, how not to spread it, and your options once it arrives.

As with many of our major pests, this one was introduced (so is termed “exotic”) to the U.S. from its native Asia, likely in wooden cargo packing materials.  Since its introduction and first spotting in July, 2002 near Detroit, it has gradually spread to most all states east of the Mississippi river, and some west of it, as well as eastern and central Canadian provinces.  It has spread on its own, and with the help of humans, particularly in firewood.  Many states now prohibit bringing firewood, such as for camping, from out-of-state—a key way to slow the spread of this serious pest—the most destructive forest pest in U.S. history.

Although the good news is that this pest only attacks ash trees, the bad news is that tree canopies (leaves) are gone within a couple years, and the trees die within three to four years.  To date, 150 to 200 million ash trees have died in at least 30 states from this pest.  Left unchecked, it is expected to kill most of the 8.7 billion ash trees in North America.  This is a major impact, both for landscapes where ash species are used widely, and forest products.  Ash are valuable for their timber which, being tough, is used for tool handles and sports equipment.  The green and black ash are common along river corridors, so their loss will impact water quality.     

Some of the better native trees in our home and city landscape plantings are ashes, available in several species and cultivars (cultivated varieties).  Most common are the white ash (Fraxinus americana), rounded up to 60 feet tall when mature, and the green ash (Fraxinus pennsylvanica), oval to upright reaching 60 feet tall.  Both have a moderate to fast growth rate, with yellowish fall leaves.  If you’re not familiar with ash trees, these have compound leaves (divided into several leaflets), arranged opposite each other on branches, and often shiny green.  Make sure you don’t confuse ash trees with elms, boxelders, mountain ash, walnut, and hickories.

The adult beetle of the ash borer is bright, metallic green, about one-half inch long.  When they emerge from the bark in late spring to early summer, they leave a characteristic D-shaped exit hole.  While they nibble on leaves, this doesn’t cause major damage.  They are active until the end of summer. 

Beetles lay eggs between bark layers, and in crevices.  In about a week these hatch, and the larvae bore into the tree.  It is the four feeding stages of these larvae that cause serious damage, disrupting the movement of water and nutrients in the tree, creating S-shaped galleries.  These larvae then overwinter as “prepupae” in a “pupal chamber” they carve in the tree, emerging in spring as adults.  Woodpeckers and their damage on ash trees is a good sign that there are larvae within. 

When the water and nutrients don’t flow within the tree, leaves yellow and wilt.  Shoots may appear from the lower trunk (below the damage, termed “watersprouts”), or from the roots (termed “suckers”) —the result of the tree trying to start new healthy growth and survive.  So even if you don’t see the bright green beetle, if you see woodpeckers and wilting on ash, look for the D-shaped holes in the trunk, and these lower shoots (termed “epicormic branching”).    One problem is that unless you look closely, you may not see serious visible damage for up to three years.

Another problem is that other stresses may cause similar symptoms to this borer.  Trees may be stressed by drought, or diseases such as ash yellows (descriptive of the symptom).  The redheaded ash borer causes similar symptoms, only it has a round emergence hole.  Several other insects may be mistaken for this beetle, including honeysuckle flat-headed borers, green ground beetles, click beetles, and the six-spotted green tiger beetle.  This latter one is actually a harmless beetle, eating small insects.  If you think you have the emerald ash borer, or see its symptoms but aren’t positive, it’s best to check with a trained professional at a nursery, your state agriculture department, county forester, or state university plant diagnostic clinic. 

If you find this pest on ash trees, there are few options.  If trees are high-value landscape and street trees, there are effective insecticide injection treatments that arborists can provide for temporary control for one to three years.  If there are too many trees, or this is not warranted or feasible economically, then trees will need to be cut down. 

If you cut down trees, chances are they are large and you’ll need to hire a reputable tree service or arborist.  The borer doesn’t affect the inner wood of the tree, so it can be milled locally into lumber, or even into garden art or furniture by woodworkers.  You also can cut it up for firewood.  Just make sure you do not transport any cut wood outside your immediate area.

One promising control of this borer is the use of nature itself, in the form of parasitic wasps.  Shortly after this pest was found in the U.S., scientists explored for natural controls in its native range.  Three natural wasp enemies of the pest were found in northeast China, their release in this country was approved in 2007, and they have now been introduced into 22 states. These wasps, one of which attacks the eggs and the other two the larvae, have resulted in up to 90 percent decline in larvae in trees (agresearchmag.ars.usda.gov/2016/may/wasps).

If you have ash trees in your landscape or community, you may want to follow the lead of some towns which are interplanting replacement tree species now.  These will then be larger and established when the ash trees need removing.  Whether planting ahead of the borer damage, or to replace already dead trees, there are many good tree options such as maples, linden, and oaks.  Check with your local nursery for these and more options.  If planting several trees, try to plant different species to hedge your bets against any future pests or problems.

With this being such a major pest, there are many government, university, and collaborative websites where you can find more information.  A couple of these resources are a series of factsheets from Penn State University (www.paemeraldashborer.psu.edu), and the Emerald Ash Borer Information Network (www.emeraldashborer.info).

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