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
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
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.
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