This column is in response to a number of questions I have
received about treating ash trees for the emerald ash borer. This
accidentally introduced pest has been present in the Pittsburgh area
since 2007, and now many untreated ash trees are dying.
The emerald ash borer (EAB) was
first discovered in the United States in 2002 near Detroit,
Michigan. Entomologists believe this pest was accidentally
introduced in wood crates or pallets that were used to ship products
to the United States from its native Asia. EAB larvae feed just
under the bark on the vascular system of host plants.
If larvae were present when the
tree was felled, they could survive and go on to hatch out as adults
as long as they were not injured in the process of turning the tree
into lumber. It is likely that the infestation was present for ten
to twenty years before the insects were identified.
tree infested with EAB has started to send
out new shoots low on the trunk
called epicormic sprouts
Despite quarantines on the movement of ash nursery stock, logs and
wood chips, designed to slow or stop the spread of this pest, EAB
has been confirmed in 19 states as well as Ontario, Canada. The
first infestation in Pennsylvania was identified in 2007, in
Cranberry Township, Butler County. Currently, 34 counties in
Pennsylvania have confirmed infestations, with more likely to join
the list through the growing season. EAB has killed over 40 million
ash trees to date. Only trees in the genus Fraxinus are susceptible
to damage from this pest.
Entomologists from Michigan State, Ohio State, Purdue and University
of Wisconsin have been working together since emerald ash borer was
first identified to develop treatment protocols and management plans
to help communities prepare to deal with the aftermath of having so
many trees die in a short period of time. One of the group’s
recommendations is that valued ash trees should begin treatment when
EAB has been discovered within 15 miles.
Emerald Ash Borer video
Pittsburgh area passed that threshold five or six years ago. Trees
that have not been treated all along should be carefully evaluated
to make sure they are worth treating before spending the money. The
systemic insecticides that provide the most effective control of EAB
move in the tree’s vascular system. If larval feeding has damaged
the vascular system too much, the insecticide cannot get to the
crown of the tree where it is needed. Treating trees that are too
far-gone will not save them and is a waste of money.
Trees are worth treating if more than half the crown is still alive
and the tree appears to be healthy with few signs of EAB
infestation. Not all trees are worth treating, even if they do not
show symptoms of heavy EAB infestation. Treatment is an ongoing
expense and should be reserved for those that significantly enrich
the landscape or have sentimental meaning to the owner. Trees are
not worth treating if less than half the crown is still alive, the
tree has significant woodpecker damage, or it has started to send
out new shoots low on the trunk (epicormic sprouts).
"D-shaped" exit holes and woodpecker damage
on the trunk indicate an EAB infestation
Homeowners can successfully treat trees that are less than 20 inches
diameter-at-breast-height (dbh) with products that contain 1.47
percent imidacloprid as their active ingredient. These are applied
as a soil drench. Calculate the dbh by measuring the circumference
of the trunk 4˝ feet above the ground, then dividing the result by
3. Soil drenches are most effective when applied mid-late spring.
Larger trees – those over 20 inches dbh – should be left to
professional arborists because they can use products such as
emamectin benzoate and dinotefuran that are not available to home
gardeners. These materials have shown improved efficacy in treating
very large trees. Certified arborists also have the training and
experience to inject insecticides through the bark of the tree using
specialized equipment. Trunk injections are best made from early May
treatment is not option, be prepared to remove infested trees as
soon as possible because they become hazardous very quickly once
they have been killed by EAB.