We have
gotten to the end of our list of the Most Unwanted Insects. For #1 we chose the
Bronze Birch Borer (Agrilus anxius).
Why did we
choose the Birch Borer for the #1 spot rather than the insects that can destroy
an entire tree species? (i.e. the Emerald Ash Borer or the Asian Longhorned
Beetle) Because it’s HERE. NOW. And because, as arborists, we’re dealing with
damage – often fatal -- from this insect all the time.
We all
love birch trees. The bright, white, striped paper bark birch is often the
first tree we recognize as children. As an understory tree, white birch can
light up the forest. Seeing a birch grove in the snow is breathtaking. And
other birches, such as river birch with its curling peek-a-boo bark revealing a
salmon-colored trunk, are popular planting choices as well.
The
problem with birch is that they are really not meant for our landscapes. Under the best circumstances, birch trees don’t
have lives as long as most of our other trees. And our yards and parks are not
the best of circumstances. Birch trees grow best in slightly acidic, moist,
well-drained soil. They like sun on
their faces but not on their feet. They don’t like pollution. When they don’t
have the right conditions, they become stressed; this makes them more vulnerable
to both insects and disease. The attacks of insects like the birch leafminer
and aphids or diseases like rust and mildew further compromise the tree’s
health.
This is
where the Bronze Birch Borer comes in. The Birch Borer lays its eggs beneath
the bark of the birch tree – and prefers to have its path cleared by another
insect or other damaging agent first. So
Birch Borers rarely attack healthy trees, but can be deadly to trees already experiencing
some decline.
As the Birch
Borer larvae emerge they ravenously eat the underside of the bark, gouging out
galleries as they munch. These channels cut through the phloem of the tree,
interrupting the tree’s ability to transmit water and nutrients.
 |
| Larva and gallery of the Bronze Birch Borer Source: David G. Nielsen, The Ohio State University, bugwood.org |
Because
the larvae grow beneath the tree bark, often the infestation goes unnoticed
until the tree canopy starts to yellow, at which time it can be too late to
save the tree. It takes aggressive
treatment to halt an attack of Bronze Birch Borers. In an infested tree, the
trunk can be injected with a control that can help to stop the larvae from
developing. The trunk can also be sprayed with a substance that helps to
prevent new insects from colonizing the tree. The correct treatment depends
upon the health of the tree, the degree of infestation and the time of year.
Not all
birch species are equally vulnerable to Bronze Birch Borer. The River Birch, in
particular, seems to resist the insect, while the Silver Birch is particularly
susceptible. Ultimately, the best way to prevent the Bronze Birch Borer from
killing your trees is to keep your Birches as healthy as possible. Birches need deep watering -- weekly, at least. Although the leaves need sun, the roots must
stay cool, so a layer of ground cover or mulch will help to insulate them from
the heat.
We recommend checking Birch trees frequently for signs of
pests and disease. Any yellowing leaves or dieback could indicate a potentially
fatal problem; early intervention can sometimes avert a fatal infestation.
