There is little, if any, threat of the Gambel oak borer (Agrilus
quercicola) harming your 50-year-old bur oak in the back yard.
By By: Jennifer Bousselot
Beetles attacking your oak trees?!? You’ve probably seen this
sensationalized story in the press recently.
There is little, if any, threat of the Gambel oak borer (Agrilus
quercicola) harming your 50-year-old bur oak in the back
yard.
And anyway, they’ve totally missed the boat.
Green industry circles were buzzing last year about the
unidentified – at the time – borer that was attacking ornamental
oaks at nurseries and garden centers.
It was a serious problem, especially for these business owners,
but it looks like that was an isolated, one-time occurrence. The
cosmos aligned, so to speak, for this unprecedented attack. Let me
explain.
The Gambel oak borer is a species native to Colorado. Before
last year it wasn’t even studied because it was considered an
unimportant pest of Gambel oak (Quercus gambelii) – also known as
scrub oak.
When species co-evolve through the millennia, like Gambel oak
and the Gambel oak borer, there is little threat of catastrophic
consequences unless the balance of nature tips the scales in favor
of one of those species.
Recently this has been the case for our oaks, with a slight
variation that I will get to in a minute.
The real danger is when a non-native species is introduced into
a new environment and, because it no longer has natural controls
from its native environment helping to keep the population in
check, it has the opportunity to become aggressive.
Dutch elm disease is the perfect example. When it was
inadvertently introduced from Europe our native elm, the American
elm. had no natural defenses against the disease.
Getting back to our Gambel oaks, the balance between the tree
and the borer was disturbed by the weather in 2002 and
2003.
Whitney Cranshaw, Colorado State University Cooperative
Extension entomology guru, generalized what was going on perfectly:
“Many native insects behaved extremely oddly last year, likely due
to the combination of 2002 drought, cool wet 2003 spring, and hot
2003 summer.”
The borers’ “odd” behavior came in the form of host substitution
by laying their eggs in 2003 on alternate hosts. Most pests –
insects and diseases alike – attack only one species of host. There
are exceptions of course, but the Gambel oak borer had never before
been one of them.
In 2004 when the horde of larvae hatched – about the same time
damage was noted – the theory was that a new Agrilus borer species
had come to Colorado.
Once the first adult in captivity emerged, experts like Cranshaw
had enough information to positively identify it as our native
Gambel oak borer.
That was the first recorded instance of this borer attacking any
other species than Quercus gambelii, which was why the experts were
thrown off the trail.
Enough entomological mystery? I bet you are wondering how this
affects your backyard bur oak. The quick answer: not at all.
Guessing from the borers’ activities in recent years, only nursery
stock and very recently transplanted oaks – which will be stressed
compared to established trees – have any risk at all.
Healthy oaks, including Gambel oak, should be able to readily
weather any damage. The recent press on this subject suggests
applying preventative insecticides.
Except for the tree nurseries, which have probably already taken
precautions, insecticidal applications are virtually unnecessary.
Plus, it is highly unlikely that this beetle will ever show up on
the pest radar again.
Also, please do not let this leave a sour taste in your mouth
about oaks. Cranshaw, entomologist extraordinaire, could not have
said it any better: “Oaks are magnificent trees that greatly
enhance the urban landscape of Colorado and have shown very little
susceptibility to serious pest problems.”
If you have any remaining concerns, or would like to inquire
about other insect problems, please contact the Douglas County
Colorado Master Gardeners at mgardenr@douglas.co.us or
720-733-6935.
Jennifer Bousselot is the horticulturist for Colorado State
University Cooperative Extension in Douglas County. Among other
duties, Jennifer manages the Colorado Master Gardener volunteer
program for the county.