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Oak Borers
Mary Small
Extension Agent, UIP
Jefferson County
Click Photo to Enlarge
Three samples of oak branches
infested with borers were recently submitted to the Jefferson County Plant
Diagnostic Clinic. After examining the insect galleries, a few larvae were
removed. They appear to belong to the genus Agrilus.
There are two native and one non-native Agrilus beetles that
are under suspicion. However, the larvae of these insects are very similar. They
must be reared to adults for correct identification. Adults are expected to
emerge from their rearing chambers next spring.
While correct identification is not possible at this time, we
do know this much about the insects and their damage.
- The borers involved are not especially aggressive. They successfully
attack only stressed trees, either in the wild or in the landscape.
- The recent drought appears to be a major stressor of native and landscape
oaks.
- Damage appears as foliar browning, thinning and dieback, beginning in the
upper portions of the tree and working downward. In some cases, there is an
area of wet bark. Symptoms can easily be confused with drought stress.
- After removing bark, the insect’s galleries become apparent. They are
1/32 inch to 1/8 inch wide and zigzag. Usually the galleries are found in
the cambial layer, right under the bark, although they may be slightly
deeper.
Lightly infested trees may be treated with insecticides such
as imidacloprid or permethrin from May through early June. Newly-planted trees
should be similarly treated soon after planting as a preventive. Maintain good
plant health and vigor through proper watering, fertilizing and pruning.
Vigorous trees are the best defense against borers.
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