Flat-headed borers in the genus Agrilus infest native gambel oak stands as well as landscape oaks. Affected trees may not show obvious signs of infestation until the foliage browns, branches die back and the crown thins. Infestations typically occur in the upper crown, then progress downward.
Some infested trees may have areas of wet or gummy bark. Trees vigorous enough to fight the infestation may have raised areas of bark above the winding larval galleries. The adult exit holes in the bark are D-shaped and are a little over 1/16 inch wide. The lack of external symptoms can be deceptive and a small amount of bark should be peeled back to confirm an infestation.
If infested, trees will have zig-zag larval galleries. The galleries range from 1/32 to 1/8 inch wide and may completely encircle small diameter stems. There may be patches of wet inner bark and some dark staining of the wood near the gallery. The galleries can be directly beneath bark or under 1 or 2 rings of wood. Larvae of these beetles are very narrow and have flattened, segmented white bodies up to 1 inch in length. The head end is as narrow as the rest of the body.
Lightly infested trees may be treated with bark sprays to prevent further infestation. Systemics can also be applied to kill larvae under the bark. Stressed landscape trees should be soil treated with imidacloprid or the bark sprayed with permethrin.
Properly water and otherwise maintain trees. Oak trees that seem particularly at risk are those that have been transplanted within the last few years or are stressed from drought, compacted soil, improper planting or some combination of these. These flat-headed oak borers are not aggressive and are only capable of damaging stressed trees. Vigorous trees should not be at risk.
For more information, see the following Colorado State University Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s).
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Updated Monday, February 01, 2016