A relatively new insect pest is troubling crape myrtle in north Texas cities and backyards. Although an official name has yet to be given, this insect is thought to be an exotic pest that has somehow found its way to north Texas.
In the genus Acanthococcus (formerly Eriococcus), the scale is a type of felt or bark scale. It’s closest cousin is the Azalea bark scale, but that species has never been associated with crape myrtle before. Current evidence suggests that the scale may be a migrant from Asia, Acanthococcus lagerostroemiae, which is common on the timber tree Lagerostroemia indica in Japan and China.
One of the first signs of an azalea bark scale infestation is a black (sooty mold) coating that appears on the bark of the trunk and on the branches of crape myrtles (see picture). Leaves and limbs may feel sticky from byproducts of the insect’s feeding.
The insects appear as white, waxy encrustations likely to occur anywhere on the plant, but often near pruning wounds or in branch crotches. Up close, the azalea bark scale-insect is white to gray in color. Larger female scales “bleed” a pink liquid when crushed. Careful examination may reveal dozens of pink eggs under some of the larger white scale covers.
Control recommendations for azalea bark scale are still being developed; however, our current best suggestions for control of this insect include:
- For heavily infested plants wash the trunk and reachable limbs with a soft brush and mild solution of dishwashing soap. This will remove many of the female scales and egg masses and make insecticide control more effective. Also, washing will remove much of the black mold that builds up on the bark on infested trees.
- Horticultural oil has not yet been shown to be effective against this insect, however a winter application of dormant oil to the bark and crotches of the plants where scales shelter may be beneficial. Winter is an especially good time to treat for scales because a higher (winter) application rate can be used without damaging the plant. Thorough coverage of the tree is especially important when treating with oil.
- Application of systemic insecticides as a drench applied to the root zone of plants to be protected has shown the most promise in tests to date. Imidacloprid (Merit® or Bayer Advanced™ Garden Tree and Shrub Insect Control) or dinotefuran (available under various trade names online) have shown best control when applied between May and July. When drenching the soil with a systemic insecticide, allow several weeks for control as the products needs time to spread throughout the plant.
As of the beginning of 2014, in Texas the crape myrtle bark scale has been observed infesting crape myrtles throughout the Dallas-Fort Worth-Denton-Rockwall area, and in Tyler, TX. In 2012 the scale was confirmed in Ardmore, OK and in 2013/14 it was confirmed in Houma and Shreveport, LA; Germantown, TN; and Little Rock, AR. If you live outside these cities and believe you have an infestation of this scale, please let us know. Pictures or samples can be submitted through your county Extension office or you can leave a message for me through the comments section of this update.
For more general information about scale insects and their control, see Extension publication B-6097, Scale insects on ornamental plants. Note: Since this post was published, we have published a YouTube version of a 44 minute webinar on what we know about the biology and control of crape myrtle scale.