Suppliers of crape myrtle should be aware of a relatively new insect pest that was first found in Texas about 10 years ago and since has spread across much of the Southeast at an alarming rate. Crape myrtle bark scale (CMBS) was first identified in a north Dallas suburb in 2004. By October 2013, it had made its way to the metropolitan Memphis area, and earlier this year sightings were confirmed in Little Rock, Ark.
“Heightened concern about this new pest is based on the speed at which it is spreading and the common use of crape myrtles in landscapes across a large part of the U.S.” says Jim Robbins, a professor and Extension horticulture specialist in ornamentals at the University of Arkansas.
How To Recognize The Pest
Since CMBS is the first and only known bark scale to occur on crape myrtles, it should be easy to identity.
Adult females appear as white or gray felt-like encrustations on small twigs to large trunks, often appearing near pruning wounds or in branch crotches on older wood. On the most current flush of growth and under heavy infestation, distribution may be more uniform. Up close, CMBS is white to gray in color. Careful examination may reveal dozens of pink eggs or crawlers under some of the larger white scale covers.
The most obvious red flag, says Robbins, will likely be black sooty mold, which appears on the bark.
As female nymphs mature, they secrete a white thread that becomes felted or matted into a thick, whitish to grayish scale covering the entire body. Adult females under this covering are wingless and incapable of moving. It is thought that for this species of scale, eggs are laid under the covering (probably late April to mid-May in Arkansas) and the female then dies. When the eggs hatch into first instar nymphs, these nymphs have legs and antenna and are mobile, dispersing over a short period (about one to two days).
It is thought that this emergence occurs beginning mid-May to early June in Arkansas, according to Robbins; however, crawlers were observed on a warm day in January 2014.
At present in the Southern U.S., the number of generations completed in a year for this species is unknown, but it is suspected that CMBS may complete at least two generations in Arkansas. It is possible that adult females and eggs overwinter, but crawlers and later stage nymphs have been observed overwintering in Arkansas under loose bark and in cracks and crevices.
So far, there has been no research to evaluate whether there is a range in susceptibility to CMBS across the hundreds of crape myrtle cultivars.
Which Methods Control CMBS
Dr. Mike Merchant, an entomologist with Texas A&M University AgriLife Extension, notes that while CMBS will likely not kill healthy crape myrtles, it can overstress the tree. Perhaps more problematic, the intense black sooty mold might be a big turn-off for consumers. “In some years, and in certain locations, the scale can detract significantly from the appearance of crape myrtle plantings,” says Merchant.
Current best suggestions for control of CMBS 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. Be sure to use sufficient volume to allow for penetration behind loose bark and into cracks and crevices. Thorough coverage of the tree is especially important when treating with oil.
• Application of systemic insecticides as a drench applied to the root zone has shown the most promise in tests to date. Imidacloprid (Merit or Bayer Advanced Garden Tree and Shrub Insect Control), thiomethoxam (Meridian) and dinotefuran (Greenlight Tree and Shrub Insect Control with Safari) have shown best control when applied between May and July. When drenching the soil with a systemic insecticide, allow several weeks for the product to be distributed throughout the plant.
Crape Myrtle Fact Sheets Available
Additional fact sheets on crape myrtle bark scale, and other crape myrtle pest management tips, are available at the following locations:
• Texas A&M: http://bit.ly/MBlyAJ
• The Crape Myrtle Trails of McKinney (otherwise known as the “crape myrtle city”): http://bit.ly/1bmJW4f
• LSU Ag Center: http://bit.ly/1fAdLtT
| February 28, 2014; http://www.todaysgardencenter.com/departments/watch-for-crape-myrtle-bark-scale-in-the-southern-u-s/