OSU Extension coordinator offers tips for local crape myrtle owners

Local crape myrtles have had a full summer of blooming, with weeks left of the trees still to show off their unique colors.

For the past several years however, crape myrtles across the region have had to deal with an issue affecting their bark — crape myrtle bark scale.

OSU Extension Educator John Haase said the problem is newer to Rogers County and, although not fatal to infected trees, can affect their lifespan.

“Crape myrtle bark scale is an exotic insect pest from Asia that was first detected in Texas in 2004,” Haase said. “Since then, it has spread throughout much of the southern U.S. including several counties in Oklahoma.

“There was major attention to this pest when it was documented in the two larger population areas — Oklahoma and Tulsa Counties,” he said. “This season, I found some on landscape plants in Rogers County.”

Crape myrtle bark scale (CMBS) is not deadly to most crape myrtles, but can affect the quality of the plants and greatly the health and longevity in the landscape, according to Haase.

“There are few natural enemies here in the U.S., and this allows the insect to build to unusually high numbers on infested plants,” he said. “It’s not unusual for heavily infested plants to have branches and twigs that are completely encrusted with scale.

“The nymphs produce large quantities of honeydew, and this results in heavy accumulations of sooty mold on the leaves, twigs, and trunks of infested plants, as well as on nearby low-growing plants and surrounding grass and mulch,” he continued. “The end result is a crape myrtle that is black and ugly — because of heavy sooty mold accumulation — and produces fewer and smaller blooms. Unless they are treated, crape myrtles that are heavily infested with crap myrtle bark scale will become a landscape eyesore.”

Effective treatments are available, Haase said, but they are “relatively costly” and do not provide 100 percent control, which means that treatments must be reapplied annually.

“So far, crape myrtle bark scale is difficult to control without the use of systemic insecticides, which are used to control most pests of this mode of feeding,” he said. “However at this time it’s not recommended to use systemic, neonicotinoid insecticides (i.e., products containing imidacloprid or dinotefuran) to control of CMBS because of the risk these active ingredients pose to pollinating insects such as honey bees and bumble bees and the long flowering period of crape myrtles that extends throughout most of the growing season.”

As such, Haase offered the following tips towards management of CMBS infestations in Oklahoma:

• Carefully inspect crape myrtles prior to purchase for signs and symptoms of CMBS, including the presence of white to gray scale bodies on bark, honeydew, and/or black sooty mold. Always buy plants that are free of mechanical damage such as bark wounds that may serve as “points of entry” for CMBS.

• If this is a common pest on your crap myrtles then horticultural oil may be effective when applied during the winter at a dormant application rate. Ensure adequate coverage of the entire tree and use enough oil to reach behind loose bark, branch crotches, and other crevices.

• Lady beetles in the genus, Chilocorus are effective predators of many scale insects. However, predation of CMBS occurs too late in the season for effective reduction in the growth of black sooty mold. Over time, additional predators and parasitoids may be discovered attacking CMBS and contributing to natural control of this pest.

For additional information, contact Haase at 918-923-4958.

or by email at john.haase@okstate.edu.

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