Heather N. Kolich, ANR Agent, UGA Extension Forsyth County

A section of crape myrtle branch showing a scaly growth
A heavy crape myrtle bark scale infestation can cause delayed leafing, small bloom clusters, and general lack of vigor in affected plants.

Crape myrtles are popular ornamental trees throughout Georgia. Properly pruned, they’re elegant flowering trees with relatively few pests. In our highly mobile society, however, pests get opportunities to spread across natural barriers, like oceans. Such is the case with crape myrtle bark scale (Acanthococcus lagerstroemiae), an insect native to Asia that has invaded several southern U.S. states.

Signs of CMBS infestation during the growing season include quantities of ants and wasps eager to collect sugary honeydew for food; tree trunks, branches, and leaves that are blackened with sooty mold growing on honeydew deposits; delayed leafing in the spring; smaller bloom clusters; and general lack of vigor in tree growth. In addition to crape myrtle trees, CMBS infest American beautyberry, boxwood, pomegranate, and fig plants.

Bark scale insects themselves can be difficult to detect on trees and shrubs while leaves are present. They don’t cause visible damage like leaf-chewing insects do, and only the first instar, the crawlers, are mobile. These tiny, pink nymphs hatch during warming trends in spring and crawl about to find a bark crevice to tuck themselves into while they suck up sugary plant sap. During the next life stages, male CMBS insects develop wings and lose their mouthparts. Female CMBS lose their legs and anchor down under a protective, felt-covered, wax shield, where they feed, excrete large quantities of honeydew, and lay up to 300 eggs each. A second generation of crawlers emerges 2-3 months later, and a third generation can overwinter on crape myrtles and other host plants.

Now that most leaves are off the trees, CMBS may be visible as white or gray, waxy bumps or crusts on twigs and branches, in bark cervices, and under pealing bark. These insects also tend to colonize susceptible plants in shady spots and cluster around pruning cuts, particularly on over-pruned trees.

While many insects become inactive during winter, a newly published study (https://journals.ashs.org/hortsci/view/journals/hortsci/58/10/article-p1237.xml) found that CMBS populations increased from October to March on a set of 34 potted ‘Natchez’ crape myrtle saplings in North Carolina. The saplings were exposed to twig cuttings from infested crape myrtles in the summer of 2021, and then evenly distributed throughout a research station field. In October, researchers made a count of settled crawlers, female ovisacs, and male cocoons on each sapling. From November 2021 to March 2022, they recorded temperature data and made weekly observations for crawler activity on the saplings.

These observations revealed that CMBS crawlers were active on 30 percent of the saplings even when temperatures were around 26 degrees Fahrenheit. Crawler activity was present on more saplings on warmer days and after accumulation of several days with temperatures above 50 degrees F. In short, CMBS crawlers were present or emerging from egg sacs throughout the winter. By March 2022, the number of scale insects present on the saplings had increased from around 28 to over 550.

While we can’t draw conclusions from a single research study, we can use this information as a basis for making our own observations and potentially to make effective winter applications of low-risk pesticides, such as horticultural oil, to reduce CMBS populations.

First, scout for CMBS on susceptible plants. Sample for crawler activity by wrapping branches near scales with double-sided tape. Use a hand lens to check the tape for crawlers. If crawlers are present, spray horticultural oil on twigs, branches, and trunk sections near anchored CMBS. Oils in these products suffocate crawlers that contact them; however, the effective window is short as the oils evaporate. Treatment may need to be repeated several times throughout the winter in conjunction with crawler activity. Be sure to remove the sampling tape from branches.