By Melinda Heigel, ExtensionSM Master Gardener Volunteer of Wake County
(Left to right) A healthy crape myrtle (Lagerstroemia spp.) in full bloom, an early infestation of a crape myrtle trunk, and a limb with crape myrtle bark scale (note the white felt-like structures). (Image credits: Melinda Heigel)
Travel down most streets in central North Carolina during the summer and you will see myriad crape myrtles (Lagerstroemia spp.) in bloom. With its dazzling flower display, this ornamental is nothing short of a staple throughout the southern landscape. These trees are typically vigorous, versatile, and low-maintenance, unless someone has damaged the tree with a poor pruning job, you may know as “Crape Murder.”1 And while they are susceptible to some common pests and pathogens,2 you should learn to identify and understand a new pest: crape myrtle bark scale (Acanthococcus lagerstroemiae).
Origins and Spread of Crape Myrtle Bark Scale
Although likely introduced to the US earlier, crape myrtle bark scale (CMBS) reports first occurred in 2004 in Richardson, Texas, and in McKinney, Texas (the self-proclaimed “crape myrtle city”) in 2005. Crape myrtle bark scale is an invasive species originating in Asia. In its native ranges of China, Korea, and Japan, researchers have also seen this pest affect a wider host of plants including persimmons and pomegranates. Since the initial US discoveries, CMBS has continued to spread and affect crape myrtles in most of the south and southeast. In North Carolina, some of the heaviest concentrations have been in the Mecklenburg County area among others, but the Triangle region in central North Carolina is now also impacted.
Over long distances, CMBS likely spread through plant material across the professional nursery trade. But it may also have spread by wind, other insects, birds, and animals. While this pest is primarily affecting crape myrtles in the US, recent reports also confirm the species on the American beautyberry (Callicarpa americana). Although CMBS has not been widely affecting other plants in North America, researchers report that a number of other species have been impacted in Asia and Hungary. This raises concern among the scientific community that other plants may also become hosts.
Insect Description and Life Cycle
There are several stages during this insect’s lifecycle. In its immature or nymph stage, the insects are difficult to see as they are pink and quite small. During this time, you can often best observe them with magnification. The insects are mobile and able to crawl (hence called “crawlers”). Crawler activity peaks in the spring. Later in their development, they plant themselves into a stationary position and feed with sucking mouth parts on the phloem (sap) of the bark of the crepe myrtle. At this stage when feeding, they secrete a substance called honeydew–a sweet and sticky liquid.
The mature female pest is a small white felt-like insect but pretty easy to spot on the tree–especially the bark. After successfully mating, when the females’ egg sacs are protected by this white filamented, felt-like covering, the mature females die and leave up to 300 eggs per scale insect. A second generation of crawlers emerges in late summer.
(Left to right) Nymph crawlers are challenging to scout with the naked eye; crape myrtle tree in Raleigh, NC, with visible signs of CMBS such as spots of sticky honeydew, dark sooty mold, and white scale; and close up of fluffy white-filamented scale bodies. While not related to one another, the common mealy bug and CMBS resemble one another. (Image credits: Jim Robbins, University of Arkansas Extension, CES, Bugwood.org; M. Heigel)
Signs, Symptoms, and Effects of Crape Myrtle Bark Scale
In their early life stages, these insects may be difficult to see. And given the mature size of many crape myrtles, some topping out at 30 feet, seeing “crawlers” in the upper parts of the tree can be nearly impossible. However, in their later stages, the white felt-like, waxy structures stand out in contrast to the tree bark on the trunk and limbs. When scouting for this pest, pay special attention to areas around branch collars and rough areas of the tree as these are often sites of initial infestation.
Expect to see wet-looking spots of honeydew visible along the bark thanks to these scale insects. While honeydew doesn’t cause harm to the tree itself, this substance is attractive to dark fungi called sooty mold, which can make parts of the bark and even the leaves appear blackened in color. Sooty mold can reduce both the vigor and aesthetics of the crape myrtle. Moreover, CMBS infestations reduce the size and amount of blooms on trees. Affected trees may also exhibit premature bark peeling and be slower to leaf out annually.
Another symptom to watch for is ants and other flying insects on and around the crape myrtle who are attracted to honeydew that comes with infestation. Researchers are still determining if ants are acting as predators or merely tending the scale insects to ensure continued access to honeydew.
If there is any good news about this increasingly wide-spread threat, it’s that the effects are mostly cosmetic. While this pest won’t likely kill your tree, it will at least make it less attractive and healthy. It is important to note, however, that stressed plants are more attractive targets for other pests and plant diseases.
Management
The earlier an infestation of CMBS is detected, the more effective control strategies will be. It is important that infestations be managed during the first, smaller generation.
Cultural Management
One of the best ways to control the impact of CMBS or any pest on your crape myrtle is to keep the tree healthy. Practice proper pruning techniques at the right time of year, mulch the plant, ensure it has the right growing conditions (those planted in full sun are often less affected), moisture, and nutrition. Healthy plants are less susceptible to pest problems. Encourage beneficial insects in your landscape by taking an Integrated Pest Management approach.3 Lady beetles are a natural predator of this scale, but with heavy scale infestations, natural predators are likely to be outnumbered.
Mechanical Management
Some experts suggest that pruning affected side branches of an infested tree may help control the pest population. To help contain the spread of CMBS, you should handle any parts of the tree that are impacted with care. Destroy any plant material you prune. Do not shred or place in compost piles. Do not put this material out for yard waste pick-up. Double bag the plant parts and dispose in the regular trash. If you choose to avoid chemical controls, Clemson Cooperative Extension’s Home & Garden Information Center offers detailed instructions on renewal pruning of the tree, which may take up to 3 years for revitalization. See their online factsheet on CMBS for details of this approach (https://hgic.clemson.edu/factsheet/crapemyrtle-bark-scale/).
Chemical Management
Chemical controls, both systemic and direct contact, are effective in managing CMBS. Many experts recommend soil drenching (placing liquid chemicals in the soil near the roots for root uptake into the plant). Understanding the timing of pesticide application based on the pest’s life cycle is important. Likewise, because crape myrtles produce copious and continuous flowers that attract pollinators, pay careful attention to any products you use, what they impact, and read and follow all label instructions. Also, consult the NC Agricultural Chemicals Manual when considering chemical options. (https://content.ces.ncsu.edu/north-carolina-agricultural-chemicals-manual) Horticultural oils, a lower-toxic choice, can also be somewhat effective but must be applied during peak crawler emergence for the best results.
Be alert. Regularly monitor your crape myrtles for this fast-spreading pest. Help identify and contain CMBS. Summer wouldn’t be summer without these trees’ stunning blooms.
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Notes
1–“Crape murder” is a common term for improperly-pruned crape myrtle trees that are lopped off severely at the crown. This severe pruning can cause overgrowth at the pruning site and negatively affect the tree’s natural growing habit.
2–Some common crape myrtle pathogens and pests are powdery mildew, leaf spot, and aphids. Like CMBS, aphids have sucking mouth parts. Their presence can also lead to displays of sooty mold on the tree.
3–Integrated Pest Management (IPM) is an effective and environmentally-friendly approach to managing pests by employing multiple tools and strategies in tandem. For additional information, see NC State Extension Gardener Handbook’s online source.
https://content.ces.ncsu.edu/extension-gardener-handbook/8-integrated-pest-management-ipm.
Reading and Additional Information
For a list of potential additional plant hosts of CMBS, see Texas A&M University Extension’s entomology site.
https://extensionentomology.tamu.edu/insects/crapemyrtle-bark-scale/
For a more in-depth discussion on the management of CMBS, see Clemson Cooperative Extension’s Home & Garden Information Center fact sheet.
https://hgic.clemson.edu/factsheet/crapemyrtle-bark-scale/
If your crape myrtle is heavily infested with CMBS and languishing, another strategy is replacing it with a native tree. See former blog posts on trees native to central North Carolina that may offer alternatives that aren’t affected by this pest.