Managing Crape Myrtle Bark Scale

Crape Myrtle Bark Scale (CMBS) is an invasive felt scale insect that primarily affects Crape Myrtle trees. After first appearing in Texas in 2004, this insect is now relatively common throughout the southeastern U.S., and, more pointedly, in Central Virginia. In recent years, the PMG Horticultural Helpdesk has received inquiries regarding CMBS and what to do about it. This article examines the biology, impact, and management of CMBS and shares some practical advice on how to mitigate it, based on my own experience dealing with a CMBS infestation over an 18-month span.

How Do You Know When You Have CMBS?

CMBS is a soft scale insect that feeds on the sap of Crape Myrtle trees. Adult females have a white, waxy coating, making them appear like small cotton-like lumps on the branches and trunks of infested trees. Once a tree becomes infested, sooty mold growth appears on the bark due to a sugar-rich sticky liquid excreted by the insects. Along with sooty mold growth, the presence of ants on infested trees can be a telltale sign of CMBS as they scavenge the sugar-rich sticky liquid given off by the insects. Other symptoms of an infestation include yellowing leaves, reduced flowering, and overall tree decline. 

One simple way to identify whether you have CMBS is to use your fingernail to scratch the white cotton-like lumps on an infested tree. If you see a red, blood-like liquid just below the cottony surface, there’s a good chance it’s CMBS. If you’re still not certain, consider these steps:

• Reach out to the PMG Horticultural Helpdesk and share a photo of an infested tree. This is often the quickest way to get help identifying CMBS. 
• Work with the Helpdesk to submit a sample for identification at the Virginia Tech labs. Although this method takes a little longer, it is the best way to a receive a definitive diagnosis.

How Does CMBS Spread?

There are numerous ways that CMBS can spread. Here are some of the most common:

• Human transport: When infested plants are moved, the scale can spread to new areas. Be sure to carefully inspect prospective Crape Myrtles in the plant nursery or garden center, before making a purchase.
• Wind: Wind can blow crawlers, the mobile stage of the scale, to new plants. 
• Birds and insects: Crawlers can be transported on birds and pollinators. 

In my case, frost damage to one backyard Crape Myrtle in the early spring of 2023 made the tree especially susceptible to CMBS infestation. From there, the disease spread quickly to six other nearby Crape Myrtles in a backyard stand. Soon all were showing signs of cotton-like lumps and various amounts of sooty mold growth. Fortunately, none of the other trees were nearly as adversely affected as the first, making mitigation slightly easier for them.

Impact on Crape Myrtles

The impact of CMBS on Crape Myrtles can be severe. As sap-sucking insects, the scales weaken the trees by extracting essential nutrients, leading to stress and increased susceptibility to disease and environmental stress. Infected trees typically exhibit stunted growth, leaf drop, and diminished flowering.

As I discovered firsthand, the need for increased pest management practices, including chemical or biological treatment and consistent monitoring necessitated a significant commitment of time and resources to overcome the infestation.

Management Strategies

Managing Crape Myrtle Bark Scale effectively requires an integrated approach. University studies emphasize the importance of early detection and monitoring. Regular inspections during the growing season can help identify infestations before they become severe. If you’re like me, though, and fail to address the problem in its early stages, CMBS is likely to have a significant impact in a matter of a few weeks.

If you’re seeing a CMBS infestation and have grown concerned that it may spread to other Crape Myrtle trees in your yard or neighborhood, it may be best to remove the infested trees entirely, placing them in plastic bags, and disposing of the bags properly in the trash. Alternatively, if there are relatively few neighboring Crape Myrtle trees and you’re committed to saving your trees by managing the infestation, there are some steps you can take.

To begin with, cultural practices play a vital role in the management of CMBS. Maintaining tree health through proper watering, fertilization, and pruning can enhance resilience against pests. Additionally, reducing humidity around the trees can help minimize conditions favorable for the scale’s growth. For this reason, it is best to plant Crape Myrtle trees in full sun if possible.

When chemical control is necessary, research indicates that systemic insecticides can be effective. Soil-applied insecticides are the most effective treatments currently available for CMBS and the easiest to apply. These insecticides contain active ingredients such as imidacloprid, imidacloprid + clothianidin, dinotefuran, and thiamethoxam. These insecticides are absorbed by the plant and provide long-lasting protection against CMBS and other scale insects. This was the approach I used after experiencing an extensive CMBS infestation in the spring of 2023. Applying a combination insecticide and fertilizer via soil-drench in the late fall of 2023 and 2024 has yielded positive results in my case. Note that if you choose this route, it is critical to read the instructions on the product label and follow them closely. If you have any doubts about working with insecticides, consider contacting a certified arborist who has experience and equipment necessary to mitigate CMBS.

Caution: The active ingredients in the insecticides mentioned above, including imidacloprid, are known as “neonics,” which have a deleterious impact on pollinators. Details can be found in this Garden Shed article by Ralph Morini: Another Pesticide Controversy: Neonicotinoids and Pollinator Decline. Since neonics are lethal to pollinators, you should never apply them when pollinators or predatory ladybird beetle adults or larvae are present. What’s more, as stated in the article Crape-myrtle Bark Scale by the Henrico County Virginia Cooperative Extension, “Avoid applying any foliar insecticide when Crape Myrtles are blooming. Drenching at the base of the tree will be a safer approach to bees and other non-target organisms, when compared to a foliar application.”

Biological control measures are also being explored, with some studies highlighting the potential of natural predators, like lady beetles and parasitoids, to help manage scale insects. While these methods may not eliminate CMBS, they can help keep populations at manageable levels.

The Road to Recovery

The road to recovery from CMBS can be a long one, requiring diligent effort over the course of months. But mitigation is possible.

The photos below show the results of my 18-month effort to mitigate CMBS in a cluster of seven infested backyard trees. After applying the mitigation strategies described above in the fall of 2023 and 2024, none of the trees are currently showing any signs of white cotton-like lumps or sooty mold. Further, the ants that were so prevalent during the height of the infestation are no longer present. As the photos below show, the original, most heavily infested tree is a bit stunted from its heavy bout with CMBS, but numerous signs of new growth are now evident. Although this tree may never catch up to the height and fullness of its neighbors, it remains an esteemed member of our backyard garden.

In general, by employing an integrated pest management approach that includes monitoring, cultural practices, and appropriate use of chemical or biological controls, it is possible to mitigate the effects of CMBS and protect these popular and cherished ornamental trees.

   SOURCES:

• Crape-myrtle Bark Scale, Virginia Cooperative Extension, Henrico County.
• Another Pesticide Controversy: Neonicotinoids and Pollinator Decline, Ralph Morini, May 2019-Vol.5 No.5 of the Garden Shed.
• Crape Myrtle Bark Scale, Clemson Cooperative Extension, Joey Williamson, PhD, HGIC Horticulture Extension Agent, Clemson University. Updated: Sep 13, 2024.
• Crapemyrtle Bark Scale in Home Gardens, University of Maryland Extension. (Updated: Sep 24, 2024).