Recognizing and Treating Crepe Myrtle Bark Scale
“What is that white stuff on my crepe myrtle tree?”
That’s a question we are seeing more and more these days, either asked through our website or on various social media platforms like FaceBook and NextDoor. And the reason we’re seeing that question is that Richmond’s crepe myrtles (Lagerstroemia spp.) are under attack from a rather nasty invasive insect known as crepe myrtle bark scale or cmbs for short (Eriococcus lagerstroemiae). Over 7% of Richmond’s tree canopy is crepe myrtle trees, and it’s getting harder to find one that hasn’t been infected than vice-versa.
Crepe myrtle bark scale originated in Asia but has now spread to many parts of the United States. It’s a small, soft-bodied insect that primarily infects the bark of crepe myrtles, especially the trunks and branches. The scale appears as a white or grayish patches that resemble a crust or waxy coating on the tree’s bark. These patches can vary in size and may become more prominent over time as the scale population grows.
“Will crepe myrtle bark scale kill my tree?”
Maybe, or maybe not, but regardless, the presence of crepe myrtle bark scale will have detrimental effects on the tree’s health. As the scale feeds on the tree’s sap, it weakens the tree, leading to stunted growth, branch dieback, and yes, even tree death if left untreated. Additionally, the secretion known as “honeydew” produced by the scale as a byproduct from it feeding on the tree’s sap, can attract ants, bees, and wasps. The secretion supports the growth of black sooty mold, which further compromises the tree’s aesthetic appeal. To make matters worse, the black sooty mold doesn’t confine itself to the tree, but will cover anything under the tree: other plants, decks, patios, lawn furniture, etc.
If you have a crepe myrtle on your property, take a close look at it. Apart from the white or gray patches on the bark, look for the presence of black sooty mold, honeydew on leaves and nearby surfaces, and the appearance of ants crawling on the tree. Don’t see any signs of cmbs? Congratulations! Now put a note on your calendar to check it again in a couple weeks. It is crucial to regularly inspect the crepe myrtle for any signs of infestation, especially during the spring and summer when the scale populations are most active.
“Can crepe myrtle bark scale be treated?”
Yes, although to be honest, treatments aren’t always 100% effective. There are several methods available for managing this pest. An option that may help reduce the population but will not completely eradicate the scale is scrubbing them off with soapy water and a soft-bristled brush. Another mildly effective option is to use horticultural oils or insecticidal soaps, which can be sprayed directly onto the infested bark. These products suffocate and kill the scales. It is important to thoroughly cover all infested areas with the spray for effective control. Be aware that some of these direct-bark treatments will also kill beneficial insects like pollinators. Consider lightly pruning flower clusters and heavily infested branches before treatment to reduce tree attractiveness to pollinators and physically remove large numbers of the pest. Double bag the infested branches before disposing them in the trash to prevent spread.
Another treatment method is the introduction of natural predators, such as ladybugs or lacewings, that feed on crepe myrtle bark scale. These beneficial insects can be purchased and released into the garden, providing a natural and sustainable solution to the problem. However, it is crucial to ensure that the release is done at the appropriate time and in the proper conditions for the predators to establish and thrive. Ladybugs, e.g., seem to only feed on the larvae of cmbs, so they may have little to no effect if used when the insect is not in the larva stage. In Richmond, cmbs may go through anywhere from two to four generations in a single season, and the larva stage is only one part of each generation.
Another alternative treatment is known as a “soil drench”, which involves injecting chemicals into the soil around the tree in the fall and spring. Some experts recommend using a different chemical in fall than in spring, essentially providing a “one-two punch” to the invader. However, it is essential to use these products with caution and follow the instructions provided by the manufacturer. You may want to consult with a Certified Arborist to provide this service.
“Is there anything else I can do?”
Prevention is always better than cure when it comes to crepe myrtle bark scale. Regularly inspecting crepe myrtles for signs of infestation, maintaining tree health through proper watering, and pruning off dead/infected branches can help limit scale outbreaks. Additionally, planting crepe myrtles in full sun and avoiding planting new trees near infested areas can reduce the risk of scale infestations.
But the bottom line is that it looks like this invader may be with us for some time.