THE LEAFLET

This article is the second in a series of featured stories this spring/summer with our partners at Bartlett Tree Research Laboratories. This partner series aims to educate the DC region about its insect inhabitants and provide practical tree care advice to residents.

A line of crape myrtle in NE DC. Crape myrtle have become a popular tree in the DMV region.

When people think about trees and Washington, DC, their first thoughts almost certainly go to the iconic cherries surrounding the Tidal Basin. After that, they might think about the majestic American elms lining the Mall or the historic American beech trees found throughout Rock Creek Park. Crape myrtles, a traditionally southern-grown tree (think of the beautiful streets of Charleston, South Carolina), likely do not come to mind. Yet the popularity of these trees has grown tremendously in recent years and today crape myrtles are in the top 10 of the most common publicly-owned tree species across the district. With their long-lasting displays of white, pink, red, or purple flowers and their tolerance for the many stressors of the urban environment, this shift is no surprise.

But as plants spread to new geographies, so do their pests. This reality has been especially true with an emerging pest of crape myrtles, the crapemyrtle bark scale (CMBS). The name “scale” refers to many different but closely related groups of insects that often don’t resemble traditional ideas of insects at all. These microscopic bugs typically have the appearance of small bumps or swellings on leaves and stems, and where there is one scale, there is usually many more. All scales make use of a piercing-sucking mouthpart (imagine a straw-like apparatus) to feed on their plant host, not completely dissimilar to how a tick might use its mouthparts to feed on the blood of its host.

CMBS is an introduced, invasive scale species that was first found in Texas in 2004 and has since spread throughout the south and up the east coast, including throughout the nation’s capital. The most obvious sign of CMBS to look for are the white, waxy masses (ovisacs) that cover the females and their eggs. A single ovisac can house many pink eggs that hatch into a nymphal life stage commonly referred to as “crawlers”. Crawlers are one of the few mobile stages of this otherwise sessile insect and once they settle, they insert their mouthparts into the tree and begin to feed.

The act of feeding by large populations of CMBS does not only stress the tree by tapping into its stores of carbohydrates, but it also results in the formation of a sugary byproduct from the insect referred to as honeydew. Another common pest of crape myrtles, crapemyrtle aphid, will also produce copious amounts of honeydew from feeding on the foliage. Honeydew is readily colonized by a black fungus commonly referred to as sooty mold which is unsightly, difficult to remove from surfaces, and can restrict the photosynthetic capacity of understory plants that are covered with it. All in all, it can be quite a headache to deal with!

While CMBS outbreaks can be challenging to manage, there are options. One of the best practices when installing new material in your landscape is to screen it for signs of pests such as scales. If you find small populations, they can be removed by pruning branches or by taking a scrub brush and physically scrubbing the scale off the bark. Natural enemies such as ladybird beetles have also shown to be quite effective at controlling this pest. Finally, a variety of pesticide treatment options are available and may be warranted when populations are large or other control strategies prove ineffective.

Chris Riley is an Entomologist and Mid-Atlantic Research Scientist and Technical Support Specialist with The Bartlett Tree Research Laboratories. Chris serves as a research collaborator with Casey Trees, providing knowledge and expertise that ensures the health of our urban tree canopy.