Scale on Nashville Trees

Scale (also called bark scale) is a common pest found on both indoor and outdoor plants, including trees. While not all are specialized, some scale bugs are associated with specific plants and trees. Any tree can be affected by scale, but over the last several years, there has been a particular problem with a newly invasive scale bug being found on crepe myrtle trees. Here, we’ll go over what scale is, why these bugs are a problem, and how to deal with them if you find a tree infested with them.

What Is Bark Scale?

Scale is a small bug that feeds on the sap of plants. It isn’t very mobile as an adult, but its larvae will spread out and establish themselves on the leaves and stems of an infected plant. There are hundreds of types of scale, but they all have a similar appearance: these insects are tiny and white or gray, with some having a hard shell that looks like a scale, from which they get their name. They are also known as mealybugs or soft scale when they don’t have shells (those with a shell are known as hard scale). The bugs cover a plant’s stems, leaves, and branches and are easily visible.

Identifying Scale 

The white or gray scale insects will be noticeable against a plant’s green or brown color, like lots of small bumps across the plant’s surface. Scale bugs secrete a sweet, syrupy substance when they feed on sap, which attracts other insects like ants and aphids. As the honeydew accumulates on the plant, it often grows a type of fungus called sooty mold, which is a group of many similar-looking molds that look like ash or dark-colored dust. 

While scale usually isn’t fatal to a tree, the pest will stress it out from the damage done to leaves and bark. On top of that, the presence of sooty mold on the leaves of a tree will block sunlight from reaching the leaves’ chlorophyll, preventing photosynthesis and reducing the amount of energy the tree can create. This might result in a tree losing some of its leaves in spring or summer, which is an important sign that a tree is stressed. Since scale bugs remain on the outside of a plant, the pests are able to be seen with the naked eye upon a close inspection of a tree’s leaves and branches.

Scale and Crepe Myrtles

While there are many types of scale native to North America, a new species of scale has been present in the southeastern US over the last decade that has been targeting crepe myrtle trees. First noticed in Texas in 2004, this Asian scale was found in Tennessee near Memphis in 2013 and has since spread across the state. Even though scale bugs are mostly immobile, the small larvae can hitch a ride to other specimens from insects or birds that visit an infested tree.

Until recently, the crepe myrtle has been a relatively problem-free tree. Crepe myrtles are a popular woody plant across the southern states for their bright pink to purple flowers and multiple thin trunks. This ornamental understory tree is not native to the US, but it hasn’t been a problem since it doesn’t spread well on its own, so it isn’t classified as an invasive tree. Now that the specialized crepe myrtle bark scale has become widespread, extra care and attention must be given to these trees. However, any tree may attract one or another type of scale, and treatment is similar for any infected tree.

How Is Scale Treated?

Since scale bugs stay on the surface of a tree, they are easy to notice and relatively simple to get rid of. There are a few ways to deal with a scale infestation on a tree (and any resulting sooty mold growth), depending on the severity and time of year:

• Washing the tree

• Applying a horticultural oil

• Applying a systemic drench insecticide

A tree with a light-to-medium scale infestation might only need to be washed clear of the insects. Using a gentle soap and water mixture, a good scrubbing of a tree’s trunk, branches, and leaves can remove the scale, honeydew, and sooty mold.

Another option, applying a horticultural oil, is specifically designed to smother pests while not harming the tree. It’s important to have good, complete coverage to be effective. It can also be applied after a good washing of the tree. Oil should be applied in late summer or fall rather than in spring, when it might interfere with pollinators visiting the tree’s flowers. A fall oil application will last over the winter.

Bark scale populations grow over the spring and peak in May, so if scale was a problem the previous year, it may be recommended to apply a systemic drench of insecticide in water to the roots of a susceptible tree in early spring. This mixture is absorbed by the roots and gets mixed into a tree’s sap, which will then kill the scale when fed on. A systemic insecticide may also be recommended for particularly bad scale infestations.

What Is the Best Insecticide for Bark Scale?

Pesticides are the last line of defense to deal with pests, particularly to protect pollinators, but they may be necessary for the health of the tree. If your crepe myrtle (or another tree in your yard) has become host to a scale population, care should be taken when applying an insecticide that contains imidacloprid or dinotefuran. While these are effective systemic pesticides that will rid your tree of scale for at least a year, they can be fatal to bees and other pollinating insects as well. 

It's important to follow directions very closely when applying insecticides, measuring and diluting the solutions as instructed. Mixing these products with the right amount of water helps the tree absorb the formula in a safe amount through its roots. It should be applied at the base of the tree onto dry soil (when it hasn’t been raining) so the tree will be efficient and effective with uptake. While these treatments do the job, there is ongoing research into ways to best manage scale infestations that are safe on pollinators. If in doubt about applying insecticide, contact a professional arborist who can knowledgeably apply a treatment to your infested tree.

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