In my 22 years of hosting GardenLine, this question has only come up in the past two.
“Randy, I have white spots on the trunks of my crape myrtle that also look like they are bleeding, then they turn to a blackish mold. What is it, and what can I do about it?”
Photo: Randy Lemmon
It is an insect pest called crape myrtle bark scale. It is relatively new to our area, but it has really made a name for itself in short order. It came to the U.S. from Asia and first appeared in North Central Texas around 2004. It has subsequently spread down into Southeast Texas, likely from nursery stock and other sources of imported crape myrtles. This is the reason one should always inspect nursery crape myrtles for scale insects before purchase.
As with other scales, its life cycle begins with either female scale or eggs overwintering under loose bark on the plant. When the eggs hatch, small “crawlers” migrate around the plant and may spread to other crapes by wind or birds.
Two or three generations may be produced per year, depending on temperatures. Once the female is fully developed, she mates and attaches to stems and trunks, where she remains to lays eggs for the next generation. She dies shortly thereafter, but the eggs survive under her covering until they hatch.
This pest is the only scale insect to infest crape myrtles, so it’s easy to identify. The adult female is usually about 2 millimeters long and has a distinctive gray-white felt-like covering. When one of the females is crushed, a pink fluid is released.
As the scales feed, they produce liquid “honeydew.” This is similar to the behavior of aphids. The sugars in honeydew may support the growth of a fungus called sooty mold. This overgrowth produces large black patches on the bark of the crape myrtle. The mold is unsightly, but it is not life-threatening to the crape.Some observers may believe, because of the fuzzy covering, that it’s a mealy bug. They are not mealy bugs, but mealy bugs are part of the scale family of insects.
Spraying the trunk with malathion is one of the best methods of elimination. But malathion can defoliate crapes if leaf contact is made. So, if you use malathion, DO NOT SPRAY THE LEAVES!
If crapes are in highest state of dormancy, genuine dormant oil sprays are a good idea. But once the crapes begin awakening with springtime warmth, that organic treatment will have to wait until the following winter. And neem oil, while useful in other applications, will not be effective for this scale. However, another organic treatment can be made with a mild solution of dishwashing soap and water, applied with a long-handled brush to remove the scale and sooty mold. Just spraying on the soapy water will not be effective against scale, so physical scrubbing is needed.
The only other insecticide that's been shown to work is liquid bifenthrin.
While some websites call for systemic insecticides to treat from the inside, I do not. Systemic insecticides in the neonicotinoid family enter the blossoms of crape myrtles and would be harmful to bees and other pollinating insects.
Lastly, for those using insecticides and not a scrub-down method, you will likely need to get rid of the black sooty mold as well. You can do this with a soapy water spray, but you have to rinse everything off after the sooty mold breaks down. And it can take a couple of treatments. Good old-fashioned Consan 20 fungicide can also be sprayed on the black sooty mold to break it up. But, like a soapy water spray, it must be rinsed off several minutes after application to wash away the loosened mold.