In my 25-plus years of hosting GardenLine, this question has only come up in the past five years. And, boy, have I seen it a ton of it in my consulting business over the past three months.
“Randy, I have white spots on the trunks of my crape myrtle that also look like they are bleeding. Then, the spots turn to a blackish mold. What is it, and what can I do about it?”
It is an insect called crape myrtle bark scale. Relatively new to our area, 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 you should always inspect nursery crape myrtles for scale insects before you buy them.
As is the case 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 lay 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 myrtle trunks and limbs, so it’s easy to identify. Aphids, whiteflies, and lace bugs ravage the leaves. The adult female is usually about 2 millimeters long and has a distinctive gray or white felt-like covering. When a female 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 - large black patches that develop on the bark of the crape myrtle. The mold is unsightly, but it is not life-threatening to the crape. Some may believe, because of the fuzzy covering, that it’s a mealy bug. They’re not, 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 occurs. So, if you use malathion, DO NOT SPRAY THE LEAVES! Or, sponge-treat the trunks, limbs and nooks and crannies to keep from defoliating.
Getting rid of the black sooty mold left behind is as simple as soapy water rinsing and re-rinsing or using fungicides like Consan 20 or neem oil.
For years, neem oil has been an organic alternative to insect and disease control, but it rarely ever worked on scale. That is until now. There is a new neem on the market from Bonide called Captain Jack’s Neem Max, pictured above. It gives scale a better hit, and its fungicide component helps break down the black sooty mold left behind.
For crapes in the 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. There is, however, another organic treatment … 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 - physical scrubbing is needed.
The only other insecticide that's been shown to work is liquid bifenthrin. While some may call for such 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.