I have white spots that turn to a blackish mold on my crapemyrtles. What is it and how do I get rid of it? Tricia, Tulsa
We addressed this issue in this column last year, but some of the recommendations made by OSU regarding control have changed. The current recommendations come from Dr. Eric Rebek, state extension specialist for horticultural insects.
It is an insect pest that is called Crapemyrtle Bark Scale. This insect is relatively new to our area but has become fairly prevalent. It came to the U.S. from Asia and first appeared in north-central Texas in 2004. It has subsequently spread northward into the Tulsa area from nursery stock and other sources of imported crapemyrtles. This is the reason one should always inspect nursery crapemyrtles for scale insect before purchase.
Like other scales, the life cycle begins with either the female scale or eggs overwintering on the crapemyrtle under loose bark. When the eggs hatch, small mobile “crawlers” are produced, which migrate on the plant and may be spread to other crapemyrtles by wind or birds. There may be two to three generations produced per year depending on temperatures.
Once the female is fully developed, she mates and attaches to the stems and trunks of the crapemyrtle, where she remains fixed and lays eggs for the next generation. She dies shortly thereafter, but the eggs survive under her covering until they hatch.
As the scales feed, they release a liquid called “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 crapemyrtle. The mold is unsightly, which creates a reduction in aesthetic quality, but it is not significant in terms of the plant’s health.
This pest is easy to identify because it is the only scale insect to infest crapemyrtles. The adult female is usually about 2mm long and has a distinctive gray-white felt-like covering. When one of the females is crushed, a pink blood-like fluid is released.
The current recommendations for control are removal by hand and the use of winter dormant oils. Scrub down the trunk of the crapemyrtle with a mild solution of dish soap and water using a long-handled brush to remove the scale and sooty mold. This method is very effective. Another recommended treatment is to spray the trunks of the trees with dormant oil in late winter. Winter dormant oil spray is a stronger concentration of the petroleum-based oil that is used in summer. Neem oil, while useful in other applications, will not be effective for this scale in winter.
Previously, systemic insecticides in the neonicotinoid family were recommended. These chemicals enter into the circulation of plants and kill the pest when they feed on the sap. They have been shown to be effective against crapemyrtle bark scale but are no longer recommended. The reason for the change in policy is the concern that these insecticides enter the blossoms of crapemyrtles and would be harmful to bees and other pollinating insects.
You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at firstname.lastname@example.org.