Crapemyrtle Bark Scale

Crapemyrtle Bark Scale

Allen Robinson: Ask a Master Gardener

Tuesday, August 1, 2017

Q: I have white spots that turn to a blackish mold on my crape myrtles. What is it and how do I get rid of it? Tricia, Tulsa.

A: 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 crape myrtle 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 crape myrtles. This is the reason one should always inspect nursery crape myrtles for scale insect before purchase.

Like other scales, the life cycle begins with either the female scale or eggs overwintering on the crape myrtle under loose bark. When the eggs hatch, small mobile “crawlers” are produced, which migrate on the plant and may be spread to other crape myrtles 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 crape myrtle, 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 crape myrtle. The mold is unsightly and creates a reduction in aesthetic quality, but it is not significant in terms of the plant’s health.

Garden tips

·        Divide and replant crowded hybrid iris (bearded iris) after flowering until August. When planting, take care not to plant the rhizomes too deeply. Cover them with an inch of soil or less. Do not mulch iris

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·        Water all plants deeply and early in the morning. Most plants need approximately 1 to 2 inches of water per week. Rather than watering daily, water less often and more deeply.

·        Some trees such as sycamores and river birches lose large numbers of leaves in the heat of summer. Trees do this to reduce water loss from their leaves. It is a coping action by the tree; it is not dying.