Control of Crapemyrtle Scale

New treatment recommendations help combat crapemyrtle scale

Bill Sevier: Ask a Master Gardener

Saturday, July 30, 2016

Q: I have what I have been told is crape myrtle scale. How do I get rid of it? Lee, Tulsa.

A: We addressed the crape myrtle bark scale in this column last year, but some of the recommendations made by OSU regarding control have changed. The current recommendations come from Eric Rebek, state extension specialist for horticultural insects.

This insect is new to our area but is 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 the 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 but 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 crape myrtles. 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 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 crape myrtle with a mild solution of dish-washing soap and water using a long-handled brush to remove the scale and sooty mold. This is effective.

Another treatment recommended is to spray the trunks of the trees with dormant oil in late winter. Winter dormant oil spray is a stronger concentration of a petroleum-based oil that is used in summer. Neem oil, while 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 crape myrtle bark scale but are no longer recommended. The reason for the change in policy is the concern that these insecticides enter the blossoms of crape myrtles and would be harmful to bees and other pollinating insects.

Garden tips

§  Divide and replant crowded hybrid iris (bearded iris) when dormant — July, August and into September. When planting, take care not to plant the rhizomes too deeply. Cover them with an inch of soil or less. Do not mulch iris.

§  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.