Saturday, September 19, 2015 By: Ask A Master Gardener

Crapemyrtle Bark Scale, a New Pest

Bark scale insects can infest crapemyrtles

Brian Jervis: Ask a Master Gardener

Saturday, September 19, 2015

Q: My crapemyrtle tree has some type of insect on the trunk. They look to be covered with cotton. What is this? T. R., Tulsa

A: Insect pests continue to surprise us. As we learn to deal with the old ones, we are faced with new arrivals. This one comes from crapemyrtle’s region of origin, Asia, and is called crapemyrtle bark scale (CMBS). It was first discovered in the U.S. in central Texas in 2004 from where it spread northward.

Master Gardeners are now seeing it regularly in their office. Most of CMBS seems to have been spread by nursery stock and on plants purchased in one area and used in another. The female scales cannot fly.

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. These crawlers mature into adults. There may be 2-3 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 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, which 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.

Treatment strategies for this insect are still being evaluated. One nonchemical approach is to scrub down the trunk of the crapemyrtle with a mild solution of dishwashing soap and water using a long-handled brush to remove scale and sooty mold.

Summer horticultural oils recommended for other scales are not thought to be useful, but using heavier dormant oils applied to the trunks in late winter may be effective.

Systemic insecticides when applied as a drench to the plant’s root zone may offer the best control. The insecticide chemicals shown to be effective are imidacloprid, thiomethoxam and dinotefuran. These generics are sold under several brand names. They should be applied between May and July as it takes a few weeks for the chemicals to be absorbed into the plant’s vascular system and before protection will start.

For more information, go to OSU’s Pest E-alert website for an excellent discussion.

Garden tips

Watch for fall specials at garden centers and nurseries because fall is a great time for planting many ornamentals. Choose spring flowering bulbs as soon as available.

Fertilize established fescue lawns with one pound of actual nitrogen fertilizer per 1000 square feet now and again in November. Do not fertilize Bermuda or zoysia lawns until next spring. Late fertilization of these warm season grasses may promote disease.

Winter broadleaf weeds like dandelion will begin to emerge in late September, which is also the best time to control them with a 2, 4-D type herbicide.

September and early October is garlic planting time with an aim for harvest in June of next year. There are many varieties from which to choose. OSU suggests German Red, Inchilium Red, Silverskin and Spanish Roja for varieties that do well in our are


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