As a followup to last week’s Keeping It Green column about crapemyrtles, I want to provide some details about a new pest that has recently been messing with these beautiful plants.
It is called crapemyrtle bark scale, an exotic scale pest that was discovered in the U.S. only 10 years ago, first in the Dallas area. It is very similar to another scale that gets on azaleas and other woody plants, but was recently determined with pretty good confidence that this scale is different and is a newly introduced insect from the Far East.
It was first discovered on crapemyrtles in Richardson in a commercial planting. What drew attention to it was the blackened trunks and limbs of the trees. The black is caused by a saprophytic fungus called black sooty mold that grows on the sugary exudate that is excreted from the scale insects. While the black mold is not harmful to the tree, it sure makes it look unsightly.
The CMBS initially spread slowly in the Dallas-Fort Worth area for several years, but in the last few years it has now been found in southern Oklahoma, Arkansas, Louisiana and near Memphis, Tennessee. It was found in Tyler last year, and I have seen it on several plants in different locations this year.
The scale feeds on the sugary sap in the phloem that flows just under the bark of the trunk and branches. So far, it doesn’t seem the scales are causing a great deal of harm to the tree, but due to the copious amounts of black sooty mold that grows on the sweet substrate, the trees can look unsightly.
Crapemyrtle bark scale is relatively easy to identify. It is the only scale known to infest crape myrtles. Adult females are felt-like white or gray encrustations that stick to crape myrtle twigs, stems and trunks. When crushed, the scales exude bright pink blood-like liquid.
On new growth and in heavy infestations, the scales may be distributed uniformly on the branch. Up close, CMBS is white to gray and about 2 millimeters long. Look closely and you may see dozens of pink eggs or the smaller crawler stage.
Most gardeners are first alerted to CMBS by the presence of black sooty mold on the bark. The source of the sooty mold may be mistaken for that produced by the crapemyrtle aphid, a common insect pest the feeds on the leaves of some varieties. However, the presence of the white adult scales on the bark and twigs, and the pink blood exuded when crushed, tells you the sooty mold is from scales, not aphids.
Crapemyrtle bark scales may aggregate toward the undersides of young horizontal branches instead of the parts exposed to the sun, and I have often seen them on old pruning cuts.
Based on limited experience with this pest in the U.S., CMBS appears to be difficult to control. Soil-applied neonicotinoids have demonstrated significant suppression. There are some suggestions, however, for controlling this insect include.
Before buying crapemyrtles, inspect plants carefully for signs of CMBS infestations.
If your plants are heavily infested, wash the trunk and reachable limbs with a soft brush and mild solution of dishwashing soap and water. This will remove many of the female scales and egg masses, making insecticide control more effective. Washing will also remove much of the black mold that builds up on the bark of infested trees.
Horticultural oil during the growing season has not yet been shown to be effective against this insect. However, it may be beneficial to apply it in the winter at dormant season rates to the bark and crotches of the plants where the scales shelter. Use enough oil to reach behind loose bark and into cracks and crevices. Winter is a good time to treat for scales because the higher (dormant season) application rate can be used without damaging the plant. Cover the tree thoroughly with pesticide, especially when using oil.
Systemic insecticides have shown the most promise in tests to date. Apply them to the root zone as a soil injection or drench. The best control in tests was achieved between May and July by applications of clothianidin, dinotefuran (Safari, Zylam), imidacloprid (Merit, Bayer Advanced Garden Tree and Shrub Insect Control), and thiomethoxam (Meridian). When drenching the soil with a systemic insecticide, allow several weeks for the product reach throughout the plant.
Although some insect growth regulators are recommended for control of other types of scales in woody ornamentals, they have not yet been evaluated on CMBS.
Some lady beetle species, especially the twice-stabbed lady beetle, are effective predators of this scale. However, control by lady beetles is often too late in the season to prevent aesthetic damage from sooty mold.
Keith Hansen is Smith County horticulturist with the Texas A&M AgriLife Extension Service. His web page is http://EastTexasGardening.tamu.edu. His blog is http://agrilife.org/etg.