Crape Myrtle Bark Scale
Crape myrtles are ubiquitous in home and commercial landscapes in our area, but a new pest has the potential to effect these (arguably over-planted) shrubs. Crape Myrtle Bark Scale (CMBS), like its host, originated in Asia. CMBS was first identified in north Texas about twelve years ago and has since moved across much of the southeastern US. NC State University entomologists confirmed the arrival of CMBS in North Carolina last year.
Like other scale insects, CMBS is cousin to aphids and other piercing-sucking Hemipterans. They feed by using their specialized mouthparts to tap into the phloem cells of plants and consume the sugary sap made in photosynthesis. The waste product excreted out their tiny rear-ends (euphemistically called ‘honeydew’) still contains high concentrations of sugar compounds. This honeydew, in turn, is a favorite food of a group of saprophytic (non-pathogenic) fungi known as black sooty molds. While these molds do not directly harm plants by infection, they greatly degrade the aesthetic appeal of many species of ornamental trees and shrubs. Heavy infestations can cover leaves entirely, reducing photosynthetic efficiency and potentially stressing affected plants. The greater risk of CMBS to our crape myrtles, therefore, is not rapid decline and death, but rather the undesirable appearance of being covered in black mold.
Female CMBS produce felt-like waxy encrustations around their bodies that provide some protection against predators and most insecticides. This felt-like covering is also a characteristic of the closely related azalea bark scale, and distinguishes CMBS from their more distantly related mealy bug and armored-scale cousins. Under this covering, females remain attached to feed at the same location until they lay eggs and die. In late spring or early summer, first instar (molt) nymphs called ‘crawlers’ hatch and move to a new location to feed, molt, mate, and repeat the cycle. At least two overlapping generations per year are possible in our climate.
In low densities, CMBS favor feeding on the bark around branch crotches (the area where two stems intersect), but in higher densities they can be found all over the bark of large and small diameter branches and trunks. Individual scales are white-gray in color and about the thickness of a dime (2 mm). When crushed, CMBS oozes a pink blood-like fluid. Often though, most homeowners don’t notice CMBS until the tree is covered in the black sooty mold growing on honeydew. Black sooty mold can also grow on honeydew of aphids, so if you observe mold on your crape myrtles, assume it is caused by aphids unless you can find CMBS.
Because this is a relatively new pest, NC State and other universities are still doing research to pinpoint the best control measures. Unfortunately, the systemic insecticides that are often effective on piercing-sucking insects are particularly difficult to use properly on crape myrtle because of its long bloom period. According to the label (and remember, ‘the label is the law’), many of these pesticides cannot be legally applied when plants are in bloom in order to protect bees from exposure. Instead, try applying dormant-grade horticultural oils in the winter, or using a soft brush and water to scrub off heavy infestations.
For more information, contact your local N.C. Cooperative Extension County Center.
Matt Jones is Horticulture Agent for North Carolina Cooperative Extension in Chatham County.