CRAPEMYRTLE BARK SCALE
By: Joey Williamson, PhD, HGIC Horticulture Extension Agent, Clemson University
The crapemyrtle bark scale (Acanthococcus lagerstroemiae) is a recently introduced pest from Asia that initially infested crapemyrtles (Lagerstroemia spp.) in Texas during 2004. Since then, it has spread rapidly through Oklahoma, Arkansas, Louisiana, Mississippi, and Georgia. Now it has been discovered in North Carolina and Virginia, and the distant spread of this pest has likely been through the movement of plant material. With the recent appearance of the crapemyrtle bark scale (CMBS) in Mecklenburg County (Charlotte) in North Carolina, this insect pest may appear soon in South Carolina.
The CMBS is a bark or felt scale, which is slightly different from soft scales. However, they have a waxy coating and exude honeydew, as do soft scales. Bark scales are in a different scale insect family (Eriococcidae) than soft scales (Coccidae), and they look very similar to mealybugs.
The CMBS infestations appear as white or gray, waxy crustations on stems, large twigs, and trunks, but rarely on foliage. They especially congregate in branch crotches and at pruning sites. This scale will settle to feed under loose, exfoliating bark of the crapemyrtle, which makes control by both predators and pesticides more difficult.
These bark scales produce copious amounts of honeydew, the sugary waste the scale produces as it feeds on the plant’s phloem. As a result, the leaves, branches, and trunk become covered with black sooty mold, which grows on the honeydew.
Insect Life Cycle
The small CMBS males are winged and will fly to find females and to mate. Once the mated females produce their ovisacs (egg-containing capsules) and lay eggs, they die. The eggs remain protected within the white colored ovisacs until the crawlers (immatures) hatch and disperse onto the branches. Each female lays about 60 to 250 eggs, which may over-winter within their ovisacs, and then hatch during mid- to late April to May. The crawlers are pink, very small, and may not be noticed without a hand lens. A second peak in crawler activity occurs in late summer. Double-sided sticky tape wrapped around small branches can be used to trap the crawlers to see when they hatch and to base the timing of additional contact insecticide applications. These mobile crawlers move out to new twigs and branches to settle down and begin feeding on the sugary phloem layer beneath the bark.
Crapemyrtles suffer aesthetic damage because of the CMBS infestations. These bark scales may not kill the plants, but there may likely be a reduction in plant vigor, number of flowers, and flower cluster size. Infested plants typically leaf out later than healthy plants. Branches and trunks can be covered in the white scale infestation. Another striking symptom is the extensive amount of black sooty mold that may completely cover the foliage, branches, and trunks. However, do not confuse the honeydew and resulting black sooty mold caused by an aphid infestation with that caused by the crapemyrtle bark scale. Aphids are small insect pests that feed on new tender growth on the ends of branches. With a scale heavy infestation, there may be premature bark peeling. Often there will be more female adults congregated on the lower (and shadier) sides of branches.
Cultural Control: Keep crapemyrtles healthy by properly mulching, irrigating, fertilizing (based on soil test recommendations), and proper pruning. Please see HGIC 1009, Crapemyrtle Pruning for best pruning practices. Crapemyrtles in sunnier sites often have smaller infestations than plants growing in more shade, and plants grown in shade typically have more crawlers (immatures) than in full sun. So, always plant crapemyrtles in the full sun areas of the landscape.
Several other common landscape plants are susceptible to CMBS infestation. These include pomegranate, persimmon, edible fig, boxwood, American beautyberry, cleyera, privet, and raspberries. These plants should be closely inspected for the CMBS, especially if crapemyrtles are planted nearby.
Natural predators may take a while to build up in numbers, but both lady beetles and mealybug destroyers are very effective in controlling CMBS.
Chemical Control: The most effective chemical control is a soil drench in the spring with either imidacloprid or dinotefuran. These insecticides are available in a number of brands as concentrates to use as a soil drench, and in a few brands as granular products to scatter around the plants and water into the soil. These systemic insecticides will move up into the plants and give control for at least a year. They are most effectively applied in spring as new plant growth begins. See Table 1 for examples of products containing these systemic insecticides.
Sprays for crawlers are best applied in the late April and May, then again in late summer when immatures appear. Use a bifenthrin spray mixed with 2% horticultural oil (i.e., 5 tablespoons of horticultural oil per gallon of water in a sprayer) added for best crawler control. Follow the label directions on bifenthrin products for rate per gallon. See Table 1 for examples of products containing bifenthrin and horticultural oil.
To determine if the soil drench treatments have been effective, scrape the soft bodies of the CMBS adults on a branch. If the result is the presence of a reddish body fluid of the scales, they are still alive. No “bleeding” will occur if they have been killed.