The blackish trunks on the crapemyrtles around town are caused by a Chinese insect known as crapemyrtle bark scale. It doesn’t kill the trees, but it does make them unsightly and less vigorous. This scale is causing concern because it is has spread quickly, and as you know, crapemyrtles are the most popular and common ornamental tree in the South. In its native range, this scale feeds on crapemyrtles and pomegranates. It causes extensive honeydew deposits and the growth of black sooty mold. The black sooty mold isn’t the problem. The insects on the trunks and branches are.
Identifying Crapemyrtle Bark Scale
Crapemyrtle bark scale is easy to identify. Adult females are whitish encrustations that stick to crapemyrtle trunks and twigs. When crushed, the scales exude a pink liquid. Look closely and you may see dozens of pink eggs or crawlers. Most gardeners are first alerted to crapemyrtle bark scale by the presence of black sooty mold on the bark. In more southern areas, sooty mold may be mistaken for that of the crapemyrtle aphid. However, the presence of the white scales on the bark and twigs, and the pink blood exuded when crushed, distinguishes this scale from aphids which feed on foliage and are only present during the growing season.
Treating Crapemyrtle Bark Scale
Entomologists say if your plants are heavily infested, wash the reachable trunks with a soft brush and mild solution of dishwashing soap and water. I’ve never taken the time to do this, however I have used a spray nozzle on a hose. This will remove many of the 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 has not been shown to be very effective against this insect. However, it may be beneficial to apply horticultural oil in the winter to the bark and crotches of the plants where the scales shelter.
Systemic insecticides have shown the most promise in tests to date. The most commonly available systemic insecticides contain the active ingredient imidacloprid and should be applied now according to exact label instructions. Although different brands will have different names, the active ingredient will always be listed on the label. Imidacloprid products (such as Bayer Complete Insect Killer) are available in both liquid (spray or drench) and granular (soil applied) forms. Remember that it takes several weeks for soil applied systemic insecticide to become active in the plant.
Products containing Dinotefuran (Safari) have also proved effective. It is very important to apply these products early in the season when the new leaves are emerging and before flowering occurs so as not to affect pollinators like bees and butterflies. The crawlers can be treated by spraying with a product containing the active ingredient bifenthrin. Ortho Bug B Gone is an example. It too, should only be used before bloom in the spring or after bloom in the fall. You might have to use it again in the fall depending how the scale population. I hate it, but crapemyrtle bark scale is apparently going to be like fire ants and feral hogs, something we have to live with and deal with every year. For the full fact sheet on this insect, Google “Crapemyrtle Bark Scale Texas A&M.”
Greg Grant - Smith County Horticulturist