Crepe Myrtle Bark Scale – Ann Huyler, SCFC Pee Dee Region Urban Forestry Specialist
Crape Myrtles. Crepe Myrtles. Shrub. Tree. Non-native invasive. Four-season Bomb-Proof Beauty. These topics have long garnered convictions and arguments. Now, except for one: Bomb-Proof.
Lagerstroemia indica has been cultivated for 1700 years, much of that in China. The native range is reported to be the Indian subcontinent (origin of specific epithet ‘indica’), southern China, Japan, Korea, parts of Southeast Asia, and surprise, Northern Australia.
Around 1759, L. indica landed on US shores and arrived in Charleston three to four decades later. George Washington planted them on his estate in the 1790s. In the 1960s, Donald Egolf at the U.S. National Arboretum started hybridizing L. indica with L. fauriei from Japan to increase powdery mildew resistance and cold hardiness.
Those hybrid crape myrtles were considered bomb-proof as they were immensely adaptable to harsh conditions. They withstood heavily compacted soils along streets, survived vehicle strikes and weed eater blades, being topped and otherwise very badly pruned. They thrived in the south’s high heat and humidity, resisted diseases (e.g. powdery mildew) and insect infestations (e.g. crape myrtle aphids), and still bloomed beautifully. Yes, you know what comes next.
Until now.
The crape myrtle bark scale, Acanthococcus lagerstroemiae, is a small insect, 2mm in size. Of at least 6000 species of scale insects, 681 are bark scales. The crape myrtle bark scale was first reported in Dallas, Texas, in 2004 and, as of 2021, has since been formally identified in at least 14 states plus the District of Columbia, from Maryland down to Florida and westward to Kansas and New Mexico. Of note, the native home range of this scale species is the same native home range of L. indica, and cities in China have dealt with their own outbreaks.
If your crape myrtle is infested with this bark scale, the adult female scale would be the easiest to spot, because she constructs a spiky white wax cover over herself. When adult females are impregnated, they create an ovisac, which looks like a fish scale covered in white fuzz, under which she lays her eggs. Crushing the ovisacs, or the scale insects, produces a pink fluid, called hemolymph, and that is a visual identifying marker for crape myrtle bark scales.
Unlike aphids which feed on leaves, the crape myrtle bark scales feed on bark, often on the smaller twigs and in crevices or wounds on limbs or trunks. Heavy infestations can cover entire limbs. The feeding is the problem. Large populations of bark scale drink a lot of sap. Limb dieback and reduced flowering and flower size occurs. Young trees may be killed.
In South Carolina, investigations of crape myrtle bark scale are being led by Steven Long, head of the Plant Protection Division of the Department of Plant Industry at Clemson University. Clemson University verified the presence of crape myrtle bark scale in Richland County. If you see what appears to be crape myrtle bark scale, please contact your Clemson Extension agents on their website https://www.clemson.edu/extension/index.html.
The crape myrtle bark scale is now a permanent resident of South Carolina. What can be done to minimize damage to our crape myrtles?
If you want to plant crape myrtles, please, inspect each tree and only accept clean trees. Plant the crape myrtle in full sun as infestations are more severe with shade. Limit pruning to only what is needed for the health of the tree. The crape myrtle bark scale often prefers feeding from wounds and pruning cuts. Only apply the bare amount of nitrogen fertilization to maintain a healthy crape myrtle. Excess nitrogen fertilization may increase the bark scale population. And, provide the tree with sufficient water and appropriate mulching every year.
If you have an infested crape myrtle, certain treatments can reduce the crape myrtle bark scale population. Horticultural oils, insecticidal soap, insect growth regulators, and insecticides differ in effectiveness and some only impact specific life cycle stages of the insect.
Spraying horticultural oils or insecticidal soap (not dish soap!) can decrease crawler density by killing on contact. Crawlers are nymphs that have just hatched from eggs and have not formed the protective waxy shell of an adult. When applied at peak crawler activity, horticultural oils can reduce 50% of crawler population. To be effective, crawlers must be covered with the oil or the soap before either treatment dries. Multiple applications are needed within a year to catch crawlers who avoided contact and those that hatched from ovisacs after application. Both horticultural oil and insecticidal soap must be used according to label instructions, or they can burn new leaves and buds and also kill the soft-bodied larvae of ladybugs and lacewings.
Insect growth regulators are hormone-like chemicals that prevent insects from progressing through life stages, such as hatching and molting. They do not kill on contact but can be used with a contact insecticide. Similar to horticultural oils and insecticide soaps, insect growth regulators are effective against crawlers, not adults or eggs under waxy shells. When applied at peak crawler activity, one sprayed application of insect growth regulator can reduce crawler density by 85% to 100% for 4 months. Beneficial insects, such as bees, can also be harmed by insect growth regulators.
Research has shown that neonicotinoid insecticides, such as imidacloprid, dinotefuran, thiamethoxam, and clothianidin, are the most effective at reducing crape myrtle bark scale infestations. Neonicotinoids are chemically similar to nicotine, the natural insecticide produced in the leaves of tobacco. These insecticides are sold by several brands in different concentrations and formulations and can be employed as spray or as a root drench.
Neonicotinoid insecticides sprayed on a tree can significantly decrease the number of crawlers. Sprays are not as effective for adult scales or for eggs under the ovisac. Spraying will be needed in spring and also in late summer for crawlers that hatch from ovisacs after the initial spray. These sprayed insecticides will damage or kill non-target insects on the tree, including natural predators and pollinators.
Systemic applications of neonicotinoids are when the insecticides are mixed with water and poured on the root area. The crape myrtle roots take up the insecticide, which becomes incorporated in plant tissues. The neonicotinoids kill crape myrtle bark scale feeding on the tree. Compared to sprays, systemic applications of neonicotinoids are the most effective way to decrease crape myrtle bark scale populations and minimize harm to non-target insects on the tree.
However, with neonicotinoids, a serious issue is incorporation of the insecticide into pollen. Crape myrtles produce pollen that bees and their larvae feed on. Corn crop studies involving neonicotinoids and bees have concluded that bees exposed to neonicotinoids through pollen have increased mortality during overwintering, reduced immune responses, and lower colony reproduction. Neonicotinoid insecticides, whether sprayed or drenched, can play a role in bee population declines.
Natural predators may provide some assistance. In the US, lady beetles, such as the twice stabbed ladybug and the mealybug destroyer, are natural predators of bark scale insects and have been observed feeding on crape myrtle bark scale. In the south, crape myrtle bark scales can produce 2 to 4 generations per year and can overwinter. The University of Florida extension agency states that ladybug predation does not come early enough in spring to reliably inhibit bark scale populations. Outside of the US, two green lacewing species and 16 parasitoid wasp species are known to feed on crape myrtle bark scale. As yet, no investigations have reported on successful native lacewing or parasitoid wasp predation on our bark scale. Given that crape myrtle bark scale and their native predators coexist in China where crape myrtles are being infested, biological control will only be part of the process.
Regardless of the treatment: horticultural soils, insecticidal soaps, insect growth regulators, and neonicotinoids, all will need to be applied throughout the life of the tree. The larger the tree, the greater the amount of treatment.
A more drastic approach would be to cut the tree down to the ground. The bark scale does not overwinter on the ground or on roots. When the crape myrtle sprouts, prune away the sprouts you don’t want, add appropriate mulch and water, and the crape myrtle can regrow. Burn the removed crape myrtle debris or place it securely in a bag for landfill disposal. Composting or putting the debris out for pickup allows the scale insects to spread. Re-infestation can occur if birds, large insects, or wind deposits a female and male, or a female with eggs, on any crape myrtle.
In SC, many crape myrtles, large and small, are not infested and they continue to provide us with beauty year-round, brightening our cities and homes. Urban foresters and city administrations, Public Works and Parks and Rec, need to search for substitutions for crape myrtles. More than likely, we will be replacing our crape myrtles with multiple tree and shrub species. This will increase the diversity of our urban forests, while helping keep our cities green and cool and lovely.
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