With the abundance of beautiful blooms in a multitude of yards and parks throughout South Carolina in the summer, the crape myrtle status would seem to be stable and probably, if not beneficial, at least harmless to our native biota. Sadly, however, the beloved tree has a new threat that is going to be extremely difficult to control.
French botanist Andre Michaux brought Lagerstromia indicas (commonly known to us as crape myrtles) to his herbarium near Charleston in the 1780s. Interestingly, these trees had been grown for several decades previously in England but were not a hit as it was seldom hot enough for them to bloom.
Well, surprise, surprise! Crape myrtles planted in Charleston were like Br’er Rabbit’s being thrown into the briar patch. Extremely tolerant of soil types and low fertility, only really troubled by salt exposure, and with excellent drought tolerance once established, crape myrtles grow best when planted in full sun. They are also incredibly easy to root.
Mary Jordan (pronounced “Jerdan”) Wannamaker, who moved as a bride to St. Matthews, South Carolina, in 1927, cut a few branches from her mother’s tree in Monticello, Georgia, and stuck them in the ground at her new home, where they still grow beautifully today. Her son, Luther Banks Wannamaker, inherited her skill with plants and is a renowned Southern seedsman. With up to three months of extraordinarily showy bloom during the hottest months of summer — one of the ancient Korean species was named “One Hundred Days of Red” — these trees are often found in the landscape of buildings ranging from old homesites to brand new office buildings. For the nursery industry, these trees bring in sales of $49 million a year.
In addition to their happiness at being neglected if sited correctly, crape myrtles have few disease or insect problems. Powderly mildew, a group of fungi that attack many plants when the humidity is high, usually occurs when crape myrtles are planted too closely together with poor air circulation and in shade. If a tree is properly sited, avoiding late afternoon watering and removing suckers from the base will usually keep the problem at bay. The most serious insect problem, until now, has been the crape myrtle aphid. These soft-bodied insects, whose populations reach epidemic proportions in the hottest weather, have the unpleasant feeding strategy of inserting their mouthpart into the underside of a leaf and letting the plant’s vascular system push carbohydrate-rich fluid through their bodies. They absorb just a fraction of the nutrients while the excess, called honeydew, is excreted and lands on the upper surface of leaves and branches below. Nature is not going to let that nutritious residue go to waste, and so sooty mold grows on it, covering the leaves with its black mycelium.
Japanese beetles occasionally can eat every flower, but that is localized; some people have trouble and others none at all. Avoid putting out traps for Japanese beetles as the attractant is the equivalent of an extremely expensive French perfume — it only captures the males and draws them in from neighboring yards.
These problems, although annoying, were concerns of the past for new landscapes when knowledgeable nurserymen, designers, and up-to-date homeowners began planting crape myrtle hybrids developed by USDA researcher Don Egolf to improve pest and disease resistance and cold hardiness. Beginning in the early 1960s, Don and his staff began years of work creating hybrids between the traditional L. indica and other species, especially the Japanese L. faurieri, with its much darker bark. By tediously removing pollen from male flowers of one species, transferring it to the receptive female flower part of another species, and then covering that structure to exclude other pollen, they created seeds to plant and evaluate.
After years of work, involving growing more than 200,000 seedlings and monitoring them for improved characteristics, the USDA introduced 29 hybrids with resistance to powdery mildew and sooty mold. These hybrids come in a variety of heights and colors and are among the most popular crape myrtles of all times. ‘Natchez,’ with its dark brown trunks, dark green foliage, and vivid white flowers, is probably the all-time favorite, but all have superb features. All hybrids are given names from Native American tribes as a tribute to Don’s love for his home state of Oklahoma.
Crape myrtles are not native, but they also are not invasive. Although pollinators do not preferentially feed from their flowers, the fact that they bloom in the dead of summer when few other flowers, cultivated or native, are flowering, does mean that certain insects visit them. And for our native birds, in a 2018 article, Ornithologist Gary Graves of the Smithsonian Museum of Natural History reported documenting heavy and preferential feeding on crape myrtle seeds by goldfinches, juncos, house finches, cardinals and others, which he believes is a fairly recent adaptation to the enormous availability of this food source. Fortunately, this feeding pattern is not spreading the population of crape myrtles. I seldom see a volunteer, which unfortunately does occur when birds feed on the fleshy fruits of privet, Elaeagnus, and other invasive plants.
Thomas Friedman in his book The World Is Flat writes about how economic globalization has removed barriers to trade and commerce. It has done essentially the same in eliminating the natural barriers of mountain ranges, rivers, densely shaded forests, deserts, oceans, and soil types, which kept living organisms in their native areas. The massive movement of materials in containers using wooden pallets and tons of packing materials provides harbor for certain living organisms. This has led to an explosion of non-native species appearing across the world. Fortunately, the majority of these accidental imports do not persist in their new environment or pose threats to those ecosystems. But with the sheer magnitude of these that arrive each year, we have been faced with some devastating organisms that are labeled Invasive Alien Species, or IAS, a term described by the United Nations’ Convention on Biological Diversity. A working paper from the convention states, “Since the 17th century, invasive alien species have contributed to nearly 40% of all animal extinctions for which the cause is known.” (CBD, 2006)
For those who lived primarily outdoors as children, the imported red fire ant and the Asian Tiger mosquito were probably their most life-changing IAS foes. Beyond that, the woolly adelgid that threatened to eliminate every hemlock from the Appalachian Mountains and beyond is an existential threat against which scientists are still waging a battle. If you haven’t had a population of smelly brown marmorated stink bugs invade your house in fall, count your blessings. Farmers battle Asian soybean rust with expensive fungicidal sprays plus the extra costs of fuel for application. Our native ash trees will soon be a memory due to the imported emerald ash borer, whose larvae girdle and kill trees in one to two years. That IAS insect was first detected in Michigan in 2002 and is now confirmed in 35 states, including South Carolina, and in parts of Canada. The hemlocks and ash trees can be protected if treated early enough and repeatedly with systemic neonicotinoid insecticides. A natural predator of the woolly adelgid has been extensively studied and now introduced into the United States and may be effective to the point that hemlocks may be able to survive without those pesticide applications.
The IAS crape myrtle bark scale was first detected in Texas in 2004 and is now found in Oklahoma, Arkansas, Louisiana, Mississippi, Georgia, North Carolina, and Virginia. In August, the first documented case in South Carolina was found by skilled Historic Columbia horticulturist Keith Mearns. A representative from Clemson’s Department of Plant Industry collected a sample that Meg Williamson of the Plant and Pest Diagnostic Clinic confirmed. Although this insect does not kill the trees outright, the presence of the female scales, covered with a highly noticeable textured coating of white fibers, along with black sooty mold growing on the excreted honeydew, makes the bark of these trees so disfigured that their natural beauty is completely obscured. A heavy infestation does stunt tree growth and diminish flowering.
Bark scale is coming, just like death and taxes, and cannot be avoided. It can be controlled, but the most effective treatment is expensive, time consuming, and must be repeated at least every two years at the risk of harming pollinators. The arrival of this pest is a depressing situation for Southern gardeners. There are certainly going to be situations where crape myrtles are so important in the overall landscape that a certain number of them must be treated. Many homeowners may elect to hire a professional, licensed applicator to treat their plants; those persons can purchase chemicals in more cost-efficient quantities and have all the proper application and protective clothing equipment necessary for effective treatment. Others may simply ignore the truly unsightly bark and enjoy the diminished but still colorful flower display from a distance, but those trees will continue to serve as a source of infestation that will spread to other crape myrtles in the area.
Scale insects in general are a pain to control. There are soft and hard (also called armored) scales; the crape myrtle bark scale is not exactly a soft scale but fortunately can be controlled by systemic drenches of neonicotinoids. The mature, mated female does not move and is protected by her shell-like covering. In the spring, her young, called crawlers, emerge from under that protective shell and mate; this is the easiest stage to control. Male crawlers can fly. The immature females are wingless but are transported by wind to nearby susceptible plants, and the scale infestations spread.
Clemson Entomologist J.C. Chong and Horticulture Specialist Joey Williamson have studied this new insect extensively. Joey has produced a fact sheet, number 2015, most easily accessed by simply searching “crapemyrtle bark scale Clemson hgic.” He outlines many treatment options. The most effective treatment, however, is achieved by making soil drenches containing the neonicotinoid dinotefuran, which is found in the trademarked product Safari. This product is available from an agricultural/horticultural supply store, such as W.P. Law, or specialized nurseries. The new Bayer formula BioAdvanced Tree and Shrub Insect Control Concentrate may be easier to find in a big box store, but its active ingredient, imidacloprid, is not as effective against this scale as is the active ingredient dinotefuran in Safari, according to Dr. Chong, who is a national expert on scale.
Although Clemson recommends making the soil application in May or June, this may result in concentrations of this systemic product in the pollen and nectar that could possibly harm pollinators. Label directions, which are legally enforceable, state never to make an application when the plant is in flower, but the whole idea is that the product moves and persists in the plant to effectively control at least two generations of crawlers that occur each summer. Dr. Chong’s extensive research shows that pollen and nectar concentrations vary not only from genus to genus but even within species, and research will no doubt be done on crape myrtles in the future. One option would be to add flowers more enticing to pollinators to your summer plantings to provide alternative food sources to the less attractive crape myrtle flowers to lessen the chance of pollinator damage.
For replacements, myriad choices are available, but truthfully none equals the ease of growth, longevity, and extended period of flowering that crape myrtles offer. But many have their own charm and beauty, so here are some to consider: non-fruiting crabapples (you don’t want dropped fruit on the ground to attract wasps), hawthorns, Asian cherries, all of which have lovely blooms, and the blue spikes of Vitex attract pollinators like mad. Our graceful semi-evergreen sweetbay magnolia can be grown with multiple trunks and has flowers with an incomparably delightful fragrance.
As an admirer of a well-toned arm, I think the tree that most replicates the beautiful trunk structure of crape myrtles is the native musclewood. Carpinus caroliniana is very long-lived and slowly matures to 30 feet. Wind pollinated flowers are not showy, but the fall color is attractive. It prefers afternoon sun, needs normal amounts of moisture, and benefits from regular top dressings of compost, which can be applied right over the mulch. Its nutlets are eaten by myriad birds and mammals. It also supports caterpillars, providing customized baby food for backyard birds, whose existence is threatened by habitat loss and the lack of native trees in our landscape.
Need more enticement? Just think of the nudge of encouragement you’ll feel when you walk past the sinuous trunks of musclewood on your way to the gym.