Yan Chen | 7/20/2016 4:24:09 PM
Yan Chen and Rodrigo Diaz
With showy summer flowers, attractive bark color and brilliant fall foliage, crape myrtles are the most widely planted summer landscape tree in the South (Photo 1). New varieties with improved cold hardiness, dwarf growth habits, and new foliage and flower colors are being released, including several series of burgundy- and black-foliaged varieties (Delta, Black Diamond, Ebony and Magic). More importantly, this year-round garden performer is relatively low maintenance. Common pests include aphids and late-summer leaf spots, which do not need treatments in most cases.
Over the past few years, the crape myrtle bark scale, Eriococcus lagerstroemiae, has become a new threat to the future of crape myrtles across the Southeast. After the first report from the Crape Myrtle Trails World Collection Park in McKinnery, Texas, in 2004, this pest has quickly spread to 96 counties in 11 states: Alabama, Arkansas, Georgia, Louisiana, Mississippi, New Mexico, South Carolina, Tennessee, Texas, Virginia and Washington. In Louisiana, infestations have been found in Shreveport, Bossier City, Minden and Monroe in 2012, Rayville and New Orleans in 2013, Hammond and Houma in 2014, and Baton Rouge in 2015.
The scale is easy to identify because it is the only known bark scale that feeds on crape myrtles. Branches and trunks are often covered with felt-like scales and turn black from the sooty mold fungus grown on the sugary exudes of the scales (Photo 2). Stem dieback, decreased number and size of blooms, and slow decline (reduced vigor) of trees have been observed. Bark scale prefers feeding on pruning cuts and is attracted to the excessive bark growth around the wounds of improperly pruned trees (Photo 3). Removing the top canopy of the tree, aka “crape murder,” may worsen bark scale infestation.
Eggs of bark scale are protected inside the felt-like ovisacs. The young scales, called crawlers, and their more mature nymph stage feed on tissue beneath the bark, and both are susceptible to chemical treatments (Figure 1). Nymphs either develop into winged males or become females enclosed in ovisacs. These life stages are hard to manage with traditional insecticides. However, beneficial insects such as the twice-stabbed lady beetle (Chilocorus stigma) and the lady beetles in the Hyperaspis genus can rupture ovisacs and feed on eggs. Further research on using these predators and other natural enemies may provide long-term management solutions.
Gardeners, landscapers and crape myrtle growers are concerned because of the speed at which bark scale is spreading and the limited information available on its control. Since 2014, LSU AgCenter horticulturists and entomologists have been working with scientists from other institutions in the region to develop management recommendations for this exotic pest.
With assistance from northwest Louisiana Master Gardeners, AgCenter personnel monitored the crawler population on infested trees in Shreveport during 2015 and found two or three crawler population peaks. The first peak appeared around mid-April, which correlates with the budding of leaf buds in these trees (Figure 2). Ultra-pure horticulture oil and insect growth regulators such as pyriproxyfen (Distance) and buprofezin (Talus) can be applied at this first crawler peak. They are more efficient on crawlers, and the use of these products is less harmful to beneficial arthropods and pollinators compared to long-lasting systemic insecticides. Additional research on crape myrtle phenology, which is the timing of bud break, first bloom and full bloom, if proven to be correlated to scale activities, will also help time the application on crawlers.
Crape myrtle flowers do not have nectar, but their “feeder pollens” are an important food source for bees in the summer. To manage bark scale with reduced impact on bees, AgCenter researchers recommend using systemic insecticides as a basal soil drench after the full bloom to avoid direct or indirect impact on bees. Products such as imidacloprid (Merit or Bayer Advanced Garden Tree and Shrub Insect Control), dinotefuran (Zylam, Safari, Transtect and Greenlight Tree and Shrub Insect Control with Safari) and thiomethoxam (Meridian) have shown control effects in field trials. Foliar application of systemic insecticides is prohibited by label during full bloom to avoid acute impact on pollinators.
If a decision is made to remove the infested plant, AgCenter researchers suggest trimming and carefully bagging all debris to avoid spreading the infestation. It will take efforts from homeowners, landscape professionals, nursery growers and retail garden centers to contain and mitigate this potential threat from bark scale to crape myrtles, an iconic landscape plant in the South.
Yan Chen is an associate professor at the LSU AgCenter Hammond Research Station in Hammond, and Rodrigo Diaz is an assistant professor in the Department of Entomology.
Acknowledgment: We thank LSU AgCenter Northwest Region horticulture agent Jennifer Williams, Terrebonne Parish horticulture agent Bennett Joffrion, research associate Joey Quebedeaux and research assistant Zinan Wang for their assistance to this project. This project is funded by the USDA National Institute of Food and Agriculture Crop Protection and Pest Management grant program.
Photo 1. The popular Delta Jazz crape myrtle is one of the dark-foliage varieties released in recent years. Photo by Allen Owings
Photo 2. Trunk of a crape myrtle tree infested with the crape myrtle bark scale. Photo by Yan Chen
Photo 3. Inappropriate pruning worsen the bark scale infestation on crape myrtle trees because excessive bark growth attracts scale to feed. Photo by Yan Chen
Figure 1. Life cycle of the crape myrtle bark scale includes egg, nymph (crawler), pupae, male and female. Photos by Zinan Wang
Figure 2. Number of crawlers per square inch of double-sided Scotch tape collected from three infested trees from February 27 to Sept. 17, 2015, in Shreveport.