Given crapemyrtle’s reputation for plant vigor and pest resistance, I was shocked to see Chinese crapemyrtles noticeably affected by a previously obscure pest.
Last November I had the privilege of traveling to China with Dave Creech (Stephen F. Austin University), Mengmeng Gu (Texas A&M) and Yan Chen (LSU-Hammond). Crapemyrtles, Lagerstroemia spp., are native to Asia, and China is regarded as the first to cultivate crapemyrtle as a flowering tree. As a crapemyrtle enthusiast, I eagerly looked forward to seeing Lagerstroemia collections in Chinese botanical gardens. What we saw in Beijing surprised us.
One of the few Asian pests of crapemyrtle is crapemyrtle bark scale, Eriococcus lagerstroemiae. Long considered a minor pest of crapemyrtle, our Chinese hosts indicated this scale recently exploded in numbers, significantly infesting crapemyrtle in some areas. Affected crapemyrtle had greatly reduced vigor and the infestations were just plain ugly (Fig. 1). We did not have an entomologist available to confirm identity of the scale we saw, but our Chinese scientist hosts believed the pest was crapemyrtle bark scale.
Symptoms, Appearance and Distribution in China
An early symptom of crapemyrtle bark scale is black sooty mold covering extensive areas of leaves and stems as a result of honeydew exuded by the scale. Individual scale insects are white to gray in color and ooze pink when crushed (Fig. 2). Large populations build up in branch crotches and extend up branches, appearing crusty white to gray. This scale usually is not present on new growth, leaves or slender stems unless infestations are heavy.
We found this scale on crapemyrtle in all four cities we visited, across hardiness zones roughly equivalent to USDA Cold Hardiness Zones 6b to 9 (Beijing, Zone 6b/7a; Nanjing, Zone 8a/8b; Shanghai, Zone 8b/9a; and Kunming, Zone 9). However, this scale was not found on all crapemyrtle. My personal observation is that stressed plants appeared more susceptible to this scale, as exemplified by infestations on freeze-damaged crapemyrtle in Beijing (Fig. 3) or in a poorly maintained planting in the Nanjing Airport parking lot. Our hosts indicated the problem appeared to be more severe on hybrid cultivars introduced from the U.S., and our observations mostly confirmed that.
In the U.S.?
Unfortunately, crapemyrtle bark scale or a similar scale may already be in the U.S. A new scale insect believed to be a species of Eriococcus was first discovered in the Dallas, Texas, area in 2010 where it is problematic on landscape crapemyrtles. It has not been definitively identified by entomologists yet and management recommendations are still being developed. However, this scale has since been observed in Shreveport, LA, Memphis, TN, and Little Rock, AR, undoubtedly being moved with plants. The expanding distribution of this scale and my personal observations of crapemyrtle bark scale throughout China suggest this scale could have a widespread and severe impact on crapemyrtles in landscapes.
America has a long history of nonnative organisms becoming invasive and causing significant problems in agriculture, landscapes and natural areas (i.e., chestnut blight, Japanese beetle, kudzu, dutch elm disease, granulate ambrosia beetle, laurel wilt, etc.). It is important for all of us to be vigilant in identifying potential invasiveness of organisms, preventing their introduction and spread, and taking actions to minimize or eradicate these invasive organisms before they get “out of hand.” Let’s keep an eye out for this scale and other potential invasives and work together to control or eradicate them.