Yan Chen, Mike Merchant, Erfan Vafaie, Mengmeng Gu and James Robbins
Crape myrtles are considered an essential Southern plant for good reasons. Selected from Lagerstroemia indica or its hybrid with L. fauriei, these summer-flowering trees provide us spectacular flowers, colorful autumn foliage, and handsome sculptural trunks that few other plants can match. The crape myrtle bark scale (CMBS), Acanthococcus (previously Eriococcus) lagerstroemiae, was first detected in Texas in 2004 and quickly spread to 12 states by 2016. An article on its distribution, damage, life stage and population trends was published in the Spring 2016 issue of Louisiana Agriculture and the 2016 Hammond Research Station Field Day Book.
New infestations have been reported in Covington and Mandeville in 2017. In addition, other than crape myrtles, CMBS has been confirmed on beautyberry (Callicarpa americana) in open fields and pomegranate (Punica granatum), henna (Lawsonia inermis), heimia (Heimia salicifolia) and winged loosestrife (Lythrum alatum) in greenhouse tests. This has raised concerns of this pest being capable of infesting native flora of the North America.
When it comes to managing this scale, landscape professionals often prefer using neonicotinoids. This chemical class is systemic, which means that the active ingredients get transferred into plant tissue and work whenever scales feed on the sap. They may be more effective or longer lasting than contact insecticides that work only when they get in contact with the scale. However, because of concerns on the potential impact of neonicotinoids on beneficial species and pollinators, they should be used as the last resort.
Other control options, such as insect growth regulators (IGRs), horticulture oil and other contact materials, need to be tested to target crawlers. IGRs mimic insect hormones and disrupt molting process so that scales cannot move on from one life stage into the next. Horticulture oils work by physically suffocating crawlers, and it is unlikely that insect pests can develop resistance to physical modes of action. In addition, systemic insecticides from newer chemical classes, such as IRAC Group 28, that are low in toxicity to pollinators, need to be tested on CMBS.
Six landscape chemical trials have been conducted over the past five years by researchers from the Texas A&M AgriLife extension service, the University of Arkansas, and the LSU AgCenter on crape myrtles naturally infested with the scale. Each study was designed with different objectives, either comparing efficacy of various active ingredients, application timing (mid-April, mid-May or mid-July), or application methods (foliar and trunk spray, soil drench or injection or soil surface application of granular products).
In summarizing the results across these landscape trials, soil-applied neonicotinoids (active ingredients tested included imidacloprid, denotefuran and thiamethoxam) were the most effective treatments. When compared with soil drench or injection, foliar and trunk spray (or soil surface application of granular formulations) were less effective and provided shorter residual effects. In three trials, complete control from soil drench or injection of neonicotinoids lasted for at least four months.
Monitoring crawler population using double-sided sticky tape is extremely helpful in deciding timing for foliar and trunk treatments. Foliar and trunk application of bifenthrin (as Talstar) was found to provide quick knockdown if applied right before or at crawler peak in mid-April to early May. The peaks may shift from year to year and may be a little different between Texas and Louisiana. Great controls were also provided by three applications (mid-April to early May, every 10 to 14 days) of pyriproxyfen (as Fulcrum) and tank mix of horticulture oil (as SuffOil-X) and azadirachtin (as Molt-X). Horticulture oil in combination with other contact or IGR products showed good suppression in general. When applied alone, azadirachtin, malathion and horticulture oil only partially suppressed the scale.
Unexpectedly, cyantraniliprole (as MainspringTM 200SC) did not provide satisfactory control either as stand-alone or in tank mix with CapSil or NuFilm, both of which are spray adjuvants, Being an oil-based material, application timing may need to be well ahead of crawler peak to allow its penetration into the bark tissue. Additional study of this product in a tank mix with SuffOil-X is being evaluated.
Carbaryl (as Sevin), acephate (as Orthene), and cypermethrin actually had a booming effect on scale population. A separate study on the impact of insecticides on the twice-stabbed lady beetle were conducted and results indicated that, numbers of scales in trees treated with Sevin and cypermethrin were significantly higher, and this improved survival of scales was possibly a result of lower number of lady beetles found in these treatments.
Scale management in the container production of crape myrtles are being tested by Texas A&M and the LSU AgCenter. Further landscape trials are being planned to compare more products and application techniques. Contact Dr. Yan Chen (email@example.com) or Dr. Dennis Ring (firstname.lastname@example.org) for current recommendations, and we will update management recommendations as they become available.
Yan Chen, Mike Merchant, Erfan Vafaie, Mengmeng Gu, and James Robbins