Managing Crapemyrtle Bark Scale: Part 2
My last couple of articles addressed what crapemyrtle bark scale (CMBS) is and non-insecticide aspects of managing it. Here, I discuss insecticides.
There is no ideal insecticide-based management approach for CMBS. The easiest and, to my knowledge, most consistently effective way to treat it is with a soil-applied insecticide drench containing imidacloprid or another insecticide in the neonicotinoid group, such as dinotefuran or thiamethoxam.
These insecticides move up into the plant from the roots and remain in the plant for a long time (over a year). Pest insects come in contact with the insecticide when they feed on the plant. Beneficial insects that just rest on the plant are not affected. However, they can be exposed to insecticide residues if they feed on pollen or nectar from the plant, on pest insects, or on honeydew excreted by pest insects. (Crape myrtles don’t produce nectar, but they do have pollen.)
Among the neonicotinoid insecticides, imidacloprid products are most readily available. There are several home garden insecticide products that contain imidacloprid as the active ingredient and are labeled for soil drench applications around ornamental trees and shrubs. Examples include Bioadvanced 12 Month Tree & Shrub Insect Control Concentrate, Bonide Annual Tree & Shrub Insect Control with Systemaxx, Ferti-lome Tree & Shrub Drench, Martin’s Dominion Tree & Shrub Insecticide Concentrate, and Monterey Once a Year Insect Control II. These can be applied once per year.
Sources typically recommend using these systemic, drench-applied products in the spring. They should not be applied during flowering, to comply with label requirements, even though it takes some time for them to be taken up into the plant.
Successfully treating scale insect infestations with insecticides sprayed onto the trunks and branches can be tricky. Once they have their waxy coverings, they’re largely protected from chemical sprays. The young, crawler stage of the insect that is most susceptible to insecticides is only present at certain times of the year and isn’t easily seen.
Since some common insecticides are not effective on CMBS at most times during their lifecycle but do kill beneficial insects that feed on pest insects, the use of such insecticides can actually cause an increase in CMBS populations.
The time of year at which crawler populations are greatest varies from year to year and location to location, but in Louisiana, it appears that the first peak can generally be expected in April or May. You can use double-sided tape to check for their presence, if desired. CMBS crawlers are about 0.5 mm long and pink in color.
To reduce the impact on beneficial insects in comparison to a lot of conventional insecticides, you can use an insect growth regulator (IGR) containing pyriproxyfen (Distance, Fulcrum, or another product labeled for use on ornamentals) twice during the time when crawlers are present, following label directions. These products are not sold in home garden-sized products but can be purchased by anyone. (Talus 70DF [buprofezin] is another IGR option but isn’t labeled for use in residential areas.)
Bifenthrin products such as Bifen I/T, Bioadvanced Tree and Shrub Insect Killer Spray Concentrate I, and Ferti-lome Broad Spectrum Insecticide are more readily available for home gardens but also more harmful to beneficial insects. If used, they should also be applied while crawlers are present.
Horticultural oil products are widely available and have less impact on beneficial insects than bifenthrin and other conventional insecticides with residual activity. They have not been as consistently effective against CMBS as other mentioned treatments but with repeated applications when crawlers are present, they can reduce the population to some extent.
When using any product sprayed onto trunks and branches, be sure to get good coverage, including undersides of branches and bark crevices.
Make sure any insecticide you plan to use is labeled for the type of plant on which you plan to use it, and follow label directions.
Let me know if you have questions.
Click here for previous LSU AgCenter's Weekly Messages
Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).
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