By MARY HELEN FERGUSON
LSU AgCenter
Crape myrtle bark scale (CMBS) has been present in southeast Louisiana for over ten years now. Recent questions from clients, as well as the finding of an infestation in my own yard, have prompted me to address the topic again and in more depth.
These insects are members of the felt scale family. They live on bark of crape myrtle trees, feeding on plant sap. Like other felt scales and soft scales, they excrete a sugary substance called honeydew. Sooty mold, made up of various fungal species, grows on the honeydew.
Oftentimes, the reason people notice that they have a problem is not because they see the insects themselves but because sooty mold makes the trunks of the crape myrtles appear black. Sometimes, plants under the crape myrtles have sooty mold on them, too, since honeydew drops from the crape myrtles onto other plants.
Over time, the infested crape myrtles become less vigorous and produce fewer flowers. Young trees may be more severely affected.
My recent delving into CMBS biology was prompted in part by the question of whether it feeds on plants other than crape myrtles. It does. In its native range, it's been found to infest a number of other hosts. In the US, the list of hosts it's been reported from is limited and includes spirea (Spiraea japonica and S. thunbergii), a species of St. Johnwort (Hypericum kalmianum) that doesn't commonly grow this far south, and, to my dismay, our native American beautyberry (Callicarpa americana).
To catch a CMBS infestation early, before sooty mold is widespread, I suggest checking your crape myrtles from time to time for the white, cotton-like coverings of the adult females and eggs or of the male pupae so that, if desired, you can take steps to manage them while the population is small. These white structures are roughly 1 to 3 mm long, or less than one-eighth of an inch in length. They do not move. Only juveniles (nymphs) and winged adult males move. Under the cottony coverings, CMBS bodies and eggs are typically pinkish in color.
CMBS bodies are sometimes hidden under the peeling bark of crape myrtles or under lichens, so look closely when checking for them.
There are larvae of some lady beetles, such as Hyperaspis bigeminata, that feed on CMBS and are also white and cottony looking. It's important to distinguish between the bad guys and the good guys, since if you have a lot of the latter, that may change how you decide to treat (or not treat) the plant. One way to distinguish lady beetle larvae from CMBS is that the former will likely move if they're disturbed.
Note that the presence of sooty mold doesn't necessarily mean that you have CMBS. Aphids often live on crape myrtle leaves, and they also excrete honeydew, resulting in sooty mold growth on leaves. If sooty mold is primarily on leaves, aphids are likely the culprit, but it covers much of the trunks and branches, you likely have CMBS.
I will address CMBS management in the next article.
Let me know if you have questions.
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).
