Why is my crape myrtle turning black?

One of the most common plants in the Southern landscape is the crape myrtle. These trees bloom freely all summer and have beautiful fall foliage and outstanding bark in the winter, if pruned properly. They have long been considered a bullet-proof plant for the South, being drought tolerant and seemingly indestructible. But a new insect pest was found in Arkansas back in 2013, and we are getting more and more reports of crape myrtle scale attacking our crape myrtle in Garland County.

Many homeowners have noticed their crape myrtle turning black. This black substance is called sooty mold and is a by-product of sucking insects, including scale and aphids. As these insects feed, they give off a sweet substance called honeydew. Wherever this honeydew lands, a black sooty mold will form. Aphids can be a problem on crape myrtles at the end of summer, but they are not cause for much concern. If that sooty mold is accompanied with white specks on the trunks or branches, that is the crape myrtle bark scale, and is cause for concern. If spotted early, females appear as white or gray felt-like growths on small twigs to large trunks, often appearing near pruning wounds or in branch crotches on older wood. If left undetected, the spread can become fairly extensive and heavy infestation will result in white crusted clusters of insects which may blanket small stems and be quite visible on the trunks.

If you get up close or use a magnifying glass, you will see that the adult is white to gray in color and there may be dozens of pink eggs or crawlers under some of the larger white scale covers. The covering of the adult scale is composed of white threads produced by the female that become felted or matted into a thick whitish scale covering over the entire body. The adult females under this covering are now attached and incapable of moving. She continues to feed by sucking sap out of the tree and lays eggs under the covering, after which the female dies.

In a normal season, this should occur in late April to mid-May. When the eggs hatch, the small nymphs have legs and are mobile and are called crawlers. These crawlers emerge from under the “mother scale” and begin to move. The females find their new location on the tree and become attached and start the process all over. Males develop wings and find a female, mate and then die. At this time, the actual number of generations completed in a year for this species is unknown, but it is suspected that there may be at least two generations in Arkansas.

Scale insects will not kill a tree, but they can weaken it to the point where they don’t bloom as freely or with as large a bloom. If you spot it on your tree, you should do something about it. If you have small limbs which are heavily infested prune them off and dispose of them. Do not put them in a compost pile or put them out on the curb for yard-waste pick-up. Left exposed in an open truck or put on the curb for yard waste pickup leads to a possibility that the millions of tiny crawlers could be easily spread to neighboring properties, thus accelerating the spread of this invasive insect. Instead, double bag the cuttings and put them in your regular trash pickup.

If you have scale insects on the main trunk of the tree, use a soft brush with soapy water and clean the trunk. This will get crawlers off and also remove the black sooty mold. While the tree is dormant, spray the entire tree thoroughly with a dormant oil, saturating the trunk and stems. The peeling bark of the crape myrtle is attractive, but also can harbor overwintering insects, which may not be smothered out with the oil, so pay particular attention to these areas when spraying.

In the spring, as the tree begins to fully leaf out, use a soil drench with a systemic insecticide. These products will be taken up by the tree and moved throughout the system of the tree, and have shown good results in controlling the scale. Only treat a tree that has the insect problem — don’t spray preventatively.

Here is a link to our fact sheet with more information: https://www.uaex.edu/publications/ PDF/fsa-7086.pdf.

Website http://www.uaex.edu has information on this subject and more, or contact our office at 236 Woodbine at 623-6841.