My crape myrtle has white spots on the bark and a lot of black leaves. What is going on? OS

What you are describing are classic symptoms of crape myrtle bark scale (Acanthococcus lagerstroemiae). Crape myrtle bark scale (CMBS) was first detected in northern Texas back in 2004 and spread to our area primarily through the movement of nursery plants. With wholesale yearly sales of nearly $50 million dollars a year, crape myrtles are one of our more popular plants. That’s a lot of plants moving throughout the area.

Adult male CMBS insects have wings and fly in search of a female. Once the female has mated, they produce an egg capsule, lay somewhere between 60 - 250 eggs in it, and then die. One of the indicators of CMBS is these small white puffy egg sacks.

Overwintering eggs begin to hatch in April and May. The crawlers that emerge are pink and very small… essentially invisible to the human eye without some sort of magnification. At this stage of life, they are able to gain some degree of mobility by hitching rides on birds, as well as being blown by the wind to nearby plants. A second generation emerges in late summer.

Damage from these insects occurs in a couple of different ways. The scale insects have what we call piercing/sucking mouth parts. This enables them to feed on the sap just below the surface of the bark, essentially robbing the plant of nutrients. Initially, the plants are not in danger, however it the population of CMBS goes unchecked for a period of time, they can build up to the point of damaging the plant.

Another symptom of a CMBS infestation is a black coating on the leaves and bark of the plant. This black coating is a fungus called sooty mold. The mold thrives on the excretions of the scale insect. We affectionately call these excretions “honeydew.” With the presence of this black sooty mold you may also notice a lot of ants as they love that sweet honeydew. Again, in its early stages, this black sooty mold is not a huge problem, more of an aesthetic issue. But, if the infestation goes unchecked, a crape myrtle whose leaves are covered with black sooty mold is not going to be able to thrive. 

I have a couple of crape myrtles and I keep my eye on them to look for early signs of an infestation. One day while getting the mail, I noticed one of my neighbor’s crape myrtles had the scale, so I heightened my vigilance. Sure enough, at some point, I noticed signs of CMBS on one branch of my crape myrtle. In this instance, control was quick and easy. I cut off that branch and threw it away and have not had an issue since. However, not everyone is so lucky.

Control of these insects is not difficult but may seem a little bit unorthodox. Traditional pesticides are not effective because they are not able to penetrate the scale covering of the insects. As a result, there are several things you can do to treat for CMBS.

While we do not recommend this particular strategy, there is a systemic insecticide that works pretty well on CMBS called Imidacloprid. This pesticide is what we call a drench. You mix it with water and pour it into the soil around the infected plant and wait. As the roots draw the pesticide up into the plant, the scale insects feed on the pesticide and die. The trouble is that this pesticide continues up into the plant and finds itself in the flowers. Along come our favorite unsuspecting pollinators and they become collateral damage in our efforts to get rid of the scale. Even though you will see this strategy recommended on the internet, we discourage the use of Imidacloprid as a treatment for CMBS.

So, what can you do? Your first line of defense will involve a little labor on your part. You will need a bucket, water, a little soap, and a stiff brush. Mix yourself up a little soapy water and scrub the scale off with a brush. Granted, if your crape myrtle is large, and many of them are, this will be problematic. But, in smaller plants, this is a great earth-friendly way to solve the problem.

The other strategy is to wait until a warmer day in winter when the crape myrtle is in its dormant stage and spray it with a dormant oil. For those unfamiliar with the term, dormant oil is a variation of standard horticultural oil. Dormant oil is of a thicker consistency and would not be safe to spray on an actively growing plant. So, we wait until the plant is dormant (thus the name dormant oil) and then spray the crape myrtle with the thicker dormant oil. This oil, because of its thicker consistency, is able to smother the insects inside of their scales. Plan on doing this on a winter day when it is around 50 degrees or so.

If you follow these instructions, you will be well on your way to restoring your crape myrtles to their original glory. Good luck.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.