This looks and sounds like an infestation of a relatively new pest called Crapemyrtle Bark Scale. They have multiple generations per year, though we are uncertain about particular life cycle details as it's still being studied for our specific area (Maryland and the mid-Atlantic).

Recommendations for control include treating the bark with dormant oil at a 2 to 3% concentration and possibly also (or instead) using a systemic insecticide sprayed on the trunk or soaked into the root zone. The systemic would have to be applied by a certified pesticide applicator as such products are not available to the general public and would be unlawful for them to apply. The oil could be applied by residents, but is also best reserved to application by professionals in circumstances where plants are large and spraying needs to occur over one's head. Dormant oil sprays need to be used with careful attention to seasonal timing as well as weather conditions during and immediately after the application, both so they remain effective and so they minimize risks of plant tissue damage. Thorough coverage of all bark surfaces is very important for it to be effective, and more than one treatment may be needed to bring the population under control. Dead scale insects don't always fall off of their host plant right away, so a treatment may start to work without initially changing the appearance of the infestation. If you spray anything yourself, pay careful attention to the product label directions.

Here is our current information on this pest, though its target audience is professionals in the nursery industry; it's only an introductory write-up for now since further study is ongoing. We are currently working on revising and creating new content for scale insect control which will include this pest, as it is not yet on our existing list of common scale insects.

Scale insects in this group produce sugary excrement called honeydew since they feed on plant sap, and this substance can be attractive to flies and wasps. Beneficial predatory insects (lady beetles and others) can also be drawn to the scale as prey, which fortunately the dormant oil will avoid harming. As the scale die off, the honeydew stuck to the crapemyrtle and surfaces below it will gradually weather off. Sooty mold, a blackish fungus growing on honeydew residue, will also go away in time, if it is present here and contributing to the dirty look of the tree. This fungus does not infect the plant and merely grows on the sticky surfaces, so is only a temporary cosmetic problem.

Miri

Hello Arati,

Since the trees are on city property, you are probably not allowed to treat them yourself, but you can notify the city arborist about the problem. (Perhaps you can find them through a dept. of the environment or similar division. Maybe in this case that's the forestry dept. you already notified.)

November is fine - and possibly even a bit too early, depending on weather - for applications of dormant oil. Using it too early, while foliage is still on the plant, risks plant damage, though admittedly if foliage is burnt just before it sheds, no harm will come to the plant as a whole. "Regular" horticultural oil is sometimes referred to as "summer rate" oil to better distinguish between the two. Its lower oil concentration is better tolerated by foliage.

Dormant oil is just horticultural oil that is used at a higher-than-typical concentration. The reason concentrations are used at a lower "summer rate" of hort. oil the rest of the year is due to that risk of leaf damage. During the dormant season, deciduous plants won't have that issue, having shed leaves, and evergreens are similarly more tolerant of that slightly more concentrated oil at that time. (Certain evergreens should not be sprayed with oil, but for other reasons not applicable here.)

Applying horticultural oils well can be tricky, which is why professional equipment tends to be more effective and somewhat safer; a professional applicator will also have the protective gear they need to apply it safely, particularly when spraying overhead. Factors impacting how well the treatment works include spray droplet size - fine mists from a high-pressure sprayer coat and stick to the plant surfaces more evenly than larger droplets from most over-the-counter pump or hose-end sprayers. Plus, the container usually needs to be kept well-mixed during spraying so the application doesn't end up too watery or too oily.

Horticultural oil is in the category of contact insecticides, meaning it must directly contact the pests it needs to control. Fortunately, scale barely move, so as long as bark surfaces are thoroughly coated, the oil should serve to deteriorate the scale's protective waxy shell and smother the insects underneath. The alternative treatment, a systemic, is a type of insecticide that the plant absorbs and is then ingested by the feeding insects. Since some of these chemicals could also make their way into nectar, they need to be chosen carefully to protect pollinators that visit the flowers. Even though crapemyrtle won't flower again until next summer, some systemic chemicals remain in the plant for months. A professional applicator should keep this in mind and choose a shorter-residual option where possible. Crapemyrtle is not highly valued by pollinators, at least, but honeybees and possibly others will visit the blooms. Cooler weather slows-down insect metabolism and respiration, so the process of the insects succumbing to treatment is a bit slower than in summer, but not to the point where this is a problem. Treating in the "off" season with contact sprays does help to spare numerous beneficial insects, as can systemics since the predators are not consuming the plant directly.

Miri