A licensed professional pesticide applicator (which may be the arborist themselves) will need to make treatments using one or more of the following insecticides: horticultural oil (in a "dormant" rate), an insect growth regulator (IGR), or a systemic insecticide (the neonicotinoid class being one example). Since systemics have the potential to move into nectar or pollen, they should choose one that has a shorter residual (effective period) so it's metabolized by the plant by the time it blooms. Fortunately, crapemyrtle blooms so late that this should be relatively simple to achieve. Dormant oils are typically applied in November, since there are temperature restrictions that are easier to meet that time of year, but opportunities may present themselves later this winter. The timing of any insecticide application will be determined by the weather and when the product label recommends it be applied, since different formulations - even using the same active ingredient - can have different usage specifications.

This scale species is fairly new for our area (and relatively new to the U.S. in general), so we do not yet know when the crawlers are out and what life stage the scale overwinters in. This makes monitoring for crawlers trickier since more vigilance is required. Several generations likely occur here; between two and four is the current educated guess. A strip of tape encircling a stem near the scale may help trap crawlers for detection, but one would need to know what to look for (magnified) amid dust or other debris that gets stuck to the adhesive over time. Tape is used sticky-side-out so crawlers are forced to walk over the surface as they explore. About two-thirds of the way down this webpage about scale (a different species in this case), there's a useful image showing how tiny scale crawlers on tape will look.

Scale outbreaks tend to occur more commonly or severely on plants experiencing chronic stress. Unfortunately, this applies to most street and urban trees, where dust kicked-up from road traffic, there is reflected heat from pavement, and limited root space make the plants less able to nourish and hydrate themselves. All of this culminates in trees being more vulnerable to pest attack. It also hinders the natural enemies (insects that prey on scale) from helping to suppress their numbers. All you can in these cases is to monitor such plants for moisture needs during warm weather and irrigate them as needed.

Fortunately, we have not yet encountered recurring problems with plant dieback due to this scale feeding on crapemyrtle, though honeydew production in summer and the resulting sooty mold can be a major nuisance or may stunt plant growth. Heavier scale populations may take more than a full year to bring under control with various treatments.

Miri

You're welcome.

If replacement is simpler than dealing with recurring root suckers, and if these grew too large for the space, there are dozens of dwarf crapemyrtle cultivars available today that mature at half this height or less. Alternatively, if this site doesn't receive full sun in summer (6-8 or more hours daily; it sound like it doesn't) then a substitute species that has better shade tolerance would be better suited to that location.

Miri