Question
On the trunks of my 15-year-old crape myrtles are many "scratches" or "tracks" of some kind carved into the bark.  All of them contain very small, white dusty specks...size, color of sesame seeds. Small sugar ants seem interested in them, as did a tiny black, shiny beetle.  The beetle dropped off the limb, but ants are still there. Wonder what is going on and if I need to do anything.

Answer
You may be describing scale insects.  They are oval or shield shaped and do not move when feeding. The ants are there for the sugary substance left by feeding, which we call honeydew.  I would treat the plant with Bayer Advanced Tree and Shrub insecticide.  This product is mixed with water and poured on the soil around the base of the plant.  It is systemic and is taken up through the roots and will kill the bugs within a week or so.  It also lasts up to a year and can be used on many other plants in the landscape.  Try this and see if it fixes the problem.  Good Luck.