There are two types of scale.. Aphids that attach themselves to the crape myrtle leaves and armored scale that attach themselves to the bark. Once these insects attach themselves they remain immobile and never move again. These attached insects are female and continue to lay eggs under the established armor. Armored scale needs to be treated with an insecticide and the crape myrtle needs to be covered from top to bottom. We recommend using Zylam, but depending on the maturity there is another product called Acephate.  Before treating, it is beneficial to remove any excess bark to ensure you cover all areas. You can also scrub the branches and remove some of the insects. Below we have attached a link for Zylam.  Let us know if you have any questions!