The guy was probably being rude because he didn't know anything about how to help you. Some professionals have a hard time saying, "I'm sorry, but I don't know."

The scenario is more complicated than we can solve without more information. It's a fact that scale insects may be more likely to cause serious injury to a weak, improperly sited and grown plant as opposed to a vigorous, sturdy, and healthy plant.

Sometimes, the soil can be the problem....lack of nutritional support if necessary, compaction, improper pH, poor water drainage, and excess or insufficient watering are all examples of soil related problems.

How long have the crape myrtles been planted? What is your general location? Have you noticed these scale insects on other plants? Are the plants located in the full sun? What kind of crapes are they....the cultivar name, please?

Anyway, there is no way anyone can help you with zero background information.