Thank you for reaching out to us with your question and photos. What you have here is an infestation of armored scale insects on a cherry laurel. White Prunicola Scale, unfortunately, is fairly common on these plants and it is difficult to control. The insects feed on plant cell contents, which can eventually result in leaf yellowing, leaf drop, and dieback. 

For a very heavy infestation like this, your best bet is probably removal, to minimize the chances of this pest moving on to your other laurels. Preferred host plants are laurels (and related plants in the Prunus genus, like flowering cherry), lilac, and privet. We do not have research on whether they would go to Virginia sweetspire (anecdotally, we do not have reports of it being on that plant).

If you do want to try management vs. removal, you can selectively prune out and discard heavily infested branches. Use a soft brush to brush away the white scale covers from the branches. During the growing season (now) wrap a piece of double-sided tape around one of the branches. This is a test to monitor the active crawler (juvenile) stage of these insects. They are more susceptible to insecticides when they come out in May/June depending upon temperature and there may be several generations per year. When you see crawlers sticking to the tape, that is a good time to apply horticultural or insecticidal soap according to label directions.

Take a look at our blog on cherry laurels and how to successfully grow them in your landscape. https://marylandgrows.umd.edu/2019/01/07/qa-whats-wrong-with-my-cherry-laurel-shrubs/

And more on White Prunicola Scale https://marylandgrows.umd.edu/2019/09/25/qa-what-is-this-white-growth-on-my-cherry-laurel-shrub/

Christa

You're welcome.