Ray wrote:

No mystery, I'm afraid. A soft scale.

Dip a toothbrush in some water containing liquid dishwashing detergent (1
tablespoon per gallon), and scrub away.

Ray,

I will certainly try this. Your advice
has been very helpful in the past.

Are there any insecticides, systemic or
otherwise, that are good for this? I have the
feeling that at this stage there's no way to
"get it all".

Brian