Question
I have an Angel's Trumpet (dark purple/white) that has white bugs clustered on the stems and back of some leaves. I've tried to identify but can't.  They don't seem to fit the description of aphids, mealybugs, scale, etc.  They are white, circlish but flat in shape, there is no "cotton" and they are huddled together. Easy to see with the naked eye.  Any suggestions?

Answer
Sounds like aphids. there are many types of aphids most do not have the cotton waxy material on them. Here is a web link to pictures of aphids on Angel's trumpet plants and their description.

http://www.abads.net/insects/brugmansia-aphids.htm

I would use a systemic insecticide a granular one that is added to the soil and the plant takes it up to the leaves and the aphids are killed as they feed. One is these is Ortho RosePride There are others. I would check with your nursery/garden type store and ask them for a systemic  insecticide that is applied to the soil.