James A. Bowery said:

Does anyone know for sure what bug this is?

http://video.google.com/videoplay?docid=-
8318991913753851458&hl=en

They're populating brussel sprout leaves but don't seem to be doing
much damage.

Some sort of aphid. They'll be getting inside the sprouts eventually,
probably also encouraging a black mold due to their excrement.

http://www.earthlife.net/insects/aphids.html
--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)