Could be one of a number of pests, including wooly adelgids, wooly aphids, planthoppers, etc. I'd get after them with insectidical soap, applied in the evening hours so that it stays wet longer than usual. Be sure to direct your application to the canes where the pests are located.

Perhaps flatid planthoppers?

They do seem to be some kind of wooly planthopper nymph- pure white, fairly flattened, the cotton that covers them raises upwards toward their back end, they jump several feet away when disturbed (but don't fly), and there's a residue of cottony material left on the stems after the bugs leave. Haven't found a picture yet that looks exactly like them.

Malathion got rid of most, but some survived the first spray. Interesting that these aren't listed as raspberry pests- so far I haven't found them on any other kind of plant.

The wife just showed me a crepe myrtle bush near the raspberries that's badly infested, so I guess they're not plant-specific. I'll try to catch some in a jar for the Extension agent to identify.

Any suggestions on how to photograph such a little creature?

Do any of the critters on this page look familiar?
http://bugguide.net/node/view/104/bgpage

Here is a link that might be useful: flatid planthoppers