At first I didn't think it could be a woolly aphid because I assumed the 'wool' would've been longer, but obviously there are different points in their life cycles so maybe this one has yet to become woolly? I found this picture in the unidentified woolly aphids category. They look similar to yours. And then there's these unidentified aphids that I thought looked similar to yours...and then I realized they WERE yours! lol. At the time I was thinking "wow! I found the same ones, they're even on a radish plant, too!". It was a brain fart of a moment. ;) Also I found out that when aphids suck the juices out of plants, droplets of their sugary waste (called Honeydew) leaves a residue on the leaves which in turn starts to grow a fungus called 'sooty mold'. I mention this because in both your pictures it looks as though something similar is happening on your radish leaves? I read that the only harm the mold can do is lessen the plants ability to photosynthesize and also sometimes attract ants that might eat it. I'm not an expert though, so don't take that to heart. :) Sorry for the rambling comment...but I hope it helps a little at least.

This one has a white-coated body and black legs: http://bugguide.net/node/view/195879/bgimage

But it feeds on conifers.

The teardrop body shape does point to aphids, but these bugs didn't act like the aphids I'm familiar with. They walked on the leaves at a steady rate, as if searching, the way ladybugs do when looking for prey, but slower (they're smaller). Aphids settle in one spot and suck. They don't have a reason to patrol a leaf.

The yellow specks on the leaves are pollen grains. Something nearby has rained pollen. I think it's a tall pine tree two lots away. I didn't notice a black residue. The radish leaves looked pretty clean. I'll recheck them, but today it's raining real rain, which might wash everything off.

The unidentified aphids in photo #324703 looked unlike these two from yesterday. They're gray, and they don't move fast. The current ones are clearly white, or coated in white.

It would help if I could see the mystery bugs eating something.

I found this one on the internet...its called the Mealy Cabbage Aphid (Brevicoryne brassicae). It doesn't have a guide page yet. This page has info and a nice photo that looks pretty close to yours, just the wax is a little more 'patchy'. See here. And here is another site where it actually lists radish under 'plants affected'. Even if it's not eating the radish, it could just be looking for a place to lay eggs or give live birth, whichever the season. I would venture a guess that yours is a mature female? If not female, still probably mature...so the 'babies' will look different.

The site at your first link says "A further migration from mid-September to mid-October results in egg laying in October." That matches the reality on the ground here. Yesterday's specimens were on a plant in a different bed from the heavy infestation I posted, maybe five feet apart, a distance that would be a short migration.

I'll look for shiny black eggs on the radish leaves.

I'm not sure how the process works, but if this is really the ID, then you've begun a new guide page! I guess an editor will read this and decide? It will probably take a few more experts/editors to agree on the ID I assume.