As I mentioned before, our horseradish is riddled with aphids this year. I'm not sure why -- I don't remember seeing so many in years past. I've twice sprayed the leaves with a mild solution of soapy water, which cut down their numbers significantly and at least allowed the plant to leaf out completely. (The aphids appeared nearly the same instant we got new growth this spring, and threatened to suck all the life out of it before it had a chance to reach maturity.)
As you no doubt remember, my policy toward the horseradish is to let it do its thing. It is a veritable hive of insect and invertebrate life all summer, and I just let those communities balance themselves. I've seen ladybugs near the plant this spring, so I assume they're working on the aphids.
I've also seen a few snails, which appeared and disappeared quickly. I assume something ate them -- a bird, perhaps.
I'm trying to avoid resorting to the soapy water again. But geez, we have a lot of aphids.