The radish plants were not looking good. Upon closer inspection, I discovered why.
And, as you can see below, it was destroying the radishes but hardly touching the nearby zucchini.
Turns out, it's the flea beetle. And apparently, having the radish plants there kept them away from the zucchini. Maybe I should plant radishes again next year just to keep the zucchini safe?
Update 7/3/2014: I planted radishes again near the zucchini and again it seems to be working. We have flea beetles once more but it doesn't seem to be nearly as bad as last year. I did happen to come across a new article from Grit Magazine about dealing with the pests and how to fight them.
You can read the article at: