Go out at night with a flashlight for a look see.

Consider snails, slugs, night-working caterpillars.

If the holes are tiny it is almost certainly flea beetles. None of the options by jean makes tiny holes. Arugula is such a magnet for them.

Yes they are probably flea beetles. Cook the arugula, and the holes disappear! I see far fewer flea beetles when interplaning arugula between onions, or maybe it's just that there's so much arugula by then. Grow it again in the fall, and it will be surprisingly clean. No flea beetles then.

Well, I'm also in zone 7 but having flea beetles out & about is a month or more away.

Slugs, snails and caterpillars all make small holes when young. Beyond that, how small is small, anyway?

For what it's worth, I've never had flea beetles on my arugula.

Any chance of pictures?

Flea beetles are here. But need warmer and drier weather to get more active. They make many many tiny holes, about the size small sewing needle. They dont stay at one spot to graze. I have found a solution for this probme: FRENCH NYLON TULLE. Just throw it on any thing. It is wind proof, super light. And air, light and water goes throgh it. AND it is inexpensive.
My observation is that FBs like mostly egplants, Chinese lantern, Horse nettle(a weed, eggplant relative. What is more, is that the can only harm small young seedlings.

after reading up on the flea beetles I am convinced that is what is causing my holes. I have a bunch of needle like holes spread over the leaves in a shotgun pattern.

they are 18" away from a row of onions so that didn't deter the beetles. now I have to figure out what to do about em!

nice link, planatus. It gives me ideas.

Can I still eat the leaves with holes?

I do.

tj