What's eating my mustard seedling?

[Radish]

Hi, I've been reading here for the last few months and you are all very helpful. This is my first year doing a garden in a raised bed (I've only done small container gardens on a patio in the past). I'm hoping someone can tell me what is eating my mustard and how to combat it organicially.

At first I thought it was slugs because we have tons of them in the yard and a few have even made their way inside on very rainy days . I surrounded the plants and the raised garden bed with sand to try to prevent slugs from getting at them and I've got boards down in the yard and I've been killing the slugs I find under the boards. This morning however I noticed some of the leaves have been chewed completely off and the whole leaves were scattered in squares where I had not planted anything. So far it has only attacked one group of mustard seedlings (I have not thinned them yet). I also have radish, brocoli Raab, lettuce, pak choy, carrots, Kohlrabi, beets, onions, peas, dill and parsley planted in the same 4 x 4 bed and nothing else has been nibbled yet.

I'm going to try to attach some pictures below:
[img]https://img542.imageshack.us/img542/2149/nibbledmustard.jpg[/img]
[img]https://img138.imageshack.us/img138/8671/healthymustard.jpg[/img]

The first is the nibbled plant. The second is a healthly one for comparison. Thank you for any insight!

[Kisal]

It probably is the slugs. They can crawl on the edge of a razor blade without being harmed. I doubt sand will discourage them.

However, they do love beer. I've found that the cheaper the beer, the better the slugs like it. I put it out in the evening in throw-away aluminum pie pans. In the morning, I just throw them in the outside garbage can, slugs, pans, leftover beer, and all.

[Radish]

Thanks! I guess I'll just keep killing as many as I can and hope they don't take over. I've read about the beer thing (and a bunch of other methods for killing them). I guess it's time to get busy making a bunch of traps.