Turnip/cabbage problem

[JustG]

I'm having some trouble in the garden and I can't seem to figure out which pest is causing it.

There are three plants that seem to be bothered the most. The turnips (which are becoming completely skeletonized), the purple cabbage (the leaves have all but disappeared), and the marigolds (put in one day, many of them half gone the next). (minor bites on the radish, brussel sprouts and zucchini).

I've been keeping an eye on them, but have yet to actually see anything eating them, even at dusk.

Please let me know which pest this is most likely to be and if possible, how best to get rid of it.

[wolfster]

Could you post a picture?