Slugs and earwigs will strip marigolds down to the stem. Both are night feeders so a check with a flashlight after nightfall will tell you which one is the culprit. Baby earwigs are hatching out right now and they can strip a whole patch of marigolds in a couple of nights. They are tiny grey bugs with an elongated body--they don't get the classic red color until grown. For slugs I suggest safers slug bait or surrounding the plants with something rough like broken egg shells. Slugs won't crawl over sharp objects and get their bellies cut up. It doesn't kill them just discourages them.
For Earwigs This bait works wonderfully well. I had a small bed full all different sizes of marigolds. I planted them and didn't get back to check them for 2 days. Almost every one was stripped. I found this recipe--can't remember where-- But I put it out in a baby food jar and the next morning it was full to the top with baby earwigs.

Bait for Earwigs.
Equal parts of soy sauce, cooking oil and (molasses or corn syrup). The molasses or syrup is bait to attract them--the oil doesn't allow them to crawl back out and the salt in the soy sauce finishes them off and makes the mixture dark so they think it is just a hole. Put it in a small container�A good use for all those detergent scoops. Cover 1/4 to 1/2 an inch of the bottom of the container with the mixture and bury it up to the brim in the soil where you have a problem with them. Prop a lid or something like a piece of wood over it so it doesn't get diluted by rain and so it is a nice dark place for them to hide for the night--ha. When you prop the lid, leave a small space for them to crawl into the container. You might have to wait a couple of days for it to work, but mine were full of them the next day. I was surprised by just how many I did get.