Looks like earwig feeding. My bet is during the day they are hiding under the rocks that I can see (is there mulch there too?) and coming out at night to eat. They CAN crawl over and into containers.

Vera

The containers are mostly sitting on gravel. Would setting them on something else help? Whatever it is seems to have a taste for marigolds, rudbeckia, dahlias. There are others that have not been touched.

I have also had a whole stand of marigold seedlings disappear after planting out in a bed. There one day, leaves stripped the next. Very frustrating.

If you see no slime trails it's mostly likely the earwigs. Sowbugs and slugs don't normally take up residence under gravel, but earwigs will.
Yes, I would move them up from the gravel....it supplies enough crevices for them to find daytime shelter. However before you do, I suggest you leave them as they are today and run out tonight with flashlight in hand. That way you can be sure what is in fact doing the damage. IF you find that is is earwigs, try putting out unwashed tuna cans filled with veggie oil as a trap. Also Rotting pieces of fruit/bacon works well too in a small paper sack turned on the side or in dishes...gather it up and toss in the morning; crumbled up moist newspaper tucked here and there, ect. will attract them also. The messed up thing is that they voraciously go after aphids and other soft bodies pests, but when the population is low they turn to tasty seedlings :(

If you want to know for sure what is doing that much damage - go out at night after dark with a flashlight and look. All the critters mentioned are night feeders, you may have to make more than one trip but you should be able to catch them in the act to know what you are dealing with.
Sometimes, it's not what you expect ;)

I'm glad that someone posted this because I'm having trouble with the same thing. I purchased some MG's and the other day I noticed that one of them was just a bloom on the end of a stick so I inspected everything real well and found nothing but I didn't realize that some of these little critters only come out at night. I've had real problems with MG's this year the 1st one's I WSed didn't do well at all and now these are being eaten. Fortunately I have 2 large lasagna pans all but ready to go but I think I'll wait and see what's eating my other ones.

Strouper

Doing a little reading, it says that earwigs are particularly fond of marigolds and dahlias, the two things I'm having the most trouble with. They also like moist conditions...we've had a lot of rain here, so lots of good hiding places, I guess.

I'll check tonight with a flashlight and see if I can catch them in the act.

i have white "flies" (or moths) - doing damage - I am not sure if they eat seedlings, but they feast on my leaves here.

Carrie

someone told me that you can use dunks in a watering thingi, follow the directions on the back and tha helps get rid of the gnats in your soil. ummmm ijust read your post again. duhh LOL ok. nevermind, my mind is going like a thousand miles a second, at least it feels like it!! LOL i just am so psyched i planted a bunch of stuff out yesterday and have less to plant out!!! LOL :'>> ummm also grasshoppers eat on your plants, i've seen them munching on my plants. of course not my seedlings yet.

A few drops of dish soap in a spray bottle of water kills earwigs instantly. In areas I have problems with them,I dump buckets of soapy water,it is effective this way also.Maybe dump a few buckets of soapy water,about 6 drops per gallon,into your gravel and see if that helps.