Is this outdoors or indoors? Maybe pics of those that are even worse off could give us a clue. That seedling looks pretty bad.

It's in an indoor, unheated greenhouse. The others have no leaves at all. I picked this one as it shows some munching.

I would check underneath the trays for lurking slugs. Also check after dark with a flashlight. Nibblers often work at night.

Yep, absolutely agree with Floral.

Checked for slugs, nada. Will try to get down to the greenhouse at night and snoop around with a flash light.

Rodents. Put some traps out.

I vote with Tiffany for pillbugs. Rodents, snails and slugs would devour the seedlings entirely. Pillbugs are good at attacking your most prized seedlings and newly planted anuuals. Is your Climbing monkshood Aconitum uncinatum? That's an eastern species that grows in damp woods and seeps. Do people grow it successfully on the west coast? Monkshood is poisonous so maybe whatever was eating it felt sick and never finished? What state are you in Senior Balloon? I just looked and your seedlings are probably Aconitum colunbianum which is also scandant and a west coast native. Thanks btw I just found out from this post that there were climbing monkshoods. I always thought they were all upright perennials. Even when I was trying( unsuccessfully) to germinate Aconitum uncinatum seeds I had no idea it was a vine. I have a vine obsession. Next time I'll order a plant. Their growth habit must be similar to Solanum dulcamara. I love bitterwseet nightshade. I get a sense of serenity and contentment being around beautiful, but deadly plants Lol. Pillbugs are good for cleaning up contaminated soil. They absorb heavy metels so maybe they like the toxic monkshood?

It's an Aconitum hemsleyanum, I purchased it from Far Reaches Farm, https://www.farreachesfarm.com/Aconitum-hemsleyanum-H2MD-003-p/p6468.htm They have several and I'm not certain which, but mine looks like the one in the photo here.

I also like herbaceous vines. They grow so much each year and die back to the ground. For some reason I like that. :o)

Last year I purchased seed for 'Athens Yellow Bleeding Heart' and Codonopsis lanceolata. Sowed this last winter, so far only one Codonopsis has germinated, still waiting on Athens.

I am in the Pacific northwest. Wet in Winter, and not too cold, though it can get down to 10 degrees occasionally, summers are mild and we can have stretches of 50 or 60 days with no appreciable rainfall.

I'll have to double check on pill bugs. I hope they got a stomach ache.