My first guess would be cutworms, since they would love to hide beneath all of the debris. My second guess would be earwigs, for the same reason. My third guess would still be slugs, possibly in combination with other factors.

Or... looking at the chopped off stems, it could be herbivore damage, possibly from rabbits or deer.

I would suggest scraping away all of the organic material, to see if that reduces the rate of damage. If that lessens the damage, the mulch could be restored once the plants get larger.

If you are not using a fence, you might try surrounding a few plants with chicken wire or hardware cloth... if the covered plants survive while the uncovered plants continue to be damaged, then herbivores are the most likely culprit.

Yep, I'd guess slugs and/or snails, followed by crickets.

Looks like slugs or snails to me too. Apart from the white powder of the de I'm pretty sure I can see their dried silvery mucus. There's also what looks like Gastropod faeces.

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