| I have millipedes in all my outdoor orchids, they're common here. I've never seen any detrimental impact on my orchids. I do not know whether they eat decaying plant matter (bark) or roots.
If your orchids are outdoors you could probably get away with doing nothing. If you're bringing them in for the winter, immersing the pot in water for a few minutes will force them out (or drown them) then you can dispose of them as you like.
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