My fire bellied newts love earthworms! However there might be some problems with just leaving them in there.
1. Are you talking about an aquatic set-up? Worms need to breathe air, and after a few minutes will drown and die, and begin to decompose fouling the water.
2. A full worm might be to large for them to ever eat. How large are the newts and the worms? My CFBN aren't too comfortable eating worms longer than their tails. It might be possible, but I find chopping the worms to about 1/3 - 1/4 of the newts body length makes it so much easier for them to eat.
3. Newts might be able to dig worms out of a burrowing substrate, but if they get under some large river rocks, your newts might never be able to get to them. And unless you feed the worm or provide it with adequate housing, it will die and foul your set-up.
That being said, earthworms are a terrific staple food for newts! I feed mine on a mixed staple of petstore tubifex and wild caught earthworms. For the earthworms I chop them into small bits, about an inch or less, and just throw them into the feeding area of my 100% aquatic set-up. The newts find them quite quickly while they're still wriggling and eat them quite easily. The worms stop moving after about 3-5 mins and newts lose interest so I remove them.
If you want to keep LIVE FOOD in your tank, I would suggest finding some clean tubifex! They are quite nutritious as well, and small amounts can live in your tank no problem! The newts will definitely have a great time hunting them down too
