Have you tried a row cover yet? Oddly, fireants go after okra. A row cover won't stop them. Are the remains of the leaves ragged or neatly eaten in half circles. The neat damage is usually done by caterpillars. Any tent caterpillars around? Are you also growing corn? Might be an early crop of corn ear worms hatched. I googled and didn't see anything specific except the ants and the ear worms.

I've had tomatoes look perfect until I pull them off the vines, then see that something has entered right underneath where the stems attach. I caught a worm in the act today. If you are seeing damage from caterpillars, there is likely other damage that is not visible yet.

The seedlings are anywere from 2 days to 2 weeks in the ground, making them 8-10 weeks old. Some of the plants were very frail and others were pretty tough. As we've gone along, the plant killer has moved higher on the the stems. My most recent attempt, which was right before the mouse theory here, was to put a small cone of plastic rabbit fence around each plant. It created about a six inch diameter circle around the 11 remaining seedlings. I figured that if it is a small rabbit, I would either keep it out or at least be able to see the way the animal got around the barrier. I'm down to 9 plants now. two more have been cut off right above the 8 inches of vinyl tubing that I put on the protect the stems. There was no visible disturbance of the cone. It's like it either came down from the sky or up from the ground. I planted the remaining five seedlings in large pots and put them by my front porch. At least they'll get me a few nice tomatoes this season, but it looks like I won't be needing my canning supplies in August. As for next year, I'm going to start working on a plan for some sort of wire mesh cones that will be burried in the ground around the plants. If it is a mouse, it is more than likely that this particular one will be dead by this time next year, and that others won't share it's desire to kill my tomato seedlings. However, I'll be prepared just in case.

Yeah, very familiar to me here in NY. They seem to knock them to the ground then just eat the middles until they get to the skin on the back side. Pretty much narrowed it down to squirrels as the culprits. I find a combination of pepper spray, "waiting till the tomatoes grow higher than the squirrels care to climb", and just letting them have some until the plant outproduces what they eat is the least stressful solution for me. Haven't noticed any remains on the ground now for a couple weeks and enjoying all the tomatoes we can eat! The pepper spray I use: https://www.amazon.com/gp/product/B000BOB7UW/ref=oh_aui_search_detailpage?ie=UTF8&psc=1