First, ants don't eat roots.

The white stuff they were carrying were likely pupal cases.

Next time you plant, regardless what the directions/seller says, remove the peat pot. It restricts the root system unless continually moist. And if any part is above the soil, it wicks moisture out of the rootball. A very bad thing.

I've read about ants making nests in the roots of a tomato plant, with the result being a dead plant. I think the problem is that they remove enough soil that the roots dry out.

Here are some home remedies which shouldn't affect the tomatoes:

= Sprinkle diatomaceous earth (DE) in a ring around the nest. When ants walk through this, it pierces their exoskeletons and they will die. [DE is a rough white powder, the fossilized skeletons of diatoms, a type of algae. It is sold in some garden centers. You could also try agricultural supply places or stores that feature organic foods and other natural products.]

= Mix 1 cake of yeast with 1/3 cup of sugar and 1/3 cup of molasses. Put it in low lids near the ant nest. Ants love sweets and will take it back to the nest; the yeast will expand inside their bodies and kill them. [Freeze whatever you don't use.]

= Crush grains of white rice with a mortar and pestle. Sprinkle it around the anthill. The ants will take it inside the nest. It will expand inside their bodies and kill them. (Some people use grits instead of rice. Powdered tapioca should work too.)

= Mix 1 part sugar with 2 parts borax. Add enough water to make a gel. Put this bait on squares of cardboard or low plastic or metal lids (low enough that ants can enter and leave easily; you can also raise the dirt around the sides of the lid to make a ramp). The ants will take this back to the nest and they will all die. [Too much borax isn't good for tomatoes, so don't use a lot of this in the garden.]

I am having the same problem was looking up the problem on Google and got an answer back that it could be firework when we took the plant up it was chewed off at the base and we saw a hole in the stem of the tomato plants we have lost 2 so far it says to cut up a potato and spear it with a stick and blurry it 2 to 3 inches to trap the firework and do it a couple of times a week to trap them.

Ants do not eat solid, dry food. The whole thing about them bursting from eating grits or rice or whatever is a myth.

Solid, dry food gets taken back to the nest, where the larvae eat it and then excrete a nutritious liquid that the worker ants can eat.

The yeast thing is also a myth.

Grits. cornmeal, flour, and yeast are attractive to ants as a food source for the colony, so they're often used as bait to carry poisons that do the actual killing. I guess somewhere along the line, people got confused and thought the bait was doing the killing.

Ants can be encouraged to move if you flood their home frequently until they get fed up. You might try diatomaceous earth sprinkled religiously around their nest entrances. The DE is abrasive to critters with exoskeletons, damages their shells, and they're prone to disease and dessication.

But unless they're biting ants that interfere with your ability to garden, ants usually don't cause harm to plants. They do take advantage of certain conditions that are not good for your plants so that it can appear they're the agent of harm instead of a third party taking advantage of the situation.

The one possible exception is that they often farm/protect aphids. They'll carry the aphids back onto the plant if you knock them off and protect the aphids from their predators. This is because aphid poop is full of sugars that the ants eat. But even that is at heart an aphid problem more than an ant problem.