It could actually be that what you sprayed around the yard might have effected these birds - IF they got into it. Or the rain brought that in. Rule that one out if you can.
Alternately, it could be mites. Check carefully all over the body of the bird, and the feathers and their shafts. The mites and lice are nearly microscopic so a flashlight and a white sheet sometimes help. They also like the area around the vent because of warmth and moisture so double-check there. Sometimes you'll only find evidence of them by eggs on the feathers.
As for the fireants, we used poultry dust for lice, etc, on our fire and sugar ants that were coming for our babies' food, too. You should have that handy in case you have a case of mites or lice anyway. So pick that up at the feedstore for your chicken cabinet.
What about the weight of the bird? did she feel thin to you? Sharp-keeled? She should have a good bit of meat on her chest/body. Anything less and I'd suspect she's just not getting her share of the food. That's really common, especially at that age as they start to really sort out their pecking order.
We had to seperate ours into two groups because I had some more gentle hens who looked fat, but weren't if you pick them up. You couldn't ever see them being bullied, but when put alone with food (like yours) they really chucked it down. Nutritional deficiencies and anemia is very quickly hard on a young bird and is one of the causes of the ataxia (inability to keep balance) that you saw.
It's good that she keeps eating - facilitate that any way you can. While you're at it, pick up all the birds and examine them since you've lost two. Some birds and some breeds just aren't as hardy as others in their little group.
As for the possibility of Mareks, it's more common that the bird will experience paralysis and inability to walk rather than ataxia. I would suspect a combination of heat, possibly parasites (to be ruled out), a lack of food (not because of you - just because that's how they do), and maybe a little nutritional deficiency.
What exactly are your babies eating right now? Did you switch them to grower? (If you do shortly, switch over a week - mixing the feeds.) If you smell the food, does it smell strongly of fresh ingredients? Or kind of like cardboard? Crumbles lose their nutritional efficacy rather quickly because there's a lot of surface area exposing the vitamins to air/heat/etc even in the bag. Sometimes less vigorous chicks will need a bit of a nutritional booster.
Their diet should be at least 90% of their age-appropriate complete ration. (In this case starter, starter/grower.) You might consider using the baby vitamins (without the iron) that you can get from the grocery store or
Walmart. Once a week with that might make up for any odd deficiencies that they might get from their feed.
I'd recommend this highly over most of the packaged vitamin/mineral/electrolyte supplements out there. There's one that apparently is decent (dlhunicorn recommends it) and that's probably the only one I'd use unless I have a very ill bird, personally, imho. But for these guys i think the baby vitamin.
Another thing to consider is using a treat of yogurt twice this week. The living bacteria will help the birds to be more healthy, disgest their food better, resist bad bacteria better, etc. But more importantly in your case yogurt is a source of riboflavin. Deficiencies of riboflavin can result in the sort of ataxia you've described. The living bacteria also produce B vitamins, the deficiency of which also causes problems walking. So really it will do a world of good and no harm.
My rule of thumb for using yogurt is to use an amount that would be comparable to the amount we'd eat in a serving if we were their height. In other words, I might eat a cup of yogurt - a small bantam would eat a thimble's worth. It's not an exact science, but you don't want to give that same small bantam a half cup - even if she begs for it. Use plain non-flavored please.
As for preening, ours do it frequently. It depends on the bird. A few of ours I've never seen preening. The others are much more vain.
As for ants, you'd see lesions on the bird particularly around the eggs. You can also use boric acid around the coop. It's pretty safe for birds. But I'd go with the poultry dust as you can literally use it on the birds.
