We have a major problem with little black ants where I live. They have little mounds and at times in the summer it looks like the ground is moving we have so many. They usually seem to be attracked to water. That's why I bought chickens, to help with the problem. They bite, but don't leave marks or those nasty little puss blisters like fire ants do. If your brooder isn't off the ground, get it up. The ants will still bite them. With the poults that little, I wonder if they don't go into some form of anaphylactic shock from the bites. They bother the larger chickens but they seem be able to handle them once grown vs. a little peep/poult.
Then, get Swat from the feed store, usually in the horse section. It's like Vaseline with insect repellent and put thick lines around the legs of the brooder.
Then, get a container, like cool whip. Put a few holes about the size of a quarter in the lid. Mix three cups of warm water to 1/4 of sugar and 1/4 white vinegar and set it around the brooder or where else you're having problems. They love that stuff and once they get in the gorge themselves and normally end up dying in there. If it gets knocked over it won't hurt anything because it's all natural. I also hang this mixture out for fly control with my horses and most insects love it. It usually takes about two days for them to find it and then they start loading it up.
This is what has worked best for us. Our black ants are so vicious that if a fire ant colony comes up, they will attack the fire ants and kill the queen/nest. No fire ants, but tons of these little nasty guys. They're even smaller than the fire ants, but apparently pack a punch.
