I've also heard that borax is effective. The environmental impact is very small with both products, especially if the gel is injected into the crevices as directed.
These are fire ants that have nest in our brick voids and are entering the house. I have a boric acid protein bait inside but need to approach the problem also from the exterior. Suspend SC is a synthetic based on a naturally occurring insecticide found in Chrysanthemums. I received it today and have already done a perimeter treatment... Hope it doesn't kill off my worm population