Boric acid is a relatively weak acid derived from boron, a naturally occurring compound. Boric acid can be used as an antiseptic as well as an insecticide. It can look like either white powder or colorless crystals. When insects ingest boric acid, it acts as a stomach poison, destroying the insect's digestive system. Boric acid is effective against nuisance insects around the house, including termites, cockroaches and ants.
To kill fire ants, place boric acid along ant trails where you have seen them. Some of the ants will eat the powder and be killed by it. Others will walk through the powder, which will stick to their legs and bodies. These ants carry the boric acid back inside the nest. There, they either ingest the boric acid while grooming themselves, or other ants will ingest it while helping them groom.
Fire ant nests usually contain about 80,000 worker ants, as well as queens and winged males. Some nests contain as many as 240,000 workers, who gather food and protect the nest. While boric acid will kill the ants that encounter it, sprinkling it around the nest will not reach enough workers to make a true dent in the number of fire ants. Boric acid can help protect your home by eliminating fire ants that come near it, persuading them to look in other areas for food.
Boric acid has low levels of toxicity for adults and children, as well as pets. If a child or a pet eats enough of the powder, the child or pet can experience gastric problems, including nausea, vomiting and diarrhea. While boric acid kills fire ants that encounter it, if you want to kill the nest itself, you will need to use other solutions, like drenching the nest and soil surrounding it with insecticidal soap, pyrethrum-based insecticides, citrus oils, or boiling water. These treatments often require repeated applications to completely destroy a fire ant nest.