Boric acid is a common ingredient in many pest control formulations and is an effective stomach poison for ants. It is nonstaining and odorless, and it is usually added to a food attractant or bait.
When ants walk through powdered boric acid, it clings to their legs, and the ant ingests it during grooming. When used in bait food, dry boric acid powder works by adhering to the exoskeleton of the insect and interfering with its metabolism, although the mechanism isn't fully understood. The resulting death of the insect is from desiccation.
Boric acid is a pesticide, and it is toxic. It can be harmful if ingested, so users should be careful when using around children and pets, and all warnings on the label should be heeded.