Ants transfer the baits by sharing food or coming into contact with one another.
Only a fraction of ants (older workers) leave the colony to forage for food. These foragers carry food back to the colony and feed it to other workers, larvae, and the queen(s). Ants share food by a process called "tropholaxis", where they regurgitate food to feed another. Interestingly, adult ants cannot digest solid food. They transport it to the ant larvae in the colony, which digest it and regurgitate part of it to feed adult ants. Lastly, some insecticides, like Fipronil, can kill by contact. Ants that walk through ant bait stations will pick up insecticide on their bodies and can transfer insecticide to other ants as they interact with them.