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Thwarting ants using boric acid-based bait

Photo: Insectarium de Montréal (Marjolaine Giroux)
Monomorium pharaonis, Québec, Canada.
  • Monomorium pharaonis, Québec, Canada.
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When you can’t find an ant nest or the nest is in a place that is hard to access, you can create poison bait to kill the queen and consequently, the rest of the colony. The bait must contain two essential ingredients: food, which attracts the ants, and boric acid, also known as borax. Boric acid is toxic to ants. When the insect ingests a small quantity of boric acid, it doesn’t kill it right away – slowly but surely, the ant will be poisoned.

The way boric acid-based bait works is simple: It attracts foraging ants, which swallow it and hold it in their stomach, without dying. When they return to the nest, these ants transfer the poison to the queen and the other members of the colony when they feed them by trophallaxy.


  • Mix equal parts powdered boric acid and a sweet food such as peanut butter, corn syrup and powdered sugar, and then add water. Make sure the mixture is viscous and still somewhat liquid so that the ants can eat it without drowning.
  • Place mixture in small containers along all active chemical trails. It is better to have too many than too few. If you find dead ants near the bait, your mixture is too strong. Add food to dilute the poison or mix a new batch with less boric acid. Repeat as needed. The aim is to get the ants to take food back to the nest. They can only do this if they are alive!
  • If needed, place bait near drains. Don’t remove the chemical trails, as ants can take several days to create them again.
  • During the first month of treatment, make sure the ants are feeding on the bait. If not, change it. Also check to see if the ants have created new trails, and add bait to the new trails if necessary.
  • Continue treatment until you are sure you have eliminated the infestation. Ant activity sometimes stops for two months or more, only to start again. Insects come into the house from well-hidden colonies that seek to expand their territory.
  • Depending on the size of the colony, this method may take several months. The procedure is very effective, but must continue until there are no more ants in the house. Bait must be changed regularly. It’s a good idea to wait before removing bait to be sure that all ants are dead.

What you should know:

  • This method is effective for all species of ants, including carpenter ants and pharaoh ants. However, the process can be lengthy.
  • Queens live longer because they are bigger and need more food than workers. However, they are very vulnerable once there are no more workers to look after them.
  • Pupae, which do not eat, survive the beginning of treatment, so it is important to continue the treatment for a long enough period of time. The operation may take anywhere from 13 to 40 weeks, depending on the severity of the infestation.
  • It is very important not to spray ants with insecticide during the treatment. They need time to bring the bait back to the nest. Using insecticides may even make the situation worse by splitting large colonies into several smaller colonies that move elsewhere inside your home. The momentary disappearance of the ants can leave the impression that the problem is solved, but the workers will eventually come back.
  • Powdered boric acid can be purchased in drugstores and hardware stores. It is not recommended that you use this product if you have pets or small children.
  • If there are not children or pets in your home, you can leave the containers open. If there is no risk of accidental poisoning, the liquid bait can be poured into saucers or metal lids and caps (such as bottle caps).
  • It is recommended that you apply preventive measures suggested in the Prevention and control section when using boric acid-based bait.

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