What Is the Ratio of Borax and Salt for Killing Fleas?

Chris Deziel

A 1-to-1 mixture of salt and borax kills fleas by desiccating them and possible causing digestive failure when they ingest it.

The collective experience of myriad pet control experts and homeowners confirms that borax is effective for flea control, although no one is quite sure how it works. No matter; mixing it with salt provides a double-whammy for fleas in your carpets and on your furniture, and there's no ambiguity about how salt works -- it dries out the little beasties' exoskeletons.

Because the exact mechanism of how borax acts isn't well understood, it's difficult to know exactly in what proportion to mix it with salt, but a 1-to-1 ratio is a safe bet.

How Borax Probably Works

Destroys Cockroach Stomach Linings

A 1995 study showed that boric acid killed German cockroaches by destroying the insects' stomach linings. Although these findings weren't conclusive, pest control experts have long known that roaches have to ingest the compound for it to work. Because it's boron-based, the same is probably true for borax. It's a larger crystal and more difficult for insects to ingest, though, which may be one reason it isn't as good an insecticide as boric acid.

Borax for Fleas

Like cockroaches and ants, fleas are biting insects -- borax won't kill sucking insects, such as mosquitoes. A flea isn't likely to ingest borax directly -- you probably have to get it to walk through a fine film of the compound and wait for it to ingest the material when it preens itself. This is why pest experts usually recommend working borax into the carpet fibers with a stiff brush, or spreading it on the carpet, leaving it for 20 minutes to an hour and vacuuming it. When you vacuum, the fine dust that is left over is probably what does the work.

Borax and boric acid -- which is derived from borax and is a more powerful insecticide -- are recommended treatments for ant and cockroach infestations. In both cases, the efficacy of the compound in killing the pests depends on their ingesting it -- in the case of ants, low-concentration baits do the job slowly enough to allow the workers to feed the bait to the rest of the colony.

Add Some Salt

The desiccating ability of salt is well-known, and the benefit of adding salt to borax for dusting your carpet and furniture isn't just that it dries out the adult fleas -- it also desiccates the larvae and eggs. Many pest experts believe that borax also has desiccating ability, so the combination makes the environment of the area you dusted doubly hostile for fleas. There's a catch, though. If you vacuum the carpet to leave a fine dust of borax, you'll end up removing most of the salt, and you'll lose the flea-control benefits it provides. There are three ways around this:

  • Adjust the ratio so that the mixture contains more salt than borax.
  • Use finely granulated salt to ensure that some gets locked in the carpet fibers and stays behind.
  • Vacuum cursorily to avoid picking up all of the borax and salt mixture. 

If you choose the first option, mix the salt in a 2-to-1 ratio with borax. Otherwise, a 1-to-1 mixture is probably sufficient.