When used on its own, borax will not kill insects such as fleas and ticks. Additionally, boric acid, which is a refined version of borax, is a popular pesticide that will kill many household pests, but not ticks or fleas, according to the Dodson Pest Control company.
Among the pests that will not be killed by boric acid are those that feed on blood, and this includes fleas and ticks. That's because boric acid works as an ingested poison. Blood-sucking pests will not consume the scattered boric acid in problem areas, so they won't ingest the poison. Boric acid and borax should never be fed to or applied to the fur and skin of pets and other animals, including humans.