Borax DOES kill fleas.
Borax has been used successfully as a non-pesticide flea treatment to rid the carpet of flea infestations. The University of Florida extension service recommends using boric acid as a dust on areas of the carpet that are flea infested, but also warns that boric acid can eventually damage carpet and upholstery. You should not use boric acid outside because it will kill most plants.
Natural Methods to Control Fleas
Metal combs: Metal combs designed specifically for pet flea removal are another method of natural flea control. Diligent combing of your pet will help to remove fleas and control the population.
B-12: Feeding your pet B-12 has not been proven to be an effective flea deterrent.
Brewer’s Yeast: Feeding your pet brewer’s yeast has not been proven to be an effective flea deterrent.
Vacuuming: Diligent vacuuming of an infested carpet has been shown to effectively kill flea populations.
Laundering: Washing your pet’s bedding and other washable that come into contact with your pet in hot, soapy water, has been shown to effectively kill flea populations.
“Non-Chemical Methods of Reducing Flea Populations.” IFAS Extension. University of Florida, n.d. Web. 12 Dec. 2011. <edis.ifas.ufl.edu/ig106>.
“Fleas Management Guidelines–UC IPM.” Home Page – UC Statewide IPM Program. N.p., n.d. Web. 12 Dec. 2011. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7419.html