Here's a link I found. I do not know how trustworthy it is, but this is why it is used in Pest Control and approved for food service.http://www.pesticide.org/boricacid.pdf
".. absorbs waxes from the outer coverings of insects"
From my organic chemistry days, I believe Borax will undergo chemical changes either with air or water and become an acid. Especially when contacting the cytoskeleton of insects, their chemistry would be reactive with the Borax. both borax and boric acid kill insects, i've seen that. i don't know about the attraction or repulsion of one over the other though.