Jonathan Anderson replied
". . . Also, don't the little beasties get to be tolerant
to it? Any view on this, Bas?"
Boric acid is extremely fine and covers the body like a fine dust making
the animal unable to breath. It is used by kill wasps nests. By
spraying, at night, boric acid into the adit of the nest, it will
anihilate the colony through suffocation.
Because of this property I don't think they become immune to it like an
insecticide.
Jack