Would it be correct to tack HardieBacker to the floor joists directly or is it necessary to lay exterior plywood when renovating/installing a new bath floor? my home is 140 years old and the floor joists are massive. My fear is that the plywood will attract termites as I have had issues with them attracted and devouring new wood used in the house and they have cost me thousands in damages. The old wood is generally left untouched by the termites since it is old growth cypress.
No, Hardie cement board is not meant to be structural, and it will not work in place of a subfloor. The board is very strong in compression and very stable so that the tiles applied to it do not crack, but it has very little shear strength.
There are treatments you can put on wood to make it less attractive to insects. I would recommend approaching the problem from that direction and use traditional, proven materials for the subfloor.