Invasion of the Bee Girls (1973)
Chosen by me from a list of public domain movies on a website David Morson linked to me.
Described on the website as a film about 'A powerful cosmic force is turning Earth women into queen bees who kill men by wearing them out sexually.' I just had to see it.
Of course like most B movies it's not quite as fun as it sounds but it had a solid confidant start and fitfully moments of wit.
It is horribly let down by it's notion of what is sexy though. At one point a couple almost literally take a roll in the hay - rolling ungainly down a hill of long grass with as much sensuality as a dung beetle. Also there is a fairly pointless and nasty attempted rape scene that would have seemed more at home in a Death Wish movie.
Written by Nicholas Meyer who also wrote some terrible Star Trek films.