I bumped into a reference to this 1984 Nebula- and 1985 Hugo-winning novella in an article today, and quickly found the full text online in couple of places; here's a link to a PDF with the story and some notes about themes from Butler herself:
I was interested in the idea of parasitic intelligent races, which is one of the big subjects of this story. I was impressed by the nuances of what Butler does with the idea. I had just been thinking mainly of the conflict: ooh, yuck, that race reproduces parasitically. Butler sets up a situation where the races need each other. I'll say no more; it deserves all the acclaim it's received.