Some of the nurseries around here sell "beneficial nematodes" to kill fleas in the yard. Never have tried them.
Discussion
Oh I didn't know they killed fleas too. Other uses they sell them here is for chafer beetles and ant colonies for peoples lawns but I found the latter didn't work particularly well as doesn't actually target the ants they just don't like them. I guess some ants are more tolerant to annoying neighbours that wont leave your house.
Yes, the instructions say that the soil needs to remain moist for the nematodes to survive, but thats not easy to do here in summer. But many folks say they really work.