Parasitic nematodes, of the right species, could possibly aid in control of fleas and they will not cause any harm to plants or beneficial insects because they are species specific. Diamotaceous Earth is also reputed to aid in flea control and may be a less expensive option.

Nematodes should last longer. I've had good results with them on grubs, not so much on fleas. But, the fleas could be more from bedding areas than soil areas where the nematodes would be.