I live in a city and have been fighting rats burrowing in my yard for about a year. I have hired a pest control service who added bait and tried a CO2 treatment, but it hasn't solved the problem, or even put a dent in it. The rats are digging underneath the plants to the point that they are threatening to destroy the plants.

Has anyone had luck with ultrasonic devices? If so, which ones?

I've also read about predator smells, but a study says this doesn't work when the rats are already comfortable with your yard.

Any other ideas???