I have a drainage ditch that runs across the front of my yard and catches rain runoff. My yard doesn't stay wet all the time, yet we have a problem with crawfish holes. It's becoming dangerous, because as the holes age or erode, they become larger and larger. I was almost injured when I stepped on what I thought was a dip in the soil and the grass gave way to reveal a hole 3 ft deep or more. How can I get rid of the crawfish?