I had existing burrowing crayfish as well and they did not control the curly leaf pondweed. I think it took 3 years for mine to get controlled. I think they start feeding as the plants emerge, so they don't need to eat thousands of pounds of foliage. I also have water lilies and emergent plants and they are doing fine. Maybe something else controlled the pondweed, but it did coincide with the papershells.