Method of dealing w/ cabbage root fly

Hello,

So i have cabbage root fly in my garden. I heard that once you have it that it stays in your garden bed. So i put some protective collars made of cardboard around them. But i read that also there is a method of dealing with them. To buy beetles and let them loose in your garden and also nematodes and disperse them in water and pour on the cabbage transplants after you put them in the ground. Has anyone ever tried this before?

Here's the article:

http://extension.umass.edu/vegetable/articles/cabbage-root-maggot-flies