Perhaps the cabbage looper's habits are dfferent in different areas. Here, I have very little trouble with them for springtime broccoli and not much for cabbage. However, they are terible in the fall for the whole season unless I do something. Last fall I kept a net handy and captured the moths and had no damage.

Cover now. If you wait, it will be too late.

Here in Northern California, I already see the moths flying around, and stopping on my favas. I've resigned to letting them have the bulk of the leaves because 1. they're getting a bit old for me to eat and 2. I don't feel like busting out with the spinosad/bt. I think it really depends on your weather, but once it started getting a bit warmer, I knew I was going to have to begin doing battle with them again.