Last year I used row covers on PVC frames to effectively keep cabbage worms off my broccoli and cabbage. I put the covers on in spring as soon as I put the seedlings in the ground, and then took them off when the plants were larger. There were some worms in the broccoli when I harvested, but they don't do much damage and come out of the heads with a good soaking when brought inside.
This year I didn't put the covers on early like I should have, and I'm wondering: is worth my time to put covers over more mature plants that already have cabbage worms? Will I just be "locking the bad bugs in"? What if I first spray with Bt, which seems to have been effective at controlling them so far, and then cover?