They will be way more of a problem in the fall because they will show up immediately and newly planted fall seedlings with fewer leaves are more vulnerable than the mature spring plants are when they show up in May. Usually by October the pressure from newly laid eggs goes away.

Use insect barrier row covers when the plants are young or treat every few days with BT-K. Remember that broccoli grows fast and puts out 3-4 new leaves a week. It is important that these new leaves are sprayed with the BT shortly after they come on or they will quickly become worm infested.

I gave a few plants to one of my neighbors and told her to keep a close eye on them and either hand pick or spray. Less than a week later I went over there and saw that the 4 plants were riddled with worms and pretty much skeletons. No more broccoli plants for that neighbor.