Usually, the imported cabbage worm feeds on my broccoli, cabbage and cauliflower. It might be a different year for pests since the cabbage looper is the one feeding. I picked them off this broccoli which is growing in a container. The others in the garden are under row cover which keeps the moths from laying eggs on them.
There are several insecticides for management. I've used products containing Bacillus thuringiensis or spinosad in the past. The insects are most often on the underside of the leaves so treatments need to be applied there. Cole crop leaves are waxy so use a spreader/sticker with the insecticide for improved results.