You should contact your cooperative extension service. The life cycle of many pests is specific to the region. Those worms could be gone tomorrow, the same day you run out and buy more chemicals. And, get yourself some bacillus thuringensis, a biological control that works well on many pests that attack the cabbage family. The Garden Safe product is not quite as safe as they want you to believe. Better than many chemicals, but still not "safe".
I try to find plants and plant them now through the first of July. Those will be ready in the fall. I usually have to start the seeds myself and plant them at the right time. I like to put in about 24 and blanch the heads after the salt soak, and freeze them. If you don't want to go to that much trouble, plant them in 2 week intervals and you can harvest until the first hard freeze.
Dwayne