I've found that tightly sealed row covers (until the plants begin to flower) help keep the early arrivals from laying their eggs. That's too late for you, though.

After removing the row covers, I try to check the underside of every leaf every day for the easy-to-spot yellow or orangey-brown eggs of this pest. You can usually scrape away the eggs with your fingernail; when the plants get large and sprawling, you can remove an entire leaf that's heavily infested. Diatomaceous earth spread around the base of the plants will destroy some of the young nymphs that congregate there, and hand-squashing will eliminate a few more.

But in my experience, once you have a major infestation, it's almost impossible to bring them under control, even with approved chemical agents. Sigh. Maybe it's not too late to start over with a fresh seeding?

Be sure to remove or till under all plant debris from this crop in the early fall so you don't allow the late larvae to mature. Plant field peas or oats as a cover crop to prevent winter soil erosion.