Cross-striped cabbage moth larvae

From the looks of the plant leaves they have been there awhile. Hard to see when young and small but a common pest for all brassica crops. Bt is the recommended pesticide tho I fear it is too late to save that one unless you planned to just let it go to seed.

For future reference floating row covers prevent the moth from laying the eggs. Or you can do egg patrol earlier in the season- small dark black dots on the underside of the leaves near the veins. You can expect them annually in various forms and colors.

Dave

I harvested all the other primary heads, and found nothing but a single green cabbage worm.

These things showed up about four days ago on one of the side shoot heads, but this final main head got obliterated in a matter of hours.

The heat of summer is here, I'm not sure if it's even worth trying to save the plants since I only have side shoots left.

When did they lay their eggs, and when should I put the row cover on going forward?

As I wrote in another post, I'm a HUGE fan of row covers. They go on as soon as the plants go in the ground or the seedlings appear. 100% effective and no spraying needed.

The "when" they arrive and lay eggs varies by location so you have to learn to track them in your location. For next year make a note that they were there approx. 3 weeks back and will be around until late July approx. Then you can fine tune the dates each year.

OR you just put the floating light-weight row covers over the plants from day one and then you don't have to worry about the "when"

You may have only seen one green looper but more were there - there always is - which is why long soaking of brassicas in cold lightly salted water before putting in the fridge is recommended. You'll be surprised what crawls out.

Dave