Probably a caterpillar called Loopers. They can eat quite a lot for their size. Go to your local nursery and ask for something to kill loopers and that the plant is for consumption.
Hope this helps
Tim

Well I have had cabbage loopers in the past and they would be on the plant when I checked it. Here there is no sign of the loopers at all. Anyone have experience with loopers hiding?

I've had them hiding numerous times. They destroy my Basil every year.

caterpillars in DECEMBER ... in z6??? ... in the northern hemisphere???

freak.. you in AU are in spring.. we are in winter ..

i suppose its possible.. but the darn thing should have pupated and laid eggs.. for i doubt there is time left this year... the end of that family ...

slugs are still around this time of year.. but they are NOT moving very fast ...

you should be able to find whatever... within a few inches of the plant.. in the soil surface...

i would not waste money on product.. this time of year.. dig them up and throw them away ..

ken

Cabbage Loopers, as well as the European Cabbageworm, can be difficult to see since they often are the same color as the plant but they do not "hide".
If you know what to look for you can find them and if you do not see them, even if the damage looks similar to what they do, it is not them.
A good defense against most all pests that go after members of the brassica family is the Floating Row Cover, especially since that family does not need pollinating.

Yeah I had row tunnels over them with row covers to protect against the harsh weather and promote growth. Whatever it is, was already there cause I had it sealed up good.

If you look at the bottom of the pic that's the folded back row covers, I finally took them off because there is no point now the plants are shot. Although I have had kale come back and flower after this type of devastation.