If you can see the green caterpillar then most likely it's the cabbage looper. Sometimes called an inchworm. I just flick them off into a coffee can with some detergent and water. You can use BT which kills them by being systemic. That means it is actually inside the plant and the plant is now deadly to the worm. I've never used it since it MUST have some effect on the rest of the world. I'm probably being a bit paranoid there. Try the detergent first. The damage to your plant, going by the picture, isn't severe as yet. By the way, those cute little white butterflies we see all summer are the culprit. Some people spray a little cheap vanilla extract on the plants and it seems to confuse the butterfly (moth). They go everywhere and apparently recognize your brassicas by aroma. Makes sense. Then they lay their eggs and along comes the hungry caterpillar. Hope all this helps somewhat. Best wishes with your plants.

Hi, re slugs - there are many low-cost home-made remedies such as beer, which work. but in case you don't drink - I used escar-go from "gardens alive" - had to spend money. :) but it worked like magic, 2 applications and they were gone and have not returned. it is safe for children pets and the environment. plus it's a very nice (if somewhat expensive) website. ellen Here is a link that might be useful: escar-go

Keep an eye out for those tiny green cabbage moth caterpillars too. They are so hard to see, but their droppings often are easily seen. I good spraying with Bt, the natural bacteria that kills only caterpillars will take care of them. They could be grasshoppers for sure too. If it's the caterpillars, the Bt will take care of them. Keep us posted! Good luck identifying the culprit, hah.

You have to be able to identify the culprit in order to treat or prevent. If you can post clear, detailed photos of the damaged plants, we might be able to narrow it down for you.