Why did you dust them with flour? What was that supposed to do? Use organic BT which is a great defense and is organic.

dirtguy should have explained that you need to look for Bacillus thuringiensis sp. kurstaki also known as Bt-k. It's a biological control for young caterpillars and only caterpillars. There are other Bacillus thuringiensis varieties on the market that are effective for other groups of insects.

Read the label.

I just hand pick the caterpillars. Flip the leaves over and look on the underside of the leaves. They are well camouflaged and often are along the veins and ribs. And then either drop them into a can of water with a small squirt of dish soap or squash them or feed to the chickens. Typically, however, my cabbage worms just chew holes in the leaves and don’t chew through stems to remove leaves.

With whole leaves cut off, it also might be cutworms which will cut through the stems of young plants, but often take lower leaves when the stems are too large. They will be buried in the soil near the base of the stem, so you can look for them by gently sifting through the first inch or so of soil around the plant base or by going out with a flashlight in the evening or very early morning when they are active. They can be anywhere in size from less than 1/4” long to up to about 1 1/2” and are grayish or tannish in color.