Cabbage worms are a real problem here in the midwest.
If you're growing cabbages and cabbage relatives, read up on these little stinkers.
Cabbage worms attack these plants:
Cabbage worms are the larvae of small white or pale yellow moths. You've probably noticed them flitting around the garden, but they aren't as innocent as they seem.
Cabbage Worm Eggs
The moths lay tiny yellow or white eggs on the undersides of leaves, and these eggs are difficult to see.
Soon thereafter the larvae emerge and start eating the leaves. They create irregular shaped holes in the leaves and in the worst case, they can strip most of the leaf, and leave only the veins.
Cross-striped cabbageworm (L) and Imported Cabbage Worm (R)
The tiny, white, fuzzy cocoons are Braconid Wasp cocoons
parasitic wasps for the cabbage worms
Two types of cabbage worms are commonly found in the midwest: the velvety green caterpillars and the darker cross-striped cabbageworms. The green cabbage worms closely match the color of the leaves they can be tricky to spot. The cross-striped cabbageworm is easier to see on a cabbage leaf.
You will notice the holes in the leaves first, and then you might notice frass (or caterpillar poop) collecting at the joints of the leaves and stem. It looks like a pile of moist, little dark brown balls. Turn the leaf over and you will probably find your culprits. Don't forget to check leaves that aren't so damaged, too. The cabbage worms move around the plant as they eat and grow.
There's another caterpillar found on cabbage leaves, called a cabbage looper. The distinguishing characteristic is that the cabbage looper doesn't have legs in the middle of its body, so it hunches up in the middle as it crawls, like an inchworm. Cabbage looper damage looks very similar to the damage caused by cabbage worms. The control methods in this article will work for cabbage loopers, too.
Severe Cabbage Worm Damage
If you use non-organic pesticides, you can dust the plants with Sevin insecticide.
Cabbage worms are difficult to control organically mainly because there are so many of them! You can pick them by hand and toss them in a small container of soapy water (or squish them). However, it's difficult to get them all, and the moths continue to lay eggs all season. In other words, hand picking them is a long battle.
Prevention is possible by covering your brassica plants. If the moths can't reach your plants, they can't lay eggs on them. You need to use a fine netting or spun polyester fabric, one that allows light and water to pass through but prevents the moths from accessing your plants. Floating row covers are one type of cover. Individual plant covers, like cloches made of spun polyester or fine mesh, but these can get expensive. Fortunately, cabbage and cabbage relatives do not need pollination to produce the crop; covers are a practical solution.
Bt is a naturally occurring bacteria that is toxic to larvae of butterflies and moths, including cabbage worms. It's available as a concentrate that you dilute and spray on the plants. It has some drawbacks, too: it degrades in sunlight and is washed off when it rains. You will need to apply it over and over again. One source I checked for this article states that it is more effective on the smaller, younger caterpillars.
Bt is toxic to all butterfly and moth caterpillars. It will kill the caterpillars of monarch butterflies, swallowtail butterflies, and more. If you aren't careful with application, the Bt could end up on the plants that desirable caterpillars feed on.
Wasp (not a Braconid) Eating a Cabbage Worm
There is a parasitic wasp that lays its eggs in the caterpillar. When the wasp egg hatches, it eats the innards of the cabbage worm which kills it. I've also seen other insects prey on the caterpillars.
Planting these near your cabbage crops can deter the moths from stopping to lay eggs on your cabbage plants. This is only a partial list as many different plants confuse or deter the cabbage moths.
A trap crop is a crop you grow to attract a pest to it. The pest likes that crop better and chooses to lay its eggs on the trap crop, not your cabbages. You don't get a harvest from the trap crop. Mustard greens and nasturtiums are recommended trap crops for cabbage worms.
I've seen some other homemade cabbage worm remedies. I don't know if any of these work, but they're inexpensive and easy. It might be worth a try!
As you can see there are many options, and each solution has benefits and drawbacks.
A pragmatic approach is to consider each solution above and implement one or more of them. Select the options that fit into your lifestyle best; after all, if you aren't able to implement it completely, it's not worth investing time, effort, and money into it.
For example, if you garden in a community garden and you don't get to the garden more than a once or twice a week, hand picking probably won't be effective enough.
Of course, you can choose not to plant the crops the cabbage worms like. Nothing wrong with growing lower-maintenance crops.