Worms are basically the Larvae, immature stage of any of a number of insects. Primarily various moth species. However some varieties of Beetles commonly produce worms fairly indistinguishable in general appearance and feeding habits from moth larvae.
Worms that feed on Cabbage, Cruciferous vegetables, Brassicas, Green leafy vegetables, Cauliflower, Chinese Cabbage, Garden cress, Water Cress, Broccoli, Brussels sprouts, kale, kohlrabi, mustard greens, radish, turnip etcetra.
Of Cruciferous Feeders the 3 primary culprits in North America are the Cabbage Worms also known as the Diamond Back Moth, Cabbage Looper and Imported Cabbage Worms.
Imported Cabbage Worm
Imported Cabbage Worms are the most commonly found species in home gardens. The larvae are a white butterfly with black spots. They are well established from Canada to Southern California.
The larvae / worms are a velvet green with faint yellow stripes along their sides.They feed on all members of the brassica family - Cabbage, Garden cress, Water Cress, Broccoli, Brussels sprouts, kale and etc.
Cross Striped Cabbage Worm
The cross-striped cabbageworm lays its eggs on the lower leaf surfaces. The worms / caterpillars [larvae] reach about 3/4 inches long. They are light blue-gray on top and green on their bottoms. There are multiple black stripes on their backs and a yellow line down their sides.
The Celery Worm also known as carrot caterpillar and parsley worm does not feed on Cabbage or Brassica but is a prolific pest of similar crops -which include carrots, celery, parsley, dill, and parsnip. They chew The foliage and stems and destroy green tops.
Parasitic Wasps, Ground beetles, syrphid fly larvae, spiders. Lady Bugs, Praying Mantid.
Spinosad is another biopesticide , correctly termed a microbial pesticide, harmless to humans. Spinosad will interfere with pollinators as well as pests - evening / night time use is advisable.
Bt - bacillus thuringiensis. - Diamond Back Moth has developed some resistance to it - has proven effective against garden worms , it is a bacterium / organic pesticide that is readily available to home gardeners. It's harmless to people and pets. It's not a miracle cure but as stated , it is highly effective. Apply it at either bloom or petal fall, or both. Bt It is a stomach poison and must be ingested . It is more effective when applied during warm, dry weather while the larvae are actively feeding. Bt breaks down quickly in nature so multiple treatments per season are necessary.
Neem and various derivatives of their active ingredient Azadirachtin are an effective Biopesticide that works by interrupting the insect's growth cycle resulting in its early death.
Diazinon can be applied at petal fall, but is not as eco-friendly as the bio-pesticides listed above as it is toxic to beneficial insects and pollinators. Other Broad-spectrum insecticides, such as pyrethrin or malathion only as a last resort, generally used commercially and not really advisable for smaller plantings or home gardeners.
Most Sprays/Pesticides must be applied in the pre-bloom stage to prevent injury. Do not apply insecticides, even bio-pesticides during bloom time to protect vital pollinating insects.