There are different kinds of cabbage worms, but the main one is the imported cabbageworm, Pieris rapae, also known as cabbage white butterfly . Cabbage loopers (Trichoplusia ni) are also green, like imported cabbageworms. Also see guilde to Diamondback moth larvae ( Plutella xylostella ) which also damage cabbage family crops.
On Crops: Broccoli, cabbage, cauliflower, kohlrabi
Most areas where cabbage family crops are grown, especially cool, moist climates
Imported cabbageworms are velvety green larvae with faint yellow stripes, while cabbage loopers are yellow-green slender caterpillars that raise and lower their backs as they move because they have no middle legs.
Adult moths of all three species lay scattered eggs on the undersides of leaves. Upon hatching, the little caterpillars immediately begin feeding. You often can see the frass they leave behind before you see the worms because their camouflage is excellent. Late in the season, hatchlings inside the bases of cabbage or cauliflower may flourish unseen, and the same thing can happen inside heads of broccoli.
You can use row cover to prevent egg laying by adults if you frequently have trouble with cabbage worms in spring. Plant lots of flowers and blooming herbs around your cabbage patch to provide a strong supply of nectar for beneficial insects, such as paper wasps, yellow jackets, shield bugs and insect eating birds.
Check plants regularly, especially for signs of frass, or cabbage worm excrement. B.t. pesticide is very effective when applied between rains and is less likely to inhibit beneficial insects than spinosad, which is also quite effective. A single treatment of B.t. in late summer about two weeks before harvesting will make a huge difference in the quality of your crop by nabbing any pests that may be hiding among florets or leaves.
Try planting red leafed varieties of cabbage and kohlrabi, which are less appealing to cabbage worms due to the lack of camouflage. More than one species of cabbage worm can be on the same plants at the same time, so be vigilant in your inspections.