Three different cabbageworms may be found on cabbage, broccoli, Brussels sprouts, collards, kale, kohlrabi, and other cole crops grown in Illinois. The imported cabbageworm is a sluggish, velvety-green worm. The cabbage looper is green with stripes along its back and sides and travels in a looping motion. The diamondback moth larva is green and very active.
In spring, white or brown butterflies may be seen depositing yellow or yellowish-white eggs on the underside of leaves of cabbage and related crops, salad crops, and leafy vegetables. Upon egg hatch, the imported cabbageworm larvae begin feeding on foliage. The adult moths of the other two species lay their eggs at night at about the same time of year.
If allowed to become numerous, cabbageworms can completely defoliate plants; they also eat their way into cabbage heads from near the base of the plant, resulting in decay and general poor appearance of the cabbage. Presence of white butterflies signals the start of infestation. It is almost impossible to raise cole crops in Illinois without controlling these pests.
Contact your county Extension office for current pesticide controls.