In many parts of North America, midsummer cole-crop plantings ought to be leafing out nicely now, and it's a pretty good bet that a sizable percentage of those tasty members of the Brassica group of vegetables will—at one time or another—be attacked by cabbage loopers. This approximately 1-1/2-inch-long caterpillar (which goes by the Latin name Trichoplusia ni) earned its more common moniker as a result of its unusual method of locomotion. Rather than keeping its body level with the surface beneath it and sort of undulating along as most caterpillars do, you see, the looper arches its body while walking (as shown in the photo), thus appearing to be "looping" itself. Although this peculiar gait (which results from the fact that Trichoplusia ni lacks two of the five pairs of stubby legs common to most caterpillars) resembles that of an inchworm (Geometridae family), the two creatures are unrelated. Loopers are, instead, members of the noctuid moth family, in which can also be found such pests as cutworms, army worms, and corn earworms.
As you'd imagine, the cabbage looper's preferred host plants include all of the coles—cabbage, broccoli, brussels sprouts, cauliflower, kale, collards, etc.—as well as lettuce, spinach, beans, peas, celery, parsley, parsnips, radishes, potatoes, and tomatoes. The considerable damage caused by this caterpillar typically takes the form of numerous irregularly shaped holes chewed through the leaves of the host vegetable. Mature and healthy plants can often survive minor infestations without suffering too much harm, but young seedlings or transplants may respond to an attack by developing a stunted, misshapen appearance and providing a poor yield.
The pest is common to the United States and to southern Canada. Its larvae hatch from minute, pale green eggs that the adult—a small, mottled brown nocturnal moth with an irregularly shaped silvery marking on each front wing—lays on the upper surface of the infested plant's leaves. In the moth's northernmost range, two generations are usually completed in a single year, while in its warmer southern realm, four (or more) generations may be produced annually.
Fortunately, options are available to those gardeners who are faced with a cabbage looper problem but would rather not resort to broad-spectrum insecticides in solving the crisis. If the infestation is still in its early stages, it should be fairly easy to remove the loopers by hand (chickens seem to savor the pests!). A better approach, if your eyesight and patience are up to it, would be to spy—and stop—an "attack" before the bugs hatch.
When a more serious infestation occurs, though, it might be time to get out the big guns, in this case the bacterium, Bacillus thuringiensis. Sold in both powder and liquid form under such names as Dipel and Thuricide, this bacterial agent can—in a gardening emergency—really save the day. Do use it only as a last measure, sparingly, and with a great deal of caution in application, though, because the substance will kill the larvae of beautiful and harmless butterflies and moths as readily as it destroys loopers.
Finally, should you find your cabbage patch slowly disappearing down the maws of a horde of wormlike chewers, remember that the pests might actually be imported cabbageworms (Pieris rapae), diamondback moth larvae (Plutella xylostella), or even garden webworms (Achyra rantalis) rather than loopers. Fortunately, the differences among these larvae are relatively unimportant, because the pests prey on the same plants, and can be controlled with identical techniques!