The imported cabbage worm, Artogeia rapae, is probably better known to the gardener as the cabbage butterfly. This butterfly was imported from Europe to eastern Canada in the mid-1800s, and has since spread to all parts of the continent. The adult is a white butterfly (wingspan 1¹/₂ inches) that has black-tipped forewings. Females have two black spots on top of each of their forewings; males have only one black spot. The hindwings are all white on the surface except for a black spot on the outer front margin. A slight yellowish hue shows on the undersides of the wings.

Adults appear in the spring, mate, and females begin to lay eggs singly on the leaves of host plants. The eggs are yellow, oblong, and deeply ridged length-wise. Eggs hatch in 3 to 7 days, depending on temperature. The larval stage takes about 2 weeks to attain full growth, about 1-inch in length. The larva is soft and velvety green with faint yellow stripes running longitudinally on its back and sides. When development is complete, the larva pupates in a pale green chrysalis, which it attaches to any handy object in the garden. Completion of the pupal stage takes from 1 to 2 weeks. Three to five generations overlap throughout the season. Overwintering occurs in the pupal stage of the last generation.

The larvae feed on the first formed outer leaves of their host plants, which often appear riddled with irregularly shaped holes. As the caterpillars become mature, they feed in the center of the head. Fecal pellets can be found between the leaves. This pest damages turnip, radish, mustard, and nasturtiums in addition to the cabbage group.

A second major caterpillar pest of cabbage and related plants is the diamondback moth, Plutella xylostella. This pest, also introduced from Europe in the nineteenth century, now has a wide distribution on this continent. The small brown or grayish moths have a wingspan of about 3/4-inch or less. When at rest the folded wings present an image of light-colored diamond shapes along the wing backs where they meet.

Adults appear in early spring. After mating, females deposit small, almost round, yellowish white eggs singly or in small groups on both sides of leaves of host plants. Hatching occurs in a few days, and the young larvae begin working as miners between the outer leaf tissues. This insect also feeds on some ornamentals, such as sweet alyssum. Larvae become external feeders within a few days of hatching. As they mature and grow larger they remove leaf tissues, creating holes or sunken areas in the leaves. Mature larvae are approximately 1/3-inch long and pale green in appearance. Pupation takes place in a delicate cocoon on leaves or in garden debris. A total life cycle may take from 2 to 7 weeks. About four to six generations occur annually. Adults overwinter in plant debris or in the soil.

The alfalfa looper adult, Autographa californica, has a wingspan of up to 1¹/₂ inches. The moth has mottled gray forewings; a distinctive, silver, commalike mark occurs in the center of each.

Appearing in the spring, adults mate, and females lay pale yellow hemispherical eggs singly and in clusters on leaves of host plants. The pale green mature larvae are about 1¹/₃ inches long, and have a dark top stripe edged with white lines, and two somewhat obscure white lateral lines. These larvae have three pair of true legs attached to the thoracic segments behind the head, and possess prolegs (false legs) attached further down on the abdomen. While the imported cabbage worm and diamondback moth larvae have five pair of prolegs, loopers possess only three pair. Because the looper prolegs are attached near the end of the abdomen, these caterpillars appear to move in a "looping" fashion.

Loopers overwinter in the pupal or adult stages. From three to five generations occur annually. While general feeders, alfalfa loopers can at times cause significant damage to cabbage and related plants. The cabbage looper, Trichoplusia ni, is another looper that sometimes attacks these plants. These two loopers are extremely similar in appearance and, therefore, difficult to tell apart.

A number of cutworms and armyworms can cause considerable damage to cabbage family plants. The adults are mostly drab moths, the same size as the alfalfa looper. In fact, they are in the same family of moths which as adults are collectively known as "Millers."

Among the many species of armyworms and cutworms that occasionally feed on cabbage, the most commonly encountered are the Bertha armyworm, variegated cutworm, spotted cutworm, army cutworm, western yellowstriped armyworm, zebra caterpillar, and the black cutworm. Many of these caterpillars have strong preferences for certain species of weeds. Gardens surrounded by, or overgrown with, weeds tend to have more serious cutworm and armyworm problems.

All species of caterpillar pests of cabbage and related plants are parasitized or otherwise preyed upon by a variety of biological entities, including parasitic wasps, predacious beetles, disease, birds, and bats. Caterpillar pests may be nearly nonexistent in your garden some years due to these predacious "good guys," adverse climate, or other factors. However, at times, one or several caterpillar pests may be numerous enough that damage becomes intolerable. Then control measures are needed. Physical removal of caterpillars may be helpful, but it is tedious work. Since many caterpillar pests are hard to locate, this method may not be satisfactory. Cages of screen door hardware cloth can be placed over developing plants and left in place through maturity to achieve maximum protection. They are effective in excluding these pests and preventing maggot damage as well (See EB0859, The Cabbage Maggot in the Home Garden).

Chemical controls available for use on this plant group include rotenone, diazinon, Sevin, Thiodan, and Bacillus thuringiensis (a bacterial insecticide).

Be sure the plant species you intend to spray is on the product label before you purchase or use it. Also, do not apply diazinon or Sevin to blooming plants or allow the spray from these materials to drift onto blooming plants. They are deadly to honeybees and to other pollinating bees. Thiodan can be used with reasonable safety at night or early in the morning when bees are not foraging. Rotenone is reasonably safe to use around bees, and the bacterial insecticide is 100% safe to bees.

While most of the chemicals mentioned here will reasonably control cabbageworm and diamondback moth, they are often only marginally effective against loopers, armyworms, and cutworms—particularly the more mature worms. For this reason, the gardener may want to strongly consider using Bacillus thuringiensis, which is a highly selective caterpillar killer and nontoxic to beneficial insects. It has demonstrated highly satisfactory results against most, if not all, of these caterpillars. The effectiveness of Bacillus thuringienses is enhanced by adding a spreader-sticker when applying this control to plants with waxy leaves. In fact, you should add a wetting agent to all control agents listed in this publication if they are not already present in the registered formulation(s) that you have purchased.