If you see these little white butterflies in your garden, take action to protect your brassicas before the cabbage worm moths lay eggs.
Illustration By Keith Ward
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Cabbage Worms (Pieris rapae)
The most common of cabbage pests are cabbage worms, which are caterpillars of various butterflies and moths such as the one shown above. They feed on the leaves and heads of cabbage, broccoli, and closely related crops. Cabbage worms weaken plants by removing plant tissue, and they can ruin beautiful heads of cabbage or broccoli by boring inside. Organic controls for cabbage worms include handpicking, excluding them with row cover barriers, or treating with a Bt pesticide. Cabbage worms are found throughout North America, and more than one species may be found in the same garden.
What Are Cabbage Worms?
These three types of cabbage worms are frequent cabbage pests:
Imported Cabbage Worms. The most common species in home gardens are properly called imported cabbage worms (Pieris rapae). The larvae of an ever-present white butterfly with black spots, imported cabbage worms are native to Europe. They were first seen in North America in 1860 and are now established from Canada to California. Velvety green with faint yellow stripes, imported cabbage worms feed on all members of the cabbage family, especially cabbage, broccoli and cauliflower.
Cabbage Loopers. If the cabbage pest in question is a green to yellow-green slender caterpillar that raises up on its back as it moves, like an inchworm, it is likely a cabbage looper (Trichoplusia ni). Cabbage loopers have no legs in their midsection. The larvae of a small mottled night-flying moth, cabbage loopers may also be found feeding on many other garden plants. Unlike imported cabbage worms, they are not exclusively cabbage pests.
Diamondback Moths. A third green cabbage worm sometimes caught rasping translucent windows in leaves, the larvae of diamondback moths (Plutella xylostella) are an occasional cabbage pest, often seen on turnips and mustard as well. Very small green worms less than a half inch long with two nubby legs extending from the tail end are the larvae of diamondback moths.
What Cabbage Worm Damage Looks Like
Velvety green imported cabbage worms eat plant leaves, and can often be detected by the trail of dark frass (excrement) they leave behind. If you see dark green to brown frass near tender new leaves, a cabbage worm is hiding nearby. As plants approach maturity, imported cabbage worms may bore into cabbage heads or infest the undersides of broccoli or cauliflower crowns where they are impossible to see.
Cabbage loopers often feed on leaf undersides when young, and only larger individuals are likely to be caught chewing holes in leaves. Like imported cabbage worms, cabbage loopers sometimes discover great food and shelter in growing heads of broccoli, cauliflower and cabbage. It is not unusual to find imported cabbage worms and cabbage loopers feeding on the same plants.
Because of their small size, diamondback moth larvae stay on leaf undersides, where they rasp away leaf tissues as they feed, creating translucent patches between leaf veins that eventually turn brown.
Cabbage Worm Reproduction
The imported cabbage worm life cycle takes three to six weeks, and goes faster in warm weather. Adult butterflies lay single tiny whitish eggs on leaf undersides, which become yellowish within a couple of days and hatch in about a week. A single adult female can lay 300 to 400 eggs in her 3-week lifetime. The tiny green larvae begin eating as soon as they hatch, and devour plant foliage continuously for about 15 days. The caterpillar then morphs into a pupae, which may be found attached to plants in the summer, or to another plant or nearby object. In summer, pupation lasts only seven days, but cabbage worms also overwinter as pupae, emerging in spring as energetic white butterflies.
Cabbage looper adults fly at dusk starting in late spring. A female may lay more than 500 eggs in her short, 12-day lifespan. Scattered on leaf undersides, yellowish eggs hatch in a few days, and the tiny green worms begin feeding. After about three weeks, the loopers wrap themselves in the cocoon-like pupal case. A new adult emerges four to seven days later.
Diamondback moth adults overwinter only where winters are mild, but south winds blow them northward in late spring. In most gardens they do not cause noticeable damage in spring, and are more likely to be cabbage pests in late summer or fall. Adult females lay about 150 eggs over a two week period, and about a month is needed for a new generation to pass.
Natural Enemies of Cabbage Worms
Numerous natural predators attack all three types of cabbage worms, and occasionally they may provide adequate control. General predators include paper wasps, yellow jacket wasps, shield bugs, and insect-eating birds, including chickens, ducks and guinea fowl. Dead imported cabbage worms with clusters of tiny white cocoons attached are the work of beneficial braconid wasps. In some areas parasitic tachinid flies are more important than wasps in controlling these cabbage pests.
Organic Cabbage Worm Control
In a small garden, you can get good cabbage worm control by checking plants regularly and handpicking cabbage worms as soon as you see them or the frass they leave behind. Chickens consider all types of cabbage worms to be great delicacies; you can even dry the collected caterpillars in the sun for feeding to chickens in winter.
If you often see numerous imported cabbage worms and cabbage loopers in spring, keep plants covered with row cover to prevent egg laying by adults. Two biological pesticides, Bt and spinosad, give excellent cabbage worm control when applied between rains. Bt is less likely to interfere with the work of other beneficial insects compared to spinosad. Organic pesticides are often needed in late summer to protect fall cabbage and broccoli from serious infestation and controls cabbage worms that are hiding among the growing florets. A single treatment with Bt two weeks before harvest can make a huge difference in the quality of cabbage and broccoli.
More Advice on Organic Cabbage Worm Control
Red-leafed varieties of cabbage and kohlrabi are less preferred by cabbage worms, probably because they provide poor camouflage.
You can increase the number of paper wasps that patrol your garden by hanging bottomless birdhouses nearby, or simply use small wooden boxes with no bottoms. They make excellent nesting sites.
Grow plenty of flowers and blooming herbs to provide a continuous supply of nectar for beneficial insects. Provide perches to increase how many cabbage pests are snapped up by birds.
More information on organic cabbage worm control is available from Ohio State, University of California, University of Maryland.