Caterpillar pests of cole crops in home gardens
Cole crops (including, but not limited to, cabbage, cauliflower, broccoli, kale, rutabaga, radish, turnip, and collard) can be hosts to many different insect pests. The most common caterpillar pests are imported cabbageworm (Pieris rapae), cabbage looper (Trichoplusia ni), and diamondback moth (Plutella xylostella). Of the three, imported cabbageworms are encountered most frequently and usually cause the most damage in home gardens. All three caterpillar species cause similar damage to cabbage and related plants.
Imported Cabbageworm adults are white butterflies with black spots on the forewings that are commonly seen flying around plants during the day. Eggs are yellow and oblong, and are laid singly on both upper and lower sides of leaves. The caterpillars are velvety green with faint yellow stripes running lengthwise down the back and sides. Full grown caterpillars are about 1 inch in length. They move sluggishly when prodded.
Cabbage looper adults are nocturnal moths with a 1½ inch wing span. They have mottled grayish brown wings with a small silvery white figure 8 in the middle of each of the front wings. Eggs are easily seen on the leaves. They are creamy white, aspirin-shaped and about the size of a pin head, and are most often laid on the undersides of the lower leaves. The caterpillars are pale green with narrow white lines running down each side. Since cabbage looper caterpillars have no legs in their middle sections, they have a characteristic looping motion as they move across vegetation. Full grown caterpillars are about 1½ inches in length.
Diamondback moths are also nocturnal flyers. The moths are light brown and slender. When the moths are at rest, their folded wings show a pattern of three white diamonds. Eggs are laid near leaf veins on leaf surfaces, and are creamy white and tiny. Diamondback caterpillars are much smaller than both imported cabbageworms and cabbage loopers. Mature caterpillars are about 1/3 inch long, light green, tapered at both ends, and wiggle vigorously when touched.
All three species have similar life cycles. The eggs hatch into caterpillars, which is the stage that damages plants. After feeding for a period of weeks on cole crops, the larvae pupate in protected areas on the plants, and then emerge as adults.
Imported cabbageworms overwinter in the upper Midwest in green pupal cases. Adults start appearing in gardens in mid-May and are a problem through the remainder of the growing season. There are 3 to 5 overlapping generations a year.
In contrast to imported cabbageworms, cabbage loopers do not overwinter in the upper Midwest. Although the time they arrive from the south varies from year to year, cabbage loopers generally migrate into Minnesota from early July to late August. In the upper Midwest, there are 1 to 3 generations a year during the growing season depending on their arrival time and late summer temperatures.
Diamondback moths overwinter in protected locations as adults. They may also be transported into the upper Midwest on transplants shipped from the South. Diamondback moths start appearing in northern gardens in mid-May. Although they can be pests through the remainder of the growing season, they are usually less severe after spring. There are generally 3 to 5 generations a year.
The caterpillars of all three species feed between the large veins and midribs of cole crops. Young imported cabbageworms and cabbage loopers produce small holes in leaves that generally do not break through to the upper leaf surface. The larger caterpillars, however, chew large, ragged holes in the leaves leaving the large veins intact. When feeding on cabbage, broccoli, or cauliflower, they often crawl toward the center to feed as they mature. They leave large amounts of frass (fecal matter) where they have been feeding. Initially, diamondback caterpillars feed inside the leaves but after a few days move to the outside of the leaves to feed. They often eat all the leaf tissue except the upper layer, resulting in a characteristic windowpane look (fig. 10).
Cole crops, like many vegetables in the home garden, can tolerate some feeding damage. Young seedlings and transplants are the most susceptible to injury. Severe defoliation of young plants can cause distorted growth or even death. Extensive feeding can also prevent the head formation of cabbage, cauliflower, and broccoli. Older plants, however, can tolerate some defoliation with little or no effect on yield. Generally, do not allow defoliation to exceed 30%. However, it may be desirable to limit feeding damage for aesthetic reasons, especially if the caterpillars start to damage the heads of cabbage, cauliflower, or broccoli or the leaves of kale or collards.
Start checking for caterpillars on cole crops soon after planting. Many transplants purchased at garden centers may be infested with diamondback moth eggs or newly hatched larvae. Inspect your plants at least once a week, and more often as the season progresses. Check both sides of leaves for caterpillars and their feeding damage. The older, larger caterpillars of all three species cause the most feeding damage.
Destroy crop residue immediately after harvest to eliminate potential overwintering sites for imported cabbageworms. Eliminate weeds from the Brassicaceae family such as wild mustard, peppergrass, and shepherd's purse, as they are alternate hosts for these pests.
Handpicking the caterpillars, especially in smaller gardens, can be an effective means of control. Drop the caterpillars into a pail of soapy water to kill them.
Floating row covers will help to prevent adult moths from laying eggs on plants. A floating row cover consists of lightweight all-purpose garden fabric draped either directly over garden plants or over metal hoops or a wooden frame for support. Fit the row covers over the cole crops at seeding or transplanting. The row covers may be removed upon harvesting the cole crop.
Whitney Cranshaw, Colorado State University, Bugwood.org
Figure 14. Imported cabbageworm larva parasitized by Cotesia
There are many natural enemies of cabbage looper, imported cabbageworm, and diamondback moth that are both native to our area and naturally occur in our gardens. These include predators, such as, paper wasps, and parasitic flies and wasps, e.g., the parasitic wasp, Cotesia glomerata.
Some wasps and flies parasitize the caterpillars while others attack the pupae. As the wasps or flies develop within the caterpillar or pupae, they eventually kill their hosts. Some wasps also parasitize the eggs of these caterpillar pests. These wasp and fly parasites are small and do not sting or bite people.
The best time to treat caterpillars is while they are still small and before they cause too much feeding damage. Insecticides are not as effective in killing older caterpillars. When possible choose a low impact insecticide that is less toxic, and "easy" on natural enemies (i.e., paper wasps, parasitic flies and wasps) and pollinators such as bees and flies.
There are several options when considering a low impact insecticide. Neem is a plant based insecticide and acts primarily as an anti-feedant. Although insects are not killed quickly, it causes them to stop feeding and they eventually die. Spinosad is derived from a naturally occurring soil-dwelling microorganism, provides excellent control, and does not harm natural enemies. Bacillius thuringiensis (Bt) is a bacterium that occurs naturally in the soil. Bt is specific to caterpillars and must be ingested in order to kill the insect which is different compared to residual products. Because of this, good spray coverage is essential.
Conventional, or broad-spectrum insecticides, are generally longer lasting but kills a variety of insects, including natural enemies. Common examples of broad spectrum insecticides include permethrin, carbaryl, bifenthrin, and lambda-cyhalothrin.
*Long residual can persist as long as four weeks. Medium -long residual can persist as long as 10-14 days.
CAUTION: Read all insecticide label directions very carefully before buying and again before using to ensure proper application. Be sure that the label specifies it can be used on the specific cole crop you wish to treat, or cole crops in general. The label is the final authority on how you may legally use any pesticide. Whenever using any pesticide, including low impact, natural or organic insecticides, follow all label directions and use standard pesticide safety practices for transporting, storing, mixing, applying and disposing of pesticides to protect yourself, your neighbors and the environment.
This publication was modified from VegEdge pest profiles entitled Imported Cabbageworm by W.D. Hutchison, P.C. Bolin, and R.L. Hines; Cabbage Looper by W.D. Hutchison, H. Hoch, P.C. Bolin, R.L. Hines, and Suzanne Wold-Burkness; and Diamondback Moth by W.D. Hutchison, P.C. Bolin, and R.L. Hines.