Species

Imported Cabbage Worm (Pieris rapae)

The imported cabbageworm (ICW), also known as the cabbage butterfly, small cabbage white, cabbage white (Pieris rapae) was originally from Europe but imported to Canada in the mid 1800's and has since spread throughout North America.

Related species

The Large White Moth is a larger version of the ICW, which also targets cabbage crops, as well as brussels sprouts. The large white butterfly is more common throughout Europe and Asia and less common in the U.S.

Host/site

• The imported cabbageworm inhabits a large part of the world and in the United States can be found from coast to coast, targeting cultivated cabbages, turnips, radish, nasturtiums and other mustard family crops. They damage both the outer and inner leaves, often feeding along the midrib, at the base of the wrapper leaves, or boring into the heads of cabbage. They also can be found on cruciferous weeds and other garden plants in this family.
• Cabbageworm larvae chew large, irregular holes in leaves, bore into heads, and drop greenish brown fecal pellets that may contaminate the marketed product. Seedlings may be damaged.

Identification

Appearance

• Eggs are rocket shaped and whitish in color. The pale yellow, bullet-shaped egg, about 1 mm long, is ribbed lengthwise and crosswise and is attached endwise to the leaf surface.
• Larvae are green and very hairy, with an almost velvetlike appearance. Older larvae may be up to an inch long and often have one faint yellow-orange stripe down their backs, a row of faint yellow spots on each side, broken stripes along the sides and five pairs of prolegs. Compared to other caterpillars, cabbageworms move slowly and are sluggish
• Pupa are sharply angled, gray, green or brown in color and about 20mm long. It is attached to the lower leaf surface by a silken loop.
• Adults of both sexes have a black area near the tip of each forewing, which spans about 1 ½ inches and small black spot on the front edge of each all white hind wing. The female has two black spots on each forewing while the male has only one. There is a slight yellowish hue to the underside of the wings. The male wingspans slightly smaller than the female. The female has a wingspan of about 50 mm; the male is slightly smaller. These are very common; seen in flight in the garden fluttering form plant to plant.

Indicators

• Good clues to cabbageworm presence include their greenish brown fecal pellets or many cabbage butterflies fluttering around.
• Cabbageworm larvae chew large, irregular holes in leaves, bore into heads, and drop greenish brown fecal pellets that may contaminate vegetables.
• Imported cabbageworms are commonly found on the undersides of leaves.
• The outer leaves of their host plants often appear riddled with irregularly shaped holes.

Life cycle

• Adults emerge in spring, as early as March, and mate. Females then lay yellow, oblong, and deeply ridged length-wise eggs singly on the leaves of host plants. After 3 to 8 days, depending on temperature, the eggs hatch.
• Larvae feed on the upper leaf surface and develop through five instars in 10 to 14 days, growing up to 1 inch length. The 5th instar larvae does the most feeding damage.
• The larvae pupate in a pale green chrysalis attach to lower leaf surfaces by silk bands. Completion of the pupal stage ranges from 1 to 2 weeks with 3 to 5 overlapping generations throughout the growing season.
• Overwintering occurs in the pupal stage of the last generation. They attach themselves to the host plant debris.
• In some places, such as the state of California, ICW are active year round.

Natural Enemies

There are numerous predators, parasitoids and diseases of ICW.

• Predators include shield bugs (Hemiptera: Pentatomidae), ambush bugs (Hemiptera: Phymatidae), vespid wasps (Hymenoptera vespidae) and many insectivorous birds.
• Apanteles glomeratus, a parasitic insect attack younger larvae as they prepare to pupate and can be found diligently searching foliage for them. Dead cabbageworm larvae are often found in clusters of 20 to 30.
• In some areas tachinids (Diptera: Tachinidae) are more effective than wasps.

Monitoring

Monitoring for cabbageworms can be done at the same time as loopers, early Spring. Cabbageworms are slightly smaller in size. Look for small larvae and eggs on the undersides of leaves. Larger worms feed toward the center of the plant often near the midribs of leaves.

Action Thresholds

If populations of medium-sized to large caterpillars are high enough to stunt growth, treatment may be needed to stop damage. Well-established plants do not need to be treated unless you find more than 9 small to medium-sized larvae per plant.

Cultural and Physical Controls

• Paper caps early in the season, and row covers later, are effective in preventing imported cabbageworm butterflies from laying eggs.
• After harvest, crop plants should be composted. Weeds such as wild mustard and shepherd's purse should be destroyed in nearby areas since the first larval stage of the imported cabbageworm may develop on these alternative weed hosts.
• High temperatures can inhibit larval development and heavy rainfall can cause high mortality in early instars. Therefore, larval populations are small during hot and wet seasons, and large in dry and cool seasons. Do not water to compensate for lack of rainfall as this can cause fungal issues.
• CLEAN UP of all debris in the area so they are not able to over winter in the soil.

Resistant Varieties

Crucifer crops differ is their susceptibility to attack by imported cabbageworm. Chinese cabbage, turnip, mustard, rutabaga, and kale are less preferred than cabbage, collards, Brussels sprouts, broccoli, and cauliflower.

Biological Control

Bacillus thuringiensis is very effective on young larvae. However, Bt does have adverse impacts on all Lepidoptera family insects which includes all butterflies. Also, some people with respiratory issues have problems with Bt.

Chemical Controls

See biological controls.

References

Capinera, John L. Featured Creatures. Publication no. EENY-126. University of Florida, Mar. 2000. Web. 5 May 2010. <http://entnemdept.ufl.edu/creatures/veg/leaf/imported_cabbageworm.htm>.

Hutchison, W. D., P. C. Bolin, and R. L. Hines. Imported Cabbageworm. Factsheet. University of Minnesota, Apr. 2009. Web. 6 May 2010. <http://www.vegedge.umn.edu/vegpest/colecrop/cabbworm.htm>.

Mau, Ronald F.L., and Jayma L. Martin Kessing. Pieris Rapae (Linnaeus). Fact Sheet. University of Hawaii, Mar. 1991. Web. 5 May 2010. <http://www.extento.hawaii.edu/Kbase/Crop/Type/pieris.htm>.

Natwick, E. T., W. J. Bentley, W. B. Chaney, and N. C. Toscano, comps. Cole Crops: Imported Cabbageworm. Fact Sheet. UC IPM Pest Management Guidelines: Cole Crops, 23 Sept. 2009. Web. 4 May 2010. <http://www.ipm.ucdavis.edu/PMG/r108301111.html>.