Imported cabbageworm, Pieris rapae
Order: Lepidoptera
Family: Pieridae
Description
Butterflies of imported cabbageworm, commonly called the “cabbage white”, are active during the day and have a wingspan of about 5 cm (2 inch). They have white and yellow wings that have a black border at the tip. Male butterflies have one black spot in the middle of each front wing, while females have two black spots. Their bodies are also covered with dense hair (setae). Larvae are green with a velvet textured appearance and have five pairs of prolegs. Newly emerged caterpillars are 3.2 mm (0.13 inches) long, while mature caterpillars reach a length of about 31 mm (1.2 inches). All instars except for the first have a thin yellow line running down the center of the back and may also have a series of yellow spots running along each side. Pupae are about 20 mm long with a chrysalis that varies in color (see images). Typically, they are shades of yellow, gray, green, and brown. The eggs are roughly 0.5 mm wide and 1 mm long. Initially the eggs are pale white but become yellow over time. The flat end of the egg is attached to the leaf and the tapered end of the egg extends upward from the leaf surface.
Quick Facts
- The imported cabbageworm is an invasive species in North America. Caterpillars can attack a wide range of cabbage and mustard plants including cauliflower, Brussels sprouts, broccoli, cabbage, kale, collard greens, and kohlrabi.
- Larvae are the destructive life stage since they feed on foliage, occasionally causing severe defoliation, and can bore into the heads of cabbage and broccoli, making them unmarketable.
- Management involves a combination of cultural, biological, and chemical controls. Monitoring is highly recommended to inform decisions related to chemical controls.
Larva of imported cabbageworm.
The imported cabbageworm is the larva of a butterfly species that attacks a wide range of cabbages and mustards. Caterpillars are green with a yellow line running along the center of their back and can defoliate entire plants when densities are high. Adults are diurnal and can often be seen flying among crops that contain suitable host plants. Image credit: David Cappaert, Bugwood.org
Adult of imported cabbageworm visiting a flower. Image credit: Mary C Legg, Mary C Legg, Bugwood.org
A single egg of imported cabbageworm (note the tapered end). Image credit: David Cappaert, Bugwood.org
Pupae of imported cabbageworm. Note the considerable variation in color. Image credit: Ansel Oommen, Bugwood.org
Cabbage with significant feeding injury due to imported cabbageworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Imported cabbageworm overwinters as a pupa on leaves of the host plant or in nearby debris. After emerging from the chrysalis, adults typically live for about three weeks and a single female butterfly can lay up to 400 eggs throughout her life. Oviposition occurs on the undersides of outer leaves and egg hatch occurs in as little as four days. The larvae develop through five instars and increase in size with each stage. Young larvae tend to migrate to the inner head to feed on younger leaves, which protects the caterpillars from predators and parasitoids. Larvae are most active on sunny days and will also feed on flowers and buds as they mature. It is estimated that the last instar is responsible for 85% of feeding. In addition to cruciferous plants, this species has been observed aggregating on flowers of blue vervain (Verbana hastata).
In addition to attacking cultivated plants in the mustard family, many cruciferous weeds serve as host plants for this pest. The number of generations produced each year is dependent on climate, with three to five generations produced in the warmer climates of California and as few as two to three generations in the cooler climates of Canada. In neighboring states like Utah there are three to four generations each year.
Injury
In closed-leaf Brassica plants, early instars initiate feeding between the large veins and midribs of leaves, while mature larvae feed on the central head. On open-leaf Brassica plants, feeding begins on the lower leaf surface in the vicinity of the egg. In the absence of management, caterpillars can completely defoliate plants, leaving only stems and large veins. The presence of irregular feeding holes that enlarge is another sign of feeding injury. In addition to attacking leaves, the larvae can bore into the heads of certain vegetables like broccoli and cabbage. The head may fail to form entirely when young plants are under significant feeding stress. In addition to feeding injury, the caterpillars defecate copiously which can stain and contaminate produce.
Monitoring
During the day, adults are often observed flying among crops containing Brassica spp. Plants should be inspected regularly for irregular feeding holes and skeletonized leaves. To do this, 10 leaves can be pulled from 10 different plants in different locations of the field, giving a total of 100 leaves. The undersides of leaves can be inspected for small larvae and eggs, as well as large caterpillars toward the midribs of leaves or toward the center of plants. Larvae are well-camouflaged and may be difficult to spot initially. The presence of dark green fecal pellets beneath feeding areas is another indicator of imported cabbageworm. To detect larvae in vegetable heads, soaking the heads in salty water will cause larvae to float to the top. To justify chemical control, a threshold of 2 or more medium-sized larvae per 10 leaves in the button stage is recommended.
Cultural control
Removing weeds and any remaining plant debris after harvest will help reduce the availability of suitable overwintering sites for pupae. Larvae can also be removed by hand picking. Placing row covers or paper caps over young plants will physically prevent adults from laying eggs. Varieties that mature quickly tend to sustain less feeding injury. Cabbage can be planted early and harvested before cabbageworm populations increase to damaging levels. After harvesting, plant residues can be destroyed or plowed. Rotating crops is recommended, in addition to distancing susceptible crops from current and previous plantings of susceptible crops. The use of row covers can help exclude butterflies from laying eggs on crops, but the covers should be removed during flowering to facilitate pollination. Some crops are less attractive to imported cabbageworm, including Chinese cabbage, turnips, mustard, rutabaga, and kale.
Biological control
There are many natural enemies of imported cabbageworm, including predators, parasitoids, and microbial pathogens. In neighboring states such as Utah, one management approach involves purchasing Trichogramma wasps and releasing them during peak moths. Two species of parasitoid wasp have been released in the United States for managing this pest: Cotesia glomerata and Cotesia rubecula (Hymenoptera: Braconidae). These wasps attack larvae of the imported cabbageworm and impact their feeding. Larvae parasitized by C. glomerata do not die until the fifth instar, after which a majority of herbivory has occurred, while larvae parasitized by C. rubecula die during the fourth instar before most feeding. Surveys have shown that C. rubecula inhabits states as far west as North Dakota, while the distribution of C. glomerata is confined to states east of Illinois.
It is important to avoid the use of broad-spectrum insecticides since they can harm naturally occurring arthropod predators and parasitoids. Two pathogens, the granulosis virus and polyhedrosis virus, will infect caterpillars and can be made into a microbial insecticide by collecting infected larvae, soaking them in water, and creating a slurry. The slurry can then be sprayed on infested plants to infect the healthy caterpillars. However, the degree of control achieved with this method is unclear, and one study suggests that insecticides containing the Bacillus thuringiensis (Bt) toxin are more effective in reducing caterpillar densities.
Chemical control
When the threshold of 2 or more medium-sized larvae per leaf is reached, two sprays are recommended at the button stage and 7-10 days before harvest. Formulations containing the Bt toxin are marketed as foliar insecticides and will provide some protection against young caterpillars. Other botanical insecticides can be effective as well, with dust formulations showing a higher degree of control than aqueous sprays.
References
Capinera, J. (n.d.). University of Florida. University of Florida – Featured Creatures. Available here.
Tsuji, J., T. Logan, and A. Russo. 2018. A Hierarchy of Cues Directs the Foraging of Pieris rapae (Lepidoptera: Pieridae) Larvae. Environmental Entomology. 47(6): 1485-1492. Available here.
University of Maryland. (n.d.). Imported Cabbageworm in Vegetables. University of Maryland – Extension. Available here.
Utah State University. (n.d.). Imported Cabbageworm. Utah State University – Extension. Available here.
Xinzhi, N., D. Riley, and A. Sparks. 2015. Aggregation and Foraging Behavior of Imported Cabbageworm (Lepidoptera: Pieridae) Adults on Blue Vervain Flowers. Journal of Entomological Science. 50(3): 252-253. Available here.