Imported cabbageworm, Pieris rapae
Adults of imported cabbageworm are active during the day and have a wingspan of about 5 cm (2 inches). They have white and yellow wings that have a black border at the tip as well as one or two black dots in the center of each wing. Their bodies are also covered with dense hair (setae). Eggs are usually laid singly on the lower surface of leaves and 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. 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 imported cabbageworm is an immature of a butterfly species that 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.
- Larvae feed on leaves and can also bore into the heads of cabbage and broccoli, making them unmarketable.
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 soil and plant debris. In neighboring states like Utah there are three to four generations each year. However, this is highly variable and 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. 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. The larvae develop through five instars and increase in size with each stage.
If left unmanaged, the caterpillars can completely defoliate plants, leaving only stems and large veins. The presence of irregular feeding holes 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.
Look for adults during the day, often they can be seen flying among crops containing Brassica spp. Plants should be inspected regularly for irregular feeding holes and skeletonized leaves. Larvae are well-camouflaged and difficult to spot initially; however, upon close examination they can be found resting on the leaf midribs. 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.
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. Some crops are less attractive to imported cabbageworm, including Chinese cabbage, turnips, mustard, rutabaga, and kale.
There are many natural enemies of imported cabbageworm, including predators, parasitoids, and microbial pathogens. 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.
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.
Utah State University. (n.d.). Imported Cabbageworm. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/veg-imported-cabbage-worm
Capinera, J. (n.d.). University of Florida. University of Florida – Featured Creatures. Available https://entnemdept.ufl.edu/creatures/veg/leaf/imported_cabbageworm.htm
University of Maryland. (n.d.). Imported Cabbageworm in Vegetables. University of Maryland – Extension. Available https://extension.umd.edu/resource/imported-cabbageworm-vegetables