Imported Cabbageworm

Larvae (caterpillars) are green with numerous fine hairs that give their skin a velvety appearance. Older larvae may be up to 1 inch long, and often have one faint yellow-orange stripe along their back and broken stripes along the sides. Compared to other caterpillars, cabbageworms move slowly and are sluggish. They feed voraciously on 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.

After feeding for about 2 to 3 weeks, larvae pupate attached to their host by a few silk strands. Pupae are green with faint yellow, lengthwise lines; there is no dense silk covering or cocoon.

The adult cabbage butterfly is white to pale yellowish with a wingspan of 1.5 inches and has one to four black spots on each forewing. Adults are often seen fluttering around fields and blossoming, low-growing plants. Adult females lay whitish, elongated eggs singly on the undersides of leaves. Eggs are pointed at the top and have pointed ridges down the sides.

The cabbageworm has several generations per year. It can be active throughout the year where hosts are present.

Cabbageworm larvae chew seedlings and large, irregular holes in leaves, bore into heads, and may contaminate marketed produce with greenish-brown fecal pellets. Most economic damage is to the marketed plant parts.

Closely watch seedlings, which quickly become damaged if the pest becomes abundant.

Between thinning or transplanting and heading, cole crops tolerate considerable damage from caterpillars eating leaves. During this time, sample frequently enough to assess numbers accurately and avoid unnecessary insecticide application that can disrupt biological control.

Once plants begin to head, even low numbers of imported cabbageworm can cause serious economic damage and may require insecticide application.

To prevent imported cabbageworms from migrating into the crop, control cruciferous weeds (such as mustards) that can host them.

Important natural enemies include the pupal parasite Pteromalus puparum, the larval-parasitic wasps Apanteles glomeratus and Microplitis plutellae, several tachinid flies, and egg-parasitic Trichogramma species. Natural viral infections may also provide effective control.

Consider installing insectary plants to attract natural enemies. Avoid the use of broad-spectrum insecticides that disrupt the biological control of imported cabbageworm by natural enemies.

Use biological control, cultural control, and sprays of Bacillus thuringiensis and the Entrust SC formulation of spinosad in an organically certified crop.

Combine monitoring of imported cabbageworm with monitoring for cabbage looper. Compared with cabbage looper, imported cabbageworm may be harder to find because its maximum size is smaller than other caterpillars and its green color is inconspicuous on plants.

In a weekly basis beginning at seedling emergence or right after transplanting, check 25 plants selected randomly throughout the field.

• Note the time when many adults (butterflies) are fluttering around the field. A few days later, check plants for eggs and small larvae on the underside of leaves.
• Look for larger caterpillars toward the center of the plant where they often feed near the midrib of leaves.
• Look for greenish-brown fecal pellets.

Base treatment on the combined number of healthy imported cabbageworm and cabbage looper larvae found.

• In the early stages of crop development, apply an insecticide if medium-sized to large caterpillars are numerous enough to stunt growth (approximately 1 caterpillar for every 5 plants).
• Prior to heading on well-established crops (except for Brussels sprouts), do not apply insecticide unless more than 9 caterpillars are found per plant.
  • For Brussels sprouts, apply insecticide just before heading or at Brussels sprouts formation if 1 or more caterpillars is found in 25 plants.
• From heading to harvest, apply insecticide if more than 10% of the sampled plants (2–3 plants out of 25) are infested with at least 1 caterpillar.

Where possible, use Bacillus thuringiensis to avoid harming natural enemies. Bacillus thuringiensis is very effective against imported cabbageworm and cabbage looper, especially when applied to early-instar (young) larvae. If significant numbers of beet armyworm, diamondback moth, or cutworms are also present, use a broad-spectrum insecticide recommended for these species to also control imported cabbageworm.