How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

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 and broken stripes along the sides. Compared to other caterpillars, cabbageworms move slowly and are sluggish but 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 2 to 3 weeks of feeding, larvae pupate attached by a few strands of silk to stems or other nearby objects; pupae are green with faint yellow lines down the back and sides; there is no spun cocoon. The adult cabbage butterfly is white with one to four black spots on the wings; they are often seen fluttering around the fields. The whitish, rocket-shaped eggs are laid singly on the undersides of leaves. The cabbageworm is active throughout the year in California.

DAMAGE

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, but most losses are due to damage to marketed parts of the plant.

MANAGEMENT

Between thinning or transplanting and heading, cole crops can tolerate considerable damage from the imported cabbageworm and other caterpillars that eat leaves. During this period the strategy is to sample frequently enough to assess population development accurately and to avoid unnecessary insecticide treatments that may disrupt biological control. Once plants begin to head, imported cabbageworms can cause serious economic damage, even when they are present in low numbers, so insecticide applications are required at much lower population densities. If cabbageworm populations build up on seedlings, damage can occur quickly so these must be watched closely.

Biological Control
Natural enemies can assist significantly in the control of imported cabbageworms. Important parasites include the pupal parasite Pteromalus puparum; the larval parasites Apanteles glomeratus, Microplitis plutella, and several tachinid flies; and egg parasites in the Trichogramma genus. Viruses and bacterial diseases are also sometimes important control factors in the field.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable management tools.

Monitoring and Treatment Decisions
Check for cabbageworms at the same time as you are monitoring for loopers; your sample should include 25 plants selected randomly throughout the field. Treatment levels combine the two species; however, cabbageworms may be harder to find because of their smaller size and their inconspicuous coloring. 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. Good clues to cabbageworm presence include their greenish brown fecal pellets or many cabbage butterflies fluttering around the field (check for eggs in a few days). Base treatment on numbers of healthy larvae present (include loopers in counts, too, if they are also present). Treat seedlings or small plants if populations of medium-sized to large caterpillars are high enough to stunt growth. Prior to heading, well-established plants do not need to be treated unless you find more than 9 small to medium-sized larvae per plant. Treat just before heading or at Brussels sprouts formation if counts show more than one caterpillar in 25 plants.

Where possible, use Bacillus thuringiensis to avoid adverse impact on natural enemies. Bacillus thuringiensis is very effective against imported cabbageworms, especially when applied to early-instar caterpillars (i.e., very young). Imported cabbageworms are also controlled with the more toxic materials recommended for use against other lepidopterous pests. If significant numbers of armyworms or diamondback moths are also present, use materials recommended for these species.

PUBLICATION

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
Insects
W. E. Chaney, UC Cooperative Extension, Monterey Co.
E. T. Natwick, UC Cooperative Extension, Imperial Co.
Acknowledgments for contributions to the insects section:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
N. C. Toscano, Entomology, UC Riverside

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