Vegetable Program
Insect Management

Imported Cabbageworm

This familiar white butterfly can be seen in daytime fluttering around cole crop fields. Each forewing has a dark border and one or two round black spots. Eggs are laid singly on the underside or top of leaves, about 1/8 inch in length, light green and slightly elongated, standing upright. The caterpillar, called imported cabbageworm, is gray-green, slightly fuzzy, and sluggish. Feeding and resting occur on the underside of leaves, and larvae feed more heavily in the head of cabbage or broccoli as they grow. The crysalis (pupa) will be green or brown, smooth with three pointed ridges on its back, and attached to a leaf. There are 3-4 generations per year in central New England.

Quick Identification Cues for the three key caterpillars of Brassica crops in the Northeast

Diamondback moth catperillar: very wiggly when poked, pointed on both ends, not fuzzy, only grows to about ½ inch. You may find white silken cocoons, with a green full-grown catperillar or a brown pupa inside.

Imported cabbageworm: gray-green, slightly fuzzy, and sluggish. Grows to > 1 inch and favors the center of the head as it gets larger. Produces wet green frass (droppings).

Cabbage looper:  light green, smooth, ‘loops’ up like an inchworm as it moves, grows 1 ½ to 2 inches.  Eats big holes in leaves.

Field Scouting for caterpillars

It is especially important to check cabbage or broccoli plantings as they begin forming heads. Greens such as collards, kale, and Chinese cabbage should be scouted earlier, since all leaves are marketed. Feeding damage can be found on the underside of leaves or in the center of the plant where heads are forming. Look for tiny feeding holes, clustered together. Often it is easier to spot the feeding damage first, and then find the caterpillar. Check 25 plants throughout the field and note how many have one or more caterpillar (ie, are “infested”), then calculate the percent of plants infested. Spray if the following threshold is reached.

Action thresholds for caterpillars in Brassicas:

Crop & Stage                                                           % Infested Plants

Cabbage & Broccoli, Cauliflower                            35%

      pre-cupping (before head formation begins)        

Cabbage & broccoli                                                   15%

      head formation to maturity        

Cauliflower 

      After heading                                                         10%

Kale, collards & other greens                                  10-15% 

These thresholds are based on research that showed that spraying whenever this level of infestation is reached results in 98-100% clean heads, the equivalent of weekly sprays but with far fewer insecticide applications.

Insecticides

See 2008-2009 New England Vegetable Management Guide for insecticides. There are many effective options. Use selective products to protect beneficials that keep aphids under control, and also consume insect eggs and small caterpillars.

Classical Biological Control

In 1988, a braconid wasp (Cotesia rubecula) was imported from China by Roy Van Driesche of the UMass Plant Soil and Insect Science Department, and released at 17 locations in Massachusetts. It has become well established; a survey conducted in 2007 found parasitized caterpillars at 100% of the twenty sites sampled. The once common parasitioid, C. glomerata, is virtually gone. For more details, see article in July 2007 Vegetable Notes.

Cotesia rubecula parasite of Imported Cabbageworm.

Updated June 2009