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Extension > Local Extension Offices > Stearns > County Horticulture Educator > Articles > Imported Cabbageworm Causing Damage

Imported Cabbageworm Causing Damage

University of Minnesota Extension, Stearns County News
August 3, 2016           

Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Imported Cabbageworm Causing Damage
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (8/3/2016) — Many gardeners have probably noticed holes eaten into their cole crops. Cole crops include cabbage, cauliflower, broccoli, kale, radish, turnip, rutabaga, and collards.  There are many insects that can cause damage to these crops, but the likely culprit at this time of the season is the imported cabbageworm, Pieris rapae.  Knowing more information about the pest’s life cycle is the first step in minimizing the damage.

The caterpillars are the life stage that causes the damage to the plants.  Of course eggs hatch into the caterpillars, and after a week of feeding on the cole crop, the caterpillar seeks protected areas on the plant, pupate, and then emerge as adults.  An adult imported cabbageworm is white to yellowish in color with one or two black dots on the front wings, depending on if it is a male or female. Their wing span is approximately 1.75-inches to 2.5-inches. Adults live for approximately three weeks and are very active during the day. Adults typically feed on flowering weeds, will mate, lay eggs on the cole crop, and then die; starting the life cycle over again. In a typical summer, one to three generations a year will occur in the upper Midwest. However, this insect does not overwinter in Minnesota and will often arrive in early July through late August.

Damage is easy to identify; the cabbageworm chews large, ragged holes in the leaves.  Heavy feeding can result in a skeleton of a leaf, only leaving the veins and midrib.  Most cole crops can tolerate some feeding. However, heavy feeding can cause distorted growth, prevent the head of cauliflower, cabbage, or broccoli to form, or even cause death to the plant.  In general do not allow more than 30-percent defoliation.

To help control the damage, begin checking your cole crops for worms soon after planting. There are several similar pests such as the cabbage looper and diamondback moth that do similar damage earlier in the season. Inspect weekly for eggs or newly hatched larvae. Simply squish or drop into a soapy bucket of water. Other helpful tips to help control damage from these pests are to remove and destroy crop residue after harvest. Floating row covers can also be installed to prevent the adults from laying eggs on the plant. Finally, when caterpillars are still small, insecticides containing Bacillus thuringiensis (Bt), spinosad, pyrethrin, bifenthrin, carbaryl, or permethrin are also effective. Use caution with all insecticides and be sure to read and follow all safety and application directions. Be sure the label indicates the pest and for use on the intended crop. Also recognize these insecticides may also kill natural enemies of the cabbageworm, including parasitic flies and wasps.

For more information about all the cole crop pests, visit www.extension.umn.edu and search for “cole crops”.

Contacts

Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
adam0062@umn.edu
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