Tuesday, April 5, 2011

Pests and diseases: Cabbage Worms

Cabbage Worm Photo by Jack Kelly Clark

I’m sure we all have our favorite cool-weather spring/fall crops. For those of you who love your brassicas, such as cabbage, cauliflower, kale, and Brussels sprouts, the Imported Cabbageworm can prove to be your arch nemesis. Yes, that’s right; an arch nemesis that you’ll battle with all growing season. One that will leave large holes on those big, beautiful, waxy seedling leaves.

Who would think such a seemingly intoxicated worm, moving around in its slow, sluggish manner could be capable of such damage? You may even mistake some of these voracious feeders to be dead. But if you watch closely, it will crawl away dropping greenish brown fecal pellets to further ruin marketable product. You think….at least… in its adult stage it’s pretty.

Life Cycle
-laid singly on the underside of leaves
-are yellowish to white and cone shaped

-green, very hairy with one faint yellow-orange 
stripe down backs/broken stripes on sides
-up to an inch long
-feed along midrib, at base of wrapper leaves, bore into heads

-green with faint yellow lines down the back and sides
-do not have a spun cocoon
-larvae pupate attached by a few silk strands

-white with 1-4 black spots on wings (often seen fluttering around fields)
-yellowish underside of wings
In the northern U.S. and Canada winter is spent as a pupa in plant debris near or in previously infested crops - yet another reason to stress a thorough fall clean up. Adults emerge from the overwintering pupae in early spring to lay eggs. The larvae hatch in 3 to 5 days and are fully grown in 2 to 3 weeks. During this period they feed voraciously. They then pupate on plant debris settled on the soil near infested plants, and the adult butterflies emerge in 1 to 2 weeks to lay eggs singly on the undersides of leaves.

Plants Affected: Brassicas such as cabbage, cauliflower, kale, and Brussels sprouts

Symptoms: Feeding larvae produce large irregular holes in leaves and heads of cabbage. Older larvae tend to feed extensively on the newer growth, but can also ruin heads by tunneling. Large populations of larvae can also result in loss of marketable product from contamination by fecal matter left by heavy larvae infestations feeding.

Controls: Weekly scouting is a key method of prevention and control. Destroy any eggs, pupae, or larvae squishing by hand or with a rock on the ground. Beneficial insects are a great method of control and are readily available from many vendors, some as local as New Hampshire. Floating row covers applied directly after transplanting or direct seeding can be a very efficient means of control and prevention. There are many “home remedies” such as garlic or hot pepper spray that can be applied on a weekly basis by using a small hand sprayer or larger backpack sprayer. This should be done as soon as butterflies appear. For large plantings, involving many row feet, or entire fields, a tank spray with a pesticide containing BT or spinosad as the active ingredient can also be used. When applying any pesticide please be sure to read and follow and label instructions thoroughly. 

Resources: "The Organic Gardener's Handbook of Natural Pest and Disease Control" Edited by Fern Marshall Bradley, Barbara W. Ellis, and Deborah L. Martin
“The Ultimate Guide to Backyard Bugs Garden Insects of North America” by Whitney Crenshaw
-“Cole Crops Imported Cabbageworm” UC IPM Online
-ALL Photographs taken by Jack Kelly Clark
-Permission for use of photographs by UC IPM Online

Article by Sonya Reynolds, Greenhouse Coordinator, Johnny’s Selected Seeds

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