Gardeners who want pest-free broccoli (Brassica oleracea) should seriously consider spraying this cool-season annual to prevent caterpillar infestations. Imported cabbageworms, diamondback moth worms and cabbage loopers are three cabbageworm species that commonly attack broccoli heads and leaves. Without spray intervention or preventative treatment, these wormlike larvae of various moth species can quickly invade plants, causing crop damage and contaminating your broccoli harvest.
About Broccoli Worms
Imported cabbageworms (Pieris rapae) have 1-inch long, velvety green bodies with light yellowish-orange stripes along the back and sides. These worms move sluggishly whenever disturbed. Diamondback moth larvae (Plutella xylostella) are greenish-yellow caterpillars with black hairs and slender, 1/3-inch bodies tapered on both ends. They wiggle when disturbed and frequently dangle from the plant by a silklike thread. Cabbage loopers (Trichoplusia ni) reach about 1 1/2 inches in length and have green bodies with white stripes running down the back. They move by arching their backs and inching along the foliage. The adult females of all moth species lay their yellowish eggs on broccoli leaves.
Signs of Broccoli Worms
Broccoli worms damage plants by eating holes in the heads and leaves. The worms are hard to spot because their small sizes and green bodies allow them to blend in with the foliage. Signs of cabbageworm infestations include numerous irregular, large holes chewed into the foliage. Young diamondback moth larvae feed on the foliage surface, leaving behind only thin layer of leaf tissue. Mature cabbage loopers also tunnel into broccoli heads to feed. Cabbageworm feeding activity can cause stunted plant growth, and the green-brown excrement they leave behind on the broccoli heads often make the crop inedible.
Treating Broccoli Worms
Cabbageworms can ruin your entire broccoli crop if you don't treat your plants as soon as you spot the pests. All cabbageworm species can be treated with insecticides containing Bacillus thuringiensis, a naturally occurring soil bacterium that targets caterpillars while not harming beneficial insects, wildlife or humans. Once a worm consumes foliage treated with Bt, the bacterium poisons the pest's digestive tract and it stops eating. Worms typically die within two or three days. Bt products come in various forms, including easy-to-use dust formulations.
Using Bt Fungicides
Following the specific instructions on the manufacturer's label, simply sprinkle the dust over your broccoli plant until it thoroughly coats all foliage surfaces. One Bt product label suggests treating cabbageworms by using 1/4 to 1/2 ounce of dust for every 50 square feet of growing space. Wait at least seven days before applying another treatment. Treat broccoli plants with Bt only on calm days when rainfall isn't expected in your area for at least 24 hours. Although Bt is an organic pesticide, the dust can still cause eye and skin irritation. Reduce your risk of exposure by wearing a face mask, goggles, long sleeves, pants and socks with shoes when dusting your broccoli plants.
Other Control Methods
Inspect your broccoli plants once a week for worms, handpicking any you see and dropping them into a container of soapy water. Spraying plants with a strong stream of water from a garden hose can wash the caterpillars off the foliage. Destroy remaining crop debris after harvest and pull up weeds about three weeks before planting broccoli to remove potential hiding and overwintering places. Draping lightweight floating row covers over seedlings can help eliminate worm populations by preventing the female moths from laying eggs on your plants.
- Missouri Botanical Garden: Brassica oleracea (Italica Group)
- University of Illinois Extension: Broccoli
- UC IPM Online: Imported Cabbageworm
- UC IPM Online: Loopers
- UC IPM Online: Diamondback Moth
- University of Florida IFAS Extension: Insect Management for Crucifers (Cole Crops)
- Clemson Cooperative Extension: Cabbage, Broccoli & Other Cole Crop Insect Pests
- Colorado State University Extension: Bacillus Thuringiensis
- Colorado State University Cooperative Extension: Organic Pest Controls That Work in the Vegetable Garden
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