Worms on a Cauliflower

The imported cabbageworm (Pieris rapae) is one the most serious pests to plague cauliflower plants. Mature caterpillars reach about 1 inch long and have fuzzy green bodies with yellow to orange stripes down the back and sides. These pests are the larval form of cabbage butterflies, which fly around cauliflower plants during daylight hours. The butterflies also reach about an inch long and have white wings with black spots. Cross-striped cabbageworm (Evergestis rimosalis) larvae emerge gray, but develop a blue tint and black and yellow stripes as they mature. These pests prefer to feed on the young terminal buds and cauliflower heads. Adult moths have about 1-inch wingspans, and their yellow-brown to brown wings bear dark brown zigzagged lines.

Cabbage loopers (Trichoplusia ni) are wormlike caterpillars that have pale green bodies with thin white lines running along the sides. Mature caterpillars reach about 1 1/2 inches in length. Because these pests don't have legs on their midsections, they crawl along the foliage with a distinctive looping motion, much like inchworms. The night-flying, adult moths have gray-brown wings with silvery markings and wingspans of about 1 1/2 inches.

Adult diamondback moths (Plutella xylostella) are slim, light brown pests that only fly at night. When they rest with their wings folded, you can see the diamond pattern that gave them their name. The diamondback moth caterpillars only reach about 1/3 inch in length and have light green bodies tapered on each end. The young larvae feed on leaves, while mature caterpillars feed on the flower heads.

According to the University of Minnesota Extension, these wormlike pests all cause similar feeding damage on cauliflower plants. Young larvae typically chew small holes in the undersides of leaves that don't go all the way through to the top surface, while the mature caterpillars chew large holes in the foliage. Imported cabbageworms and cabbage loopers move their feeding activity toward the center of plants as they mature, often boring their way into the cauliflower heads. Cross-striped cabbageworms riddle the buds and flower heads with holes, which can cause deformed growth. Cauliflowers can usually handle light pest populations, but severe infestations can kill entire plants.

Start inspecting your cauliflowers once a week for worms shortly after planting. Be sure to look beneath the leaves where the larvae like to feed. Handpick any caterpillars you find and drop them into a bucket of soapy water. Draping floating row covers over your cauliflowers until harvest time can help keep the adult pests from laying eggs on your plants. All of the common caterpillar pests can be treated with pesticides containing Bacillus thuringiensis (Bt), a bacteria that targets the caterpillars without harming beneficial insects. Following the directions on the product's label, mix 1 to 4 teaspoons of Bt-based product into each gallon of water. Spray your plant when you first notice the pests; repeat applications once a week until caterpillars are gone. The pests must actually ingest the bacterial spores for the treatment to work, so thoroughly cover all parts of your cauliflower plant. Spray your plants in the early evening when the pests are actively feeding and bees are less likely to be flying around.