Proper identification and organic control of imported cabbage worms

Cabbage worm identification and organic control

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Imported cabbage worms (Pieris rapae, syn. Artogeia rapae) can wreak havoc in a vegetable garden. If left unchecked, they’ll consume the leaves, stems, and even the flower buds of all members of the cabbage family. Thankfully, cabbage worm control isn’t difficult, if you arm yourself with the information you need to tackle these common garden pests.

What is a cabbage worm?

Technically called the imported cabbage worm, this pest is a native of Europe. It’s now found across much of North America and produces several generations per year. Adult cabbage worm butterflies are a common summertime sight in yards and gardens, my own included. The white butterflies have a wing-span of about one to one-and-a-half inches. Females have two dark spots on each forewing. Males have only one spot.

Female cabbage butterfly laying eggs.

Female cabbage worm butterflies have two dark spots on their forewings. Males have only one.

Larval cabbage worms aren’t actually worms at all; they’re caterpillars. They are difficult to spot when they’re young because they often hangout on the undersides of leaves or along the leaf veins, which helps to camouflage them. As the caterpillars grow, they become a soft, velvety green and develop a faint yellow stripe down the center of their back. There are several other caterpillar species that feed on the same family of plants, but identifying cabbage worms is easy if you look for the yellow stripe.

Cabbage caterpillars feed on members of the cole family.

If you look closely, you can see a faint yellow stripe down the back of this cabbage caterpillar.

Preferred cabbage worm host plants

Female imported cabbage worm butterflies lay eggs singly on members of the mustard family (also called the cabbage family, the brassicas, or cole crops). Some of their favorites include their namesake cabbage, broccoli, kale, cauliflower, and Brussels sprouts.

Signs of cabbage worm damage are holes in the leaves or flower stalks (as is often the case with broccoli), skeletonized leaves, and the presence of their dark green, round, pelleted excrement, called frass. If you spot signs of this type of damage on your cole crops, here are a few organic control methods for cabbage caterpillars.

Signs of cabbage worm damage

Cabbage worm damage is confused with slug damage, but the presence of the small caterpillars and/or their frass is unmistakable.

Biological controls for cabbage worm management

First, before you take up arms against any little green worms on your cabbage or broccoli plants, it’s important to realize that they are an important and invaluable food source for many other creatures, including birds and many predatory beneficial insects. I love to sit and watch the house wrens and chickadees hop across the tops of my broccoli plants every morning. They glean off the young cabbage worms and fly back to the nest to feed them to their young. According to one of my favorite books, Bringing Nature Home by Dr. Doug Tallamy, each brood of chickadees requires up to 9000 caterpillars to reach the fledgling stage. Encourage birds to take up residence in your vegetable garden by putting up nesting boxes and eliminating the use of harmful synthetic chemical pesticides that end up in the food chain.

Robberflies are a predator of many common garden pests.

Robberflies are one of many natural predators of cabbage worms. This one has caught an adult butterfly for lunch in my garden!

Beneficial insects to control cabbage pests

Cabbage worm caterpillars are also a food source for many beneficial insects that are excellent helpers in the garden. Robberflies love catching the adults mid-flight in my garden (see photo above) and enjoying them for lunch. The paper wasps fly back and forth between their tree-top nest and the garden all day long, carrying pieces of caterpillar back to feed their larvae. (Yes, paper wasps are very good for the garden!). And, I often spy beneficial spined soldier bugs and assassin bugs enjoying the cabbage worms in my garden, too. Plus, there are several different species of parasitic wasps that help manage these and other pest caterpillars.

Spiders are another beneficial creature that enjoys cabbage worm caterpillars. Hunting, or cursorial, spiders such as jumping spiders and wolf spiders, prowl the garden at night. They climb onto plants to search for their prey. I’m weird enough to go out to the garden at night with a flash light to search for these amazing creatures. I often find them eating asparagus beetle larvae, cabbage worms, and even Colorado potato beetle larvae.

Paper wasp controlling pest caterpillar

A single paper wasp nest is filled with tens of thousands of pest caterpillars which are used to feed the wasp larvae. This wasp is preparing to take a young cabbage worm back to its nest from its perch on one of my broccoli plants.

To encourage beneficial insects to naturally help you limit cabbage worm issues, plant lots of flowering herbs and annuals in and around your vegetable garden. Of particular importance are small-flowered plants, such as dill, fennel, cilantro, oregano, chamomile, thyme, sweet alyssum, and more. If they are inter-planted with the crop, some of these beneficial insect-attracting plants may also help mask the presence of the host plants from the adult cabbage worm butterflies. This can also limit egg-laying efforts and further reduce pest populations.

Physical controls

Cover cole crops with a layer of floating row cover early in the season to prevent adult butterflies from accessing the plants to lay their eggs. Place the fabric over the plants immediately after planting. Allow plenty of slack in the fabric for the plants to grow. Since cole crops do not need to be pollinated in order to produce their edible crop, leave the row cover in place up until the day of harvest.

Handpicking is another very effective method of physical control for this pest. Head out to the garden on a daily basis and examine the tops and bottoms of the leaves. Pick off any caterpillars you find and squish them. Or, simply toss them out of the veggie patch and onto the lawn. There, spiders, ground beetles, birds, and other animals will make a quick lunch out of them. We feed the caterpillars to our chickens and they go crazy for them. We call it “chicken rugby” when we watch them fight over one!

Broccoli plant with young leaf-eating caterpillar.

Cabbage worms are easily controlled by hand-picking, especially if you have just a few plants.

Product controls for cabbage worms

I highly recommend turning to biological or physical controls first. However, there are a few organic products that are useful against cabbage worm caterpillars. They are especially useful in large patches where handpicking is difficult.

  • Spinosad-based organic pesticides, such as Captain Jack’s Deadbug Brew, Entrust, and Monterey Insect Spray, are extremely effective in controlling cabbage worms. Care should be taken to avoid using them when pollinators are active or when spray drift could land on non-target butterfly host plants. Spinosad is a fermented bacterial product that manages all pests that chew on leaves. This includes dozens of different caterpillars and beetles. It does not work on sap-sucking insects, such as aphids, squash bugs, and scales.
  • Bt (Bacillius thuringiensis var. kurstaki) is another organic control for cabbage worms. These products, including Safer Caterpillar Killer and Thruicide, are made from a bacteria that, when ingested, disrupts the gut of all caterpillars. They don’t affect any other insects when used properly. But again, care should be taken to not use them around non-target butterfly host plants.
Little green caterpillar on radish leaf

If you spy little green worms on your plants, proper identification is key, before you take any action.

Management strategy matters

Managing cabbage worm issues in the garden starts first with proper pest identification. Encourage natural biological controls by diversifying your garden with plenty of flowering plants. Cover plants with floating row cover as your first line of defense. Turn to product controls only when necessary, and be sure to follow all label instructions carefully.

With these cabbage worm control tips, a successful and productive broccoli, cabbage, cauliflower, and kale harvest is definitely in the cards!

Good Bug Bad Bug book

For more on controlling vegetable garden pests, check out the following posts:
The Savvy Gardening veggie pest guide
Managing slugs organically
Prevent squash vine borers
Four lined plant bug control

Discover how to identify and control cabbage worms organically.

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