10 Organic Strategies to Manage Cabbage Worms

Have you ever walked out to your vegetable garden only to find your cabbages and kale looking like Swiss cheese? It's a punch to the gut, right?

You're definitely not fighting this battle solo. The insatiable hunger of cabbage worms has met its match. I'm here, armed with personal battles won (and lost) and strategies that work, ready to help you take back your garden from these leafy-green munchers.

Let me take you back to the first time I grew my own broccoli and cauliflower seedlings. I was delayed a bit in planting them so they were outside in a shaded area for a few weeks. One day I passed by them and all the leaves were decimated. I saw these tiny caterpillars and when I touch them, they leaped like Spiderman off the leaf hanging by a silk strand.  

That was my first introduction to diamondback moth larvae, a type of cabbage worm. Those seedlings did not recover from those tiny invaders and I ended up buying seedlings at Home Depot.

What are Cabbage Worms?

There are several species of cabbage worms that can cause problems. The usual suspects include the offspring of the cabbage white butterfly or the imported cabbageworm (Pieris rapae) and the diamondback moth (Plutella xylostella), not to mention their accomplices, the cross-striped cabbageworm (Evergestis rimosalis) and the notorious cabbage looper (Trichoplusia ni).

These troublemakers are a common guest in the gardens of cabbage, kale, cauliflower, broccoli, and their relatives from the cabbage clan. Cabbage loopers have a wider palette which also include beet, cantaloupe, celery, cucumber, lime bean, lettuce, parsnip, pea, pepper, potato, snap bean, spinach, squash, sweet potato, tomato, and watermelon.

How to Identify Cabbage Worms?

If you see holes in your plant, inspect it closely to see what is eating it. Although not all offenders stick around, cabbage worms typically do and they blend very well into the leaves. I’ve seen photos posted to Facebook where people are asking what is causing the holes and you can see the caterpillar that they didn’t spot. Also check the back of the leaves since pupae hang out there.

Imported cabbageworm larvae are velvety green caterpillars with a faint yellow stripe down its back and a row of faint yellow spots on each side while cross-striped cabbageworms are a little more obvious with light bluish-grey on top and green underneath, with numerous black bands across their backs and a yellow line down each side. Imported cabbageworms grow over an inch long while cross-striped cabbageworms grow to three-quarters of an inch.

The adults of the imported cabbageworm are white butterflies with black dots while the adults of cross-striped cabbageworms are brown moths. If you see moths hanging around your brassicas, start inspecting for eggs.

Cabbage loopers are pale green with a white stripe on the side. Since they have fewer prolegs than the others, they crawl by arching their back to form a loop and then project their body forward. Loopers are commonly called inchworms and grow to 1.5 inches long.

Diamondback moth larvae are the smallest of the cabbage worms, not exceeding a half inch. They are light green. A distinguishing feature is that if you mess with them, they wriggle and drop down on a strand of silk. I often see them as pupae on the backs of leaves where they spin a silk cocoon with the yellowish-green pupae inside.

What Time of Year are They Found?

The adults emerge from pupae in the south where they overwinter in early spring with overlapping generations throughout the US and Canada until fall. Cross-striped cabbageworms are the most abundant in late-season plantings. Cabbage loopers are unusual since they will be very abundant for one year, and then scarce for two to three years.

What Damage Do They Cause?

The damage cabbage worms cause depends on their size, as well as how many you have. When the caterpillars are small, you will start to see small holes in the middle of the leaves. For loopers, young larvae will feed on the lower leaf surface, leaving the upper surface intact. The holes gradually get larger and some of the larger caterpillars may leave merely a skeleton of a leaf left. As a fun fact, some European restaurants seek out leaves with holes as a sign of an organic salad. 

In addition to unsightly holes, cabbage worms can reduce the yield of the plants and stunt their growth especially when older larvae remove most of the leaf veins and invade the heads of cauliflower and cabbage. They can kill seedlings when they eat too much of the leaves. Lastly, they leave copious amount of fecal matter which is just gross.

The ability of a plant to recover from cabbage worm damage depends on the severity of the infestation and how quickly the plant grows. Leafy plants like mustard, kale, collards, spinach, and chard can quickly grow new leaves from the center of the plant and recover, although cauliflower and cabbage may not recover if the heads are damaged.

Early Detection is Key

The first step in any battle plan is reconnaissance. Regularly inspecting your plants for yellow or white eggs or small caterpillars can make all the difference. These pests are master hiders, but with a keen eye, you can catch them before they cause extensive damage. Remember, early detection leads to easier management.

You should inspect your plants weekly throughout the growing season, focusing on the undersides of the leaves where caterpillars and eggs are located. If you see eggs, you can remove them easily with tape.

Management Strategies for Cabbage Worms

Below are the best strategies to prevent and manage cabbage worms.

1.  Keep Plants Healthy to Prevent Pests

The best way to prevent cabbage worms is to grow strong and healthy plants. Some brassicas like cabbage, broccoli, and cauliflower are heavy feeders, meaning they need more nutrients to grow well. I add compost at the beginning of the season, along with organic fertilizer. As the plants grow larger, I supplement them with liquid fertilizer every 3 weeks.

They also tend to be moderate to heavy water drinkers, especially as they form larger leaves and heads. Make sure that they remain well-watered. By providing adequate water and fertilizer, you reduce the stress of the plant and make them less attractive to cabbage worms.

2. Floating Row Covers for Cabbage Worms

Use lightweight floating fabric row covers to physically exclude the butterflies and moths from laying eggs on your plants. Install them over hoops when you plant your seedlings. This is so you don’t trap pests in by covering them too late. The key is to anchor them down so that nothing gets under them.

3. Companion Planting

There are several companion plants that will deter cabbage moths. Interplant with celery and/or thyme to deter cabbage moths. Borage and chamomile repels cabbage moths, as well as encourage beneficial insects.

4. Trap Cropping

More than my other crucifers, I saw that my mustard plant was a holey mess. This is because cabbage moths are more attractive to mustard plants. You can use these as trap crops or indicator plants for monitoring. If you start seeing holes, then you should start inspecting.

5. Hunt and Grab Cabbage Worms

But let's get hands-on too. Regular patrols in your garden can make a world of difference. Grab a bucket of soapy water and go on a caterpillar hunt. It might sound a bit tedious, but think of it as spending quality time with your plants. Plus, finding and dunking those worms can be oddly satisfying.

6. Release of Trichogramma Wasps

You can purchase tiny parasitic wasps, such as Trichogramma spp., and release them into the garden. They work by attacking the eggs, so they don’t kill the emerged caterpillars directly, but they will keep the population in check.

7. Beneficial Bacteria for Caterpillars

Lastly, you can use the help of some beneficial bacteria. Bacillus thuringiensis (Bt) spray is one of my go-to sprays when I suspect caterpillars. The caterpillars chew on their favorite plant and the toxin in the bacteria does its job. It is recommended to use this every two weeks during caterpillar season. Spinosad is another bacterial biopesticides that works on cabbage worms.

As a note, once caterpillars are ¾ inch long or more, these biopesticides won’t work.

8. Neem Oil as a Repellent

This botanical oil, derived from the neem tree, acts as a repellent and disrupts the feeding and molting of these destructive caterpillars. While it won't necessarily eliminate existing infestations overnight, regular neem oil sprays can make your cabbage plants unappetizing to egg-laying butterflies and hinder young worms' development. Remember, prevention is key, so start spraying early in the season and reapply every week or so for optimal results. With a little neem oil TLC, you can enjoy a healthy harvest of beautiful cabbages.

9. Crop Rotation: Outsmarting the Enemy

Cabbage worms can become more problematic if they anticipate where their next meal will come from year after year. This is because pupae can overwinter in the soil and debris from your previous crop. By rotating your crops—planting them in different locations each season—you can confuse and deter these pests. This practice also promotes healthier soil and reduces disease incidence, making it a win-win strategy.

10. Clean Up Debris

As mentioned above, pupae overwinter in plant debris, so by cleaning up any fallen leaves, you remove hiding spots for the pupae and reduce the release of adults. Keep up debris before putting your garden to bed in the fall.

Embrace Resilience: The Heart of Gardening

Despite our best efforts, some battles may seem lost. However, it's important to remember that gardening is as much about resilience as it is about cultivation. Learning from setbacks and adjusting our strategies ensures that we grow alongside our gardens.

In closing, managing cabbage worms in your vegetable garden doesn't have to feel like an insurmountable challenge. By staying vigilant, embracing natural solutions, and engaging directly when necessary, you can protect your vegetables and enjoy the fruits (or veggies) of your labor.

Remember, every gardener faces pests at one point or another—it's how we respond that defines our gardening journey. So roll up your sleeves and let's give those cabbage worms a run for their money!