Cabbage worms are some of the most common and detrimental pests in the fall garden. Although they may look like cute, harmless caterpillars, they can quickly damage your plants.

I’ve watched fall cabbage worms devour brassicas, turning previously robust crops into shredded, lacey leaves. Large infestations can even consume entire seedlings in a single night!

Fortunately, dealing with fall cabbage worms is easy. Becoming familiar with the pests’ appearance and life cycle is a good place to start. Then, you can move on to preventing and controlling them in your garden.

Types of Cabbage Worms

There are a few different species of cabbage worms, and becoming familiar with each one can help you manage them in your garden. Here are a few types of these caterpillars that often become pests in the fall garden.

Imported Cabbage Worm

This is the most common type of fall cabbage worm.

When people talk about cabbage worms, they’re often referring to the imported cabbage worm (Pieris rapae). This species is originally native to Eurasia, but it’s a common pest in North America.

The small white butterflies lay tiny, single, cylindrical, yellow eggs on brassica leaves. Eventually, these eggs hatch into small green caterpillars that immediately start feeding on brassica foliage. The caterpillars continue to feed, molt, and grow until they form a light green pupa a few weeks later.

The caterpillars pupate for one to two weeks, then emerge as adults. They then start their life cycle all over. In warm environments, imported cabbage worms can form five generations each year.

Cross-Striped Cabbage Worm

These colorful worms are easy to spot.

Cross-striped cabbage worms cause similar damage as imported cabbage worms, but they differ in appearance. The caterpillars have slate gray bodies, featuring faintly striped tops and a single large yellow and black stripe running down each side. Their heads are orange.

The adults are brown moths that are mainly active at night. They lay clusters of small, round, yellow eggs on the undersides of brassica leaves. These eggs hatch within a couple of weeks, and the larvae start feeding on the crops.

Similar Pests

Other similar pests affect crops in fall.

Numerous other types of butterfly and moth larvae are pests of fall crops. Although these pests aren’t cabbage worms, they look similar and attack the same crops.

The cabbage looper (Trichoplusia ni) is a small green inchworm that’s the juvenile form of a brown moth. You can distinguish them from other larvae by the way they inch along as they move.

Diamondback moth (Plutella xylostella) larvae are light green with a body that tapers towards the head and rear. 

What Crops Do Fall Cabbage Worms Impact?

Brassicas are common victims of fall cabbage worm damage.

Cabbage worms feed on members of the Brassica family. Unfortunately, a large number of fall crops are brassicas.

Here are some of the crops that fall cabbage worms eat.

• Arugula
• Bok choy
• Broccoli
• Brussels Sprouts
• Cabbage
• Cauliflower
• Kale
• Radish greens
• Rutabaga greens
• Turnips greens

How to Control Fall Cabbage Worms

Cabbage worms can become a major pest in the fall garden, but practicing a few control and preventative measures prevents them from becoming a serious issue.

Monitor Crops

Remove worms as soon as you spot them.

The first step in controlling fall cabbage worms is regularly monitoring your garden for the pests. Small larvae won’t do much damage to crops, but even a few large cabbage worms can quickly decimate kale, broccoli, and other fall veggies. Spotting the caterpillars when they’re small allows you to remove them before they seriously damage your plants. 

Set aside at least one day a week to walk through your garden and look for cabbage worms. Remember to look for the eggs as well as the larvae. While the white butterflies don’t damage brassicas, spotting them means the caterpillars may soon appear.

Since the eggs and larvae like to hide on the undersides of leaves, turn leaves over to check for the pests. You should also inspect crevices in the middle of plants, since the larvae like to hide in these protected spaces. If you see any holes in your plants, take a closer look for the pests.

Encourage Natural Predators

Invite beneficial insects into your garden.

Large outbreaks of fall cabbage worms mean you need to take action, but you’re not the only one who can help keep pest numbers low. Natural predators feed on cabbage worms and prevent their populations from spiraling out of control.

Natural predators include birds that eat the larvae and adults, as well as insects that feed on the eggs and larvae. Parasitic wasps are the major pest of cabbage worms, but yellowjackets, green lacewing larvae, spiders, and ground beetles may also feed on these pests.

If you spot egg clusters that appear to be coated in a web, it means good bugs are at work. These clusters are the cocoons of parasitic larvae that feed on the caterpillars developing inside the eggs. After the wasp larvae consume the caterpillars, they emerge from their cocoons and continue their life cycle.

Here are a few ways you can encourage natural predators to visit and remain in your garden. 

• Plant a diversity of flowering plants that provide food for adult predators. Asters, goldenrod, cilantro, dill, and sweet alyssum are a few blooming plants you can add to your fall garden.
• Provide a source of water for the predators. A bird bath is a simple yet effective option.
• Avoid spraying broad-spectrum insecticides that indiscriminately harm insects. Instead, choose targeted options and only spray the pests you want to kill.

Physically Exclude Pests

Cover susceptible crops to protect them.

If you know that fall cabbage worms are likely to become a problem on your crops, one of the easiest ways to prevent them is to cover your plants. Row cover or insect netting will keep the adults off the crops and prevent them from laying eggs on the plants. If you want to use this method, it’s important to cover your plants immediately after seeding or transplanting.

You can use lightweight row covers for your plants during cooler fall months, but be aware that these covers can trap lots of heat on warm and sunny days. Insect netting keeps out bugs without trapping heat, so it’s often a better option if you want to keep your plants cool.

You can drape the covers directly over your plants or extend them over metal hoops. Regardless of which option you choose, use weights to secure the ends to the ground to keep the covers in place and prevent pests from sneaking in. Bricks, sandbags, and large rocks all work well.

Hand-Pick Cabbage Worms

Pick off worms you see by hand to reduce populations.

If you left your plants exposed to the adult forms of cabbage worms, you may soon see the caterpillars crawling on your plants. Regular scouting allows you to spot the worms when they’re still small and easy to control.

One of the easiest ways to remove a few cabbage worms from your plants is to pick them off by hand. Just grab the worms with your fingers, then plop them into a dish filled with soapy water. You can also try to shake the worms off the plants and into the soap solution.

Smaller cabbage worms can be difficult to remove from the plants, but you can easily squish them against the leaves. Putting gloves on before completing this task makes it more tolerable.

Spray with Organic Insecticides

Use targeted products for major infestations.

If you have a large garden or major infestation, handpicking the pests off your plants may not be feasible. Rather than spending hours removing the pests by hand, you can spray the infected plants with an organic insecticide.

Bacillus thuringiensis var. Kurstaki (Btk) is a naturally occurring soil bacterium that works as an effective control against cabbage worms. The bacteria only impact members of the Lepidoptera family, which includes all larvae of butterflies and moths. It won’t harm other insects like bees, ladybugs, and green lacewings.

Btk works by disrupting the larvae’s digestive system and preventing them from feeding. The insects must ingest the toxin for it to be effective, so it’s important to thoroughly coat all surfaces of infected plants. Cabbage worms take a few days to die after ingesting Btk, so don’t be worried if you see them on your plants immediately after you spray.