While butterfly collectors may rejoice that the cabbage butterfly now lives in all 50 states and southern Canada, people who grow cole crops and crucifers do not share their joy.
Cabbage worms can completely destroy a crop of cabbage or other brassicas if left to their devices. While a couple holes in a head of cabbage is not a big deal, these caterpillars can eventually leave cabbage plants with only the stems and large veins.
However, there are steps you can to take to protect your cole crops from the cabbage worm and cabbage moth, and we at Gardener’s Path will introduce you to these methods.
What You’ll Learn
Identification, Biology and Distribution
The parents of the cabbage worm are small cabbage white butterflies (Pieris rapae) which one of the most common butterfly species in the northeastern United States.
The 1/2-inch butterflies are white with wings edged in black that also have one or two white spots on them. While the butterflies are very attractive, they are probably laying eggs on the undersides of the leaves!
The 1-inch caterpillars (also known as the imported cabbage worm) are velvet green in color with a few faint yellow stripes. The caterpillars also have several yellow spots down their backs.
These voracious pests can dig into leaves and fruits of brassica plants, resulting in missing chunks and small gaps that are obvious to anyone eating the produce.
Also their fecal matter is scattered throughout the crop, staining and contaminating them.
An exceptionally large infestation can kill the plants.
Cabbage worms overwinter as pupae. The adults emerge in the spring, mate, and then the female lays eggs on her hosts.
The eggs hatch within a week, and the larvae begin feeding.
Over the next two weeks, the larvae go through several stages before using silk-like threads and attaching themselves to the plant to pupate.
The adults emerge in a week or two to start the cycle over again. Cabbage worms often go through three generations in the Northeast and more in warmer climates.
What’s at Risk?
The following species and cultivars are most at risk being attacked by the cabbage worm:
- Bok Choy
- Brussels sprouts
- Mustard and other crucifers
Any plant belonging to the Brassicaceae family can be vulnerable. Lettuce is also a target of these nuisance insects despite being in a different family (Asteraceae).
Organic Control Methods
You can use several types of organic methods to control cabbage worms.
A type of BT, Bacillus thuringiensis kurstaki (Btk), is an effective treatment and will immediately stop the caterpillars from feeding. The caterpillars will then die of malnutrition within several days.
Treating the plants every week or two will provide sustained control throughout the season.
Pyrethrins are another option, although you should not apply them in the heat of the day – especially if the temperatures are greater than 90 F.
Trichogramma parasitic wasps will help control cabbageworms. You can attract them to your garden by planting pollen and nectar producing plants.
The wasps lay their eggs on the surface of the worms; the eggs hatch, burrow in, and then drill back out form pupae attached the dead or dying body of the caterpillar.
Another option is to buy Trichogramma wasps and release them into your garden.
Using floating row covers early in the season before the butterflies lay their eggs is an effective means of control.
You can handpick the larvae and put them in a bucket of soapy water.
Once you have harvested your cole crops, remove remnants of the plants and till the soil under. That will plow the pupae under the soil and kill them.
Chemical Pesticide Control
While the organic control methods should greatly minimize the damage to your brassicas from cabbage worm, chemical pesticides are another option. Sevin is an insecticide that has been recommended to destroy these pests.
Ever-present Butterflies That Can Kill Your Cabbages
These attractive butterflies are mainstays in the spring and summer throughout the US. And they are as destructive as they are attractive.
Eggs laid on brassicas hatch into voracious caterpillars. Fortunately, there are many methods available to control these caterpillars ranging from treatments with Bt to parasitic wasps.
If you have a small garden, you can handpick the caterpillars and kill them.
Have you vanquished cabbage worms? Or did they decimate your garden? Either way let us know in the comments.
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About Helga George, PhD
One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the childhood discovery that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.