If you grow cole crops like cabbage, broccoli, or kale, you’ve likely encountered the cabbage worm (Pieris rapae), a destructive pest that can devastate your harvest. While these pests are also known as small white or cabbage white butterflies, their larvae, commonly called cabbage worms, are responsible for significant crop damage. These voracious caterpillars feed on brassicas, sometimes stripping them down to just the stems and veins. Luckily, there are several effective strategies to protect your crops.
What You Need to Know About Cabbage Worms
Identification and Biology
The cabbage worm starts life as the larvae of the small white butterfly. These butterflies are common in North America, easily recognizable by their white wings with black edges and occasional black spots. While these butterflies may look harmless, their eggs are typically laid on the undersides of plant leaves, where the larvae hatch.
The larvae, about an inch long, are pale green with faint yellow stripes and yellow spots along their backs. They feed on the leaves of plants in the Brassicaceae family, including cabbage, broccoli, kale, and mustard, causing noticeable damage such as holes or missing chunks in the leaves.
Life Cycle
Cabbage worms overwinter as pupae, emerging as adults in spring. After mating, females lay eggs on host plants. These eggs hatch within a week, and the larvae begin feeding immediately. Over two weeks, they mature, forming pupae from which new butterflies emerge to repeat the cycle. Depending on your climate, cabbage worms can go through multiple generations in a year, making them a persistent problem.
What’s at Risk?
Cabbage worms attack a wide range of crops, including:
- Cabbage
- Broccoli
- Kale
- Cauliflower
- Turnips
- Mustard greens
- Radishes
- And other Brassicaceae crops like bok choy and kohlrabi.
Even lettuce, though unrelated, may sometimes attract cabbage worms, as will flowers like nasturtiums.
Control Methods for Cabbage Worms
1. Organic Control Methods
There are several eco-friendly ways to tackle cabbage worms:
- Bacillus Thuringiensis (Bt)
This natural bacterium is an effective treatment for cabbage worms. It causes larvae to stop feeding and die within a few days. Reapply every week or two to maintain control. Bt is safe for humans, animals, and beneficial insects, making it ideal for organic gardeners. - Pyrethrins
Derived from chrysanthemum flowers, pyrethrins are natural insecticides that can control a range of pests, including cabbage worms. Apply these during cooler times of the day to avoid harming beneficial insects, and avoid spraying when temperatures exceed 90°F. - Natural Predators
Parasitic wasps, such as Trichogramma species, are natural enemies of cabbage worms. These wasps lay their eggs inside the eggs of cabbage white butterflies, preventing the larvae from hatching. To attract these helpful wasps, plant nectar-rich flowers like zinnias, cosmos, and alliums. You can also purchase these wasps and release them into your garden. The right species depends on your crops and region: Trichogramma brassicae works well for low-lying plants like cole crops, while T. minutum is recommended east of the Rockies.
2. Cultural Controls
- Floating Row Covers
Cover your plants early in the season before butterflies have a chance to lay their eggs. This physical barrier keeps adult butterflies from reaching your crops. - Handpicking
Inspect your plants regularly and handpick any caterpillars you find. Drop them into a bucket of soapy water to dispose of them effectively. - Soil Tillage
After harvesting, remove plant debris and till the soil. This process buries pupae, preventing them from emerging and continuing their life cycle.
3. Chemical Control
If the infestation is severe, chemical pesticides like Sevin can be used. Always follow the instructions carefully, and wear appropriate protective gear when applying these products.
Conclusion
Cabbage worms are persistent pests that can cause extensive damage to brassicas and cole crops, but there are numerous ways to manage and control them. From natural predators like parasitic wasps to organic treatments like Bt and pyrethrins, gardeners have a range of options to protect their crops. By using these methods, you can ensure your harvest remains healthy and abundant, free from the damage caused by cabbage worms.