Organic solutions for effective cabbage worm control in home gardens.
Now common throughout the United States, the imported cabbage worm (Pieris rapae) does great damage to brassica and other cabbage-family crops in fields and gardens where it gains a foothold. Although the larvae of this garden pest moves sluggishly, it is extremely destructive, especially later in the growing season when populations can build significantly.
The imported cabbageworm (1-1/4 inch long) is velvety green in color and has many short fine hairs and faint yellow strips down its side and back. It’s five sets of prolegs are easily visible. Adults are white or pale yellow butterflies (1-2 inch wingspan) with three or four black spots on their wings. They are frequently noticed fluttering about the garden from early spring to late fall.
Adult females emerge in early spring after over wintering as green pupae. They lay up to 200 tiny yellow eggs on host plants, usually on the undersides of leaves. These hatch in 7 or more days (depending on temperature) into young larvae caterpillars. The larvae feed heavily for 15 or more days, then pupate on lower leaf surfaces or nearby garden objects. During late spring and summer, the worm pupates for 10 days before a new generation of butterflies emerges. There are 3 to 5 overlapping generations each year, as many as 8 in warmer areas.
In the larval stage, cabbage worms will feed on the surface layer of leaves, leaving behind a a translucent, tissue-like scars. As they grow, they chew large, irregular holes usually beginning on the outside leaves of cabbage and other cole and mustard crops (broccoli, cauliflower, kale, turnip, radish). As the worm feeds, it commonly bores into the center of cabbage heads contaminating them with its fecal pellets. The dark-green pellets can also be found in the crook of leaves near the stem.
The imported cabbage worm is one of many worms that attack garden plants. Luckily, control tactics aimed at a particular worm, be they loopers, army worms, cut worms or diamond back moths, are usually effective against all.
- This insect has many natural enemies, including predatory beetles, spiders, yellow jackets, green lacewing and parasitic wasps. Birds also favor cabbage worms. Make sure your garden welcomes these creatures. And don’t use chemical sprays that might harm or destroy these natural predators.
- Protect plants with floating row covers to prevent adults from laying eggs.
- Use pheromone traps to determine the main flight period for moths.
- Release trichogramma wasps to destroy eggs.
- Predator wasps of various sorts, most indigenous to your garden, will seek-out the eggs of all types of worms. Help protect them by using something other than indiscriminate chemical pesticides.
- Herbalists report that moths are discouraged from laying eggs on cabbage sprayed with tansy oil or a strongly brewed tansy tea (because of the volatile oils it contains, tansy teas can be dangerous to humans, especially when consumed in quantity, and should be avoided; instead use it in your garden). Planting tansy near your cabbage crops can also discourage them. In an example of the two-way street nature of companion planting, tansy planted near cabbage does surprisingly well.
- As soon as damage is noticed (large irregular holes in leaves, fecal pellets on plants and ground), begin handpicking caterpillars and destroying them.
- Chickens can be thorough pickers of cabbage worms. Ducks, too. Of course, they might also eat things you don’t want them to, especially early in the season when plants are still small. Wait until your plants are peckable size and you’re sure you have a pest problem before unleashing the clucks and the quacks.
- Botanical insecticides — derived from plants which have insecticidal properties — have fewer harmful side effects and break down more quickly in the environment than synthetic chemicals. However, they are still toxic and should only be used after other least-toxic options have been tried.
- Once worms are apparent, apply Garden Dust (Bt-kurstaki) to leaves where they’re seen. This naturally occurring soil bacteria, listed for organic use by the Organic Materials Review Institute, will take out the worms as they feed.
- Spinosad, the active ingredient in Monterey Garden Insect Spray is made from fermentation. It doesn’t persist in the environment — crops are ready for harvest a day after application, and is a good substitute for Bt-kurstaki dusts.
- To prevent overwintering pupae from emerging as adults in early spring, till under all garden debris to which they might attach. In places with milder winters, it may be necessary to remove the debris all together. Pay special attention to plants of the mustard family. They’re a favorite place for cabbage worm pupae to spend the winter.
- Worms will also retreat to garden margins and borders. Keep them clean and short to prevent overwintering there.