Control cabbageworms with a few tricks
Question: How can I safely keep cabbage moths off of my red cabbage and broccoli? I’ve tried netting, but the insects and moths just get stuck under it. I’ve tried dousing with water in centers then adding mud as well as picking them off several times a day. I’ve read good and bad things about Bti powder and tried a concoction of chili powder and garlic but only ruined a spray bottle. Thanks for any advice you can provide.
Answer: I sympathize with your cabbageworm woes; these pests are often present in my garden, too. However, I do have a few tricks up my sleeve to limit their damage successfully.
Cabbageworms (Artogeia rapae) are caterpillars that feast on all members of the cabbage family. The adults are known as cabbage moths, though they’re actually butterflies. Adults are common across much of North America. They’re white to yellowish-white butterflies with a one to two inch wingspan. Adults sip nectar from blooms, and the females lay eggs on host plants throughout the spring and summer months.
Cabbageworm caterpillars are light green with a faint yellow stripe down the back. Just before pupation, they measure about an inch long.
Cabbageworm caterpillars chew holes in leaves of brassica crops. They may also leave round holes through flower clusters of broccoli and cauliflower. Young cabbageworms are difficult to spot because they’re so tiny. They’re often found hanging out on leaf undersides and along the leaf midribs.
I control them best by putting a barrier of floating row cover over susceptible plants the very day they are planted. Netting (such as fabric tulle) isn’t as effective because the cabbageworm adults will be able to see and “smell” their host plants through the holes in the netting, and the females can lay eggs right through the mesh if it’s resting against the plant.
Floating row cover, on the other hand, does not have perforations and forms a denser protective barrier. It’s inexpensive, and lets light and water pass through it to the plants beneath. To keep the adult butterflies from getting underneath it, when initially covering the plants, be sure to leave plenty of slack in the fabric to allow for plant growth, and pin the edges down with landscape pins or rocks.
Bt is an option as well. It’s labeled for use in organic production and is easy to find on the market. Bt is the abbreviation for Bacillus thuringiensis, a naturally-occurring soil bacterium that’s been used for many years to combat specific pest insects. Bt is applied as either a dust or a spray. When it’s ingested by insects, it disrupts their digestive system, causing death. There are many different brand names available at local garden centers.
Several different strains, or varieties, of Bt exist, each used against a different group of pests. You mention Bti in your question, but that’s actually not the correct strain for cabbage moth caterpillars. The strain for caterpillars is Bt variety kurstaki (Btk). It’s also effective against hornworms, corn earworms, fall webworms, gypsy moths and many other pest caterpillars.
Bti, on the other hand, is Bt variety israelensis. This strain targets aquatic mosquito larvae and would not be effective against cabbageworms.
When used according to label instructions, not only is Bt safe to use around non-target organisms in the garden (including pollinators and humans), it’s also highly effective.
Btk-based products kill any cabbageworms that directly consume foliage its been recently applied to. However, be careful not to let any spray drift onto surrounding plants. Btk kills all caterpillars, including butterfly caterpillars such as the swallowtails often found munching on garden parsley or monarchs feasting on nearby milkweed. To avoid impacting non-target species, be sure to apply only to the pest-ridden plants on a windless day where spray drift can be avoided.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to firstname.lastname@example.org or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.