Who Is Eating My Cabbage?
Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
MGCC Help Desk Response: Thank you for contacting the UC Master Gardener Program with your question about bugs on your cabbage plants. Thanks also for the helpful photo. Since the tiny white bugs that you previously saw are now gone, it's hard to tell exactly what they were, but possibilities include whiteflies. This UC website has pictures of the various insects likely to affect cabbages - you might be able to identify them by scrolling through the 'invertebrates' section.http://ipm.ucanr.edu/PMG/GARDEN/VEGES/cabbage.html
Regarding the damage that you now see, I think the most likely culprit is the Imported Cabbageworm. This is a small green caterpillar, the larva of the white butterflies very commonly seen in vegetable gardens.
http://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/importcabwrm.html . If you look carefully on the young leaves of your plant you may see these caterpillars - when just hatched they are quite small.
Now for how to control this pest. Once they are present on the plant there are a couple of organically approved sprays that will kill the caterpillars on contact; Spinosad or Bacillus thuringiensis (BT). The Cabbage worm website (above) has links with more information on these pesticides. You can also pick them off by hand. One very useful strategy to prevent the butterflies from laying eggs is to cover the plants with lightweight row cover http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/protectivecov.html . This material can be found at some nurseries or online - I use this myself, and have found that once the winter sets in the butterflies are no longer around and the cover can be removed. With some minimal
care you can use the row cover for many years.
I hope this is helpful, and that your cabbages will recover now. Once the head starts to form it's hard to get at caterpillars down inside the curved leaves, so you should try and kill as many as you can now! If you think that the problem is something else, and you can get a photo of any pests please do get back to us.
Help Desk of the UC Master Gardener Program of Contra Costa County (SMW)
Note: UC Master Gardeners Program of Contra Costa's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: email@example.com, or on the web at http://ccmg.ucanr.edu/Ask_Us/.
MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog (//ucanr.edu/blogs/CCMGBlog/)