If your cabbage, broccoli or kale plants have huge holes in just about every leaf, you probably can blame cabbage worms, which are caterpillars that can bore right through your cabbages and turn them into something reminiscent of Swiss Cheese. Although they are caterpillars, not worms, cabbage worms are partially true to their name: they do feed on the leaves of plants in the brassica (cabbage) family. Depending on which species of cabbage worm they are, they will turn into butterflies or moths once they mature.
Species of Cabbage Worms
There are three main species of "cabbage worms": imported cabbageworms, cabbage loopers, and diamondback moths. If you're a budding entomologist, you can identify which of these cabbage worms are devouring your crops based on their markings and behavior.
- Imported cabbageworms are about 1" long and are green with a yellow stripe down their back. The adult form of this species is a white butterfly that's about 2" across. So, those white butterflies fluttering around your broccoli plants may beautiful to behold, but they are probably laying eggs on you crop!
- Cabbage loopers are true to their name: they are pale to dark green caterpillars with thin white stripes that arch their bodies into a loop when they crawl. They can be up to 1.5" long. Adult cabbage loopers are 1.5" brown moths with silver markings on their wings.
- Diamondback moth caterpillars are up to 0.5" long, grayish green and have no stripes. Their body tapers at both ends. Adult diamondback moths are gray, small (0.75" wing span) and have cream markings on their wings that form a diamond shape along the moth's back when its wings are folded.
If you'd prefer not to get as up close and personal with the cabbage worms, that's OK too: there are some organic control strategies that will work on all three species of these worms. However, if you have identified which species have taken up residence in your garden, you will have additional organic control options, including the targeted use of various types of parasitic wasps (don't cringe--these wasps will be tiny and won't sting you!).
Control Cabbage Worms Organically
- Use floating row cover-- Diamondback moths can potentially have between 4 and 6 generations per year. Imported cabbage worms and cabbage loopers typically have anywhere between 1 and 4 generations per year. This means that your cabbage worm problem can go from bad to worse very quickly. Floating row cover (a lightweight spun polyethylene fabric that you can use to cover your plants) will "fence" those pesky adult moths out of your crops and keep them from laying their eggs on your cherished brassica plants.
- Practice crop rotation--There are as many different crop rotation systems as there are gardens: some people grow brassica crops only during certain times of the year (i.e. fall only), others grow them almost year round, but make sure that never grow more than one brassica crop in any given garden plot during that year. Almost any rotation system that you can dream up will help keep the populations of cabbage moths lower than they would be if you didn't practice crop rotation.
- Spray infested plants with Bt-- Bt is a biological insecticide (actually a bacterium that only infects caterpillars) that can help you control an existing cabbage worm problem. Use Bt by spraying infested crop's leaves with it and allowing the cabbage worms to eat the leaves and become infected by the Bt. The infection will kill the cabbage worms. However, it does not kill the adult moths or any eggs they may have laid on your plants.
- Attract insects that are enemies of cabbage worms--Cabbage worms have "enemies" other than yourself: ground beetles, various wasps, spiders, lacewings and various other insects prey on cabbage worms. Your job is to help nature take its course by providing an insecticide-free home with a diversity of different plants for these "worm-eaters."
- Remove older, "spent" plants from your garden--although some aspects of the cabbage worm lifecycle are still being discovered, it is known that these pests can overwinter (as adults or pupae, depending on the species) on the debris left over from previous brassica crops. Therefore, by removing your brassica plants from your garden when they wane, you will be interrupting the cabbage worm's lifecycle by depriving them of a home. So, move those old plants out of there and compost 'em! You can also remove them by tilling them in thoroughly, if tillage suits your management style better.
If your garden is swarming with cabbage worms, they are major roadblock in your quest for growing great cabbage (or broccoli, collard greens or Brussels sprouts). The tried and true management tips in this article are some of the most effective cabbage worm control strategies available: since cabbage worms can evolve resistance to insecticides, organic controls are often used by conventional gardeners and professional vegetable growers as well as by die-hard organic gardeners. So, unless you really love watching those pretty butterflies fluttering amongst your cabbages, go ahead and join the organic cabbage worm control club. Your cabbage awaits!