THINGS WERE GOING SO WELL. Even the most-vulnerable crops—the crucifers, or Brassicas, including cabbage, Brussels sprouts, broccoli, kale, collards—were looking beautiful. Big, strong plants grown under row covers for about six weeks (successfully defeating flea beetles, at least) are suddenly under attack by small, velvety green caterpillars. What’s up, and what can I do about cabbage “worms”?
Though I cannot see without a hand magnifying lens if they have the requisite tiny markings, I’m betting from its overall appearance and velvety surface that this is the larval stage of the cabbage white butterfly, Pieris rapae, because I have also seen its adult stage flying around, a smallish butterfly with a couple of smudgy spots on each white wing.
This article from Missouri Botanical Garden is extremely detailed on my latest visitor, also known as the imported cabbage worm, and other pests of cabbage relatives, including cabbage looper and the caterpillar of the diamondback moth. The latter two caterpillars are smooth, not velvety, among other clues to differentiating among the three.
As with all caterpillars, these can be controlled with the non-chemical biological control called b.t. (Bacillus thuringiensis), often sold as Dipel or Thuricide, but I don’t use it (tempted!). Nor do I use pyrethroids, which are also effective, apparently, but synthetic and not approved for organic production, or even natural pyrethrum/pyrethrin, which is permitted for organic use.
Instead, with my home-garden sized small number of plants, I’m making the rounds early and again late each morning, and hand-picking the sticky little beasts and—yes—squishing them. The challenging part is how well-camouflaged they are, often resting on leaf midribs as if they’re part of the plant. Go slowly, looking on top of and underneath each leaf; some will be tiny, just hatched.
After picking the pests, I rinse the plants, since the appearance of fresh tiny drops of gray-black excrement will signal the next time if, and perhaps just where, the hungry caterpillars are chewing. I wish I had left my row cover in place, to prevent the butterflies from laying eggs on the leaf undersides, or checked the undersides of leaves for eggs when I saw the butterflies; live and learn.
I’m putting out a welcome sign for various species of wasps that target and parasitize these unwanted caterpillars (there is a photo of their eggs on this page the Missouri Botanical website, at the bottom of the page). Most important: I’ll be certain to clean up extra carefully this year, to reduce the chance of overwintering pupae. I’m also reading up on weeds in the cabbage family (wild mustards, for instance, and shepherd’s purse, among others) with a sterner eye to removal.
The bad news: The imported cabbage worm will have multiple generations each season, so I guess this routine will become a familiar one. I don’t even really mind if they eat the tough outer leaves of the broccoli or Brussels sprouts plants. Somehow I don’t think they’ll respect any such boundaries, and are probably already eyeing the buds-to-be of the parts I’d hoped to serve up for my supper later this season. Damn.
P.S.–I’ve also been host to cross-striped cabbage worms (above) on occasion. More about that Brassica pest in this story.