Organic pest management: Imported Cabbage Worm
Cabbage white butterfly, Pieris rapae, is one of the most visible insects, pest or otherwise, in our garden. It looks lovely and innocuous, dozens of white yellow butterflies floating around the garden on a warm summer day. But watch them closely and find that what they're really doing is laying their eggs on host plants, which are those in the brassicacea family. For us, the larva are mostly a problem in cabbage, as the leaves they feed on are also the part we eat. They're frass fills the crevices of the growing cabbage, making it a disgusting mess. Their feeding can stunt broccoli, cauliflower, and brussels sprouts, but usually those crops are still edible even with damage because the parts we eat are not the parts they feed on. If you can focus your eye on one butterfly, it's easy to watch her land on a cabbage or broccoli plant, bend her ovipositor to the underside of the leaf and drop one or two oblong eggs there.
The cabbage worm overwinters in the soil as pupa. It pupates in the spring and emerges as an adult white butterfly. Butterflies mate, and females lay their eggs on the leaves of host plants, which are those in the brassicacea family. These include radish, cabbage, turnip, broccoli, and cauliflower. Eggs hatch larva that grow fat feeding on the host plants, then become pupa and the cycle starts again. There can be a handful of generations a season, count on them as long as the weather is decent- usually through September.
Signs and symptoms:
The easiest way to know they are present is by seeing the butterflies flying around the garden. Real fair weather friends, they only fly on mild, windless summer days, from around 10am-5pm. If there are butterflies and your garden has host plants growing in it, its a sure bet they are females laying eggs.
You can see larva feeding, usually in the folds of the leaves or on the undersides. They are bright green and grow quickly. Before you spot the larva, spot the holes they make in leaves. They don't make large holes, but move from place to place make smallish holes that simply look like something has been munching there. If you see birds taking an interest in your cabbage plants, that can also be a sign that they are finding caterpillars there.
Organic pest management
Since the butterfly overwinters as pupa in the soil, it is imperative to rotate brassica plantings. Especially if you're using row cover, emerging pupa will simply be trapped with their favorite food if you don't move your brassica plantings to an area that hasn't had brassicas for at least a year.
Row cover is your friend. When plants and surrounding soil are covered well with row cover, females don't have access to lay their eggs on the plants.
With row cover, its imperative you cover host plants right after you plant and keep the cover on as much as possible. If you cover too late, you may just be trapping newly laid eggs in with their favorite food.
Happy kid and blemish free cabbage taking a very temporary break from under its row cover.
Encouraging predatory insects:
We manage our field organically, which allows predatory insects of pests to flourish and help us with our pest management. I have huge networks of spider webs helping catch larva under cabbages in my field.
What won't work
Hand picking larva:
Some publications mention hand picking the larva as a method of control. I've tried this, and no. In my experience, if there
Trap crops are crops you don't care about that you just plant to attract pests away from valuable crops. They aren't harvested. Trap crops only work when a pest clearly prefers one kind of plant and you plant that one as the trap crop. Cabbage butterflies don't seem to prefer one brassica over another, so they won't know to go to your trap crop instead of your valuable one. In this case, uncovered trap crops just allow the butterflies to grow their population.
Hoping they'll just go away:
If you've had so much trouble with these guys in the past that you gave up on radishes or turnips or cabbage, I hope you'll now feel empowered to try again!