Know Your Enemy
The Cabbage White butterfly, Pieris rapae, seems to be one of the commonest garden pests worldwide. The caterpillars feed on crucifers/brassicas, and have a particular liking for cabbage and broccoli. The caterpillars hatch from single eggs on the underside of a leaf, and stay on the underside until they become large, when they move to the top. They work their way from outer plant leaves to inner ones. Early signs of their presence are pinholes on outer leaves, which quickly become larger until leaves are skeletonised. Green frass (chunky caterpillar poo!) is sometimes visible. Once the caterpillars are in the cabbage heart or into the broccoli head, they cannot be eradicated without chemical use.
In Sydney, the butterflies are more common in the warmer months, and you can expect to see caterpillar damage once daytime maximums move past about 20 C (that is, more than half the year!). The lifespan of the insect is about a month, with the last generation over-wintering in cocoons. The butterflies find each other -- and egg-laying sites -- by sight.
Forget it. Unless your country is killing Cabbage Whites as part of a Four Pests Campaign.
A lot of people overlook prevention, but insect plagues don't just happen. They occur when there is a confluence of favourable conditions. Here are some ways to make the lives of Cabbage Whites more difficult:
- Encourage insectivorous birds. These tend to be smaller birds, so provide dense (even prickly) bushes for habitat, and compost your cat. Provide a bird-bath. If you have chooks, consider tractoring them in your vegie patch so that they can eat any caterpillars they find.
- Encourage predatory insects. Stop spraying general insecticides in your garden, and don't kill wasps unless you really have to (when they have inconveniently-located nests). Wasps feed the caterpillars to their young. If your brassicas support braconid wasp pupae, don't pull the plants out till the wasps hatch. Remember the bird-bath -- most predatory insects need water to drink.
- Hide the cabbages. Interplant them with other vegetables. Patches and lines of identical plants are too easy for passing butterflies to see.
- Look populated. Scatter white half-eggshells around your brassicas. Apparently, the butterflies perceive these as other butterflies, and head off to lay their eggs in a less populated area. The calcium from the eggshells is good for your garden.
- Get real. If you insist on growing brassicas in warmer weather, you will get caterpillars on them. Not only are the butterflies multiplying, the brassicas themselves are not as healthy as they are in cooler weather, and therefore attract more pests. As evidence, I give you the photo below of one of my winter-grown broccoli, which is entirely unmarked.
But I Want to Kill Them!
Go out in the early morning and you will find the caterpillars sitting on top of the leaves, usually near the ribs. Pick them off and feed them to your chooks, or throw them onto the lawn for the magpies as Gavin does.
That's Yucky! I Want a Nice Civilised Pesticide!
Probably the most appropriate pesticide to use is Bacillus thuringiensis, sold as B.t. or Dipel. These bacteria attack caterpillars only, nothing else, and Dipel is therefore permitted in most organic certification systems. According to the MSDS, it's about as safe as you can get with a pesticide. Now go away and get over yourself.