If you've ever grown broccoli, cabbage, kale, or cauliflower you know this insect. We've all experienced this. You're happily washing your head of broccoli or kale leaves when you come across a green caterpillar. Worse yet, if you miss them you end up having a little protein in your veggie dish.
The cabbage worm, and related cabbage looper, are common insects that attack all the Brassica or broccoli-family crops. They start in early summer as a harmless looking white butterfly flitting around plants. What it's actually doing is laying single eggs on the underside of the leaves. The eggs hatch and the small green caterpillars start munching. They grow and eat quickly forming holes in the leaves and getting into the heads and flowers. Eventually, they pupate in the ground. There's usually a few generations a year.
Luckily, there are many ways to thwart the cabbage worm. Our best solution is to cover the broccoli-family plants with floating row covers or tulle fabric. This prevents the butterfly from laying eggs and plants mature under the fabric. Of course, check under the cover periodically to be sure no butterflies have emerged in your bed. Also, plant a diversity of flowers and herbs in your garden.
We've seen more parasitized cabbage worms lately due to wasps that feed on them. You can check the underside of the leaves every few days to crush eggs and young caterpillars so they don't get established. If they get out of hand, spray the organic Bt pesticide. This bacteria parasitizes the caterpillars. It also kills all butterfly larvae so use it carefully.