If you find caterpillars on your vegetables that look like these you’re most likely dealing with the Large White butterfly:
Photo by Lazarus Churchyard.
It’s a quite common butterfly in Europe but pay attention if you see any of these hovering around your precious rows of radish or cole crops:
Photo by jpockele.
Colourful flowers with nectar attract the adult butterflies. They emerge in April and May and one butterfly lays between 20 to 100 small yellow eggs, that hatches into caterpillars in about 2 weeks. The first generation pupates in June and the second in July.
Fortunately there are a couple of natural enemies who will help save your vegetables. The parasitic wasp called Apanteles glomeratus lays its eggs inside the caterpillars which will eventually be eaten from the inside and die. Birds like Starlings will also try to keep a natural balance, so it’s a good thing to put up nesting boxes for insectivorous birds.
You can also do some manual regulation by crushing the caterpillars with your fingers. Do this in July to keep their numbers down. If you find infected old dead caterpillars just leave them where they are. They will act as a breeding ground for more caterpillar parasites.