How to Control Carrot Worms (Black Swallowtail Caterpillars)
Carrot worms or more specifically black swallowtail caterpillars will feed on the leaves of carrots, parsley, dill, fennel, parsnips, and any other plants in the carrot family. If left unchecked, they can defoliate a carrot plant in short order. If provoked or under stress the swallowtail caterpillar will show their osmeterium, which is an orange forked gland which will release a foul odor to repel predators.
1. Alliums make great companions for carrots, which can repel the worms. Alliums would be garlic, onions, chives, and leeks.
2. Rotate your carrot crops. If you grow your carrots in the same place year after year, you will have more pest problems.
3. Grow your carrots in a polyculture. Don’t make it easy for the pests to find your carrots by growing them in nice neat grocery store rows.
4. Encourage natural predators such as predatory wasps by planting lots of flowers blooming at different times of the year.
5. Avoid insecticides, as they will knock out your caterpillars for a time, but they will also repel your natural predators.