In the back of my garden, I have a small patch of fennel (Foeniculum vulgare) that grows year-round and stands five feet tall. Sometimes I’ll harvest the fronds for a salad or a bulb for my favorite seafood stew, or even the pollen or seeds for my cooking, but for the most part, I let the fennel grow “wild” here.
Aside from being edible and beautiful with wisps of anise fragrance wafting through the air, it also happens to be a beneficial plant that functions as a trap crop.
What does it trap? Parsleyworms, the striking caterpillars that eventually turn into swallowtail butterflies.
West of the Rockies, parsleyworms are the beginnings of the anise swallowtail butterfly. Its eastern version is the black swallowtail butterfly, and while the butterflies look distinctly different, their caterpillars and chrysalises are nearly identical.
Parsleyworms are so named because they feed on members of the parsley family, Apiaceae, including parsley, dill, carrot, Queen Anne’s lace, and of course, fennel.
They can be found in all stages of life on these host plants, and it’s truly fascinating to revisit your plants each week to see their natural transformation. As the parsleyworm eats and grows, it sheds an exoskeleton and emerges with a completely different appearance. In total, it goes through five instars (stages) of life.
Here’s the first (or possibly second) instar…
The third instar…
The fourth instar (where it turns into the plump black-and-green striped caterpillar that you may be most familiar with)…
And the fifth and final instar, also known as the chrysalis stage. It’s from here that the parsleyworm pupates and emerges as an adult butterfly.
Since swallowtail butterflies lay their eggs on any member of the Apiaceae family, you might occasionally find them on your carrots or parsley as well. They have large appetites, so unless you planted enough for yourself and the butterflies, it can be a depressing sight to see your food crop munched down to its stems by an army of hungry caterpillars.
Planting a separate crop of fennel (or any related member) can help preserve the butterflies in your garden. I especially like Florence fennel, or finocchio, a bulbing variety that gives you a vegetable, an herb, and two spices over its lifespan. It’s a cool-weather crop that can be planted after the last frost for a spring harvest, or mid to late summer for an autumn harvest.
If you find a parsleyworm feeding where it shouldn’t be feeding, you can simply relocate it to your fennel patch and at the end of the season, you’ll still have some bulbs to harvest. A win all around.