The parsnip seed project looks to be a success this year. Plants left in the ground over winter are happy to turn the energy stored in their delicious roots into vigorous growth in the spring. The parsnips were one of the first things to green up in the spring, and now they have the record for the tallest plants in the garden.
Parsnips don’t have many pests because the plant manufactures its own insecticide, chemical furanocumarins. Their roots are much more resistant to pests than are carrots over the winter. Hence, I was surprised when I discovered quite a few caterpillar nests in the emerging seed blooms a few weeks ago. It turns out parsnips do have a specific pest, the parsnip webworm, that can be a problem. The caterpillars spin webs that hold the flower umbels together in a tight silk/parsnip-flower nest. The developing flowers provide food for the worms, and the nest provides protection from predators.
Prof. Berenbaum and her group at the University of Illinois have studied parsnip webworms in some detail. They have a very informative web page with lots of information on these creatures. Notable is discussion about a recent invasion of the parsnip webworm in New Zealand. Parsnips have been grown in New Zealand for about 150 years without the webworm pest. Over time, the parsnips have lost some of their chemical defenses to their enemy. As a result, the plants in New Zealand are being devastated, with plants often producing no seed at all. It is expected that this strong selection pressure will quickly produce plants with more furanocumarins in the flowers to provide some resistance to the webworms.
After figuring out what I was dealing with, I ripped apart some of the webs and picked out the worms. But I didn’t get them all. The larva eventually burrow down into the center of the main stalk where they pupate and eventually hatch out as adult moths. I am no longer seeing the larva, so presumably they are all hanging out inside the stalks at this point. I’ll investigate further when it’s time to harvest.
The worms ate some seed, but there is still plenty left to make a good crop.