The Plant & Insect Diagnostic Clinic recently received a sample of an unusual caterpillar feeding on dill flowers from Johnson County, IA. The Johnson County Extension Office correctly identified the culprit as the caterpillar stage of the purple carrot-seed moth, Depressaria depressana. We've had one earlier inquiry from Story County about caterpillars feeding on dill plants being grown for black swallowtail caterpillars. Purple carrot-seed caterpillar is a new invasive insect pest of dill that we have not previously encountered.
This invasive insect is native to Europe and was first documented in the northeastern U.S. in 2009. It has since spread over much of the eastern U.S. and Canada, including many midwestern states. For example, purple carrot-seed moth was reported in Wisconsin in 2018. Thirty-six specimens of the purple carrot-seed moth have been photographed and cataloged from seven different Iowa locations from 2017 to 2020 on the Insects of Iowa website.
Caterpillars of the purple carrot-seed moth feed on the flowers and unripe seeds of plants in the parsley family, including dill, carrot, celery, parsley, parsnip, coriander, fennel, caraway, and cumin. The caterpillar spins a messy web around the flowers where it feeds and pupates. The caterpillars may also feed on the leaves. The caterpillar is greenish-red with conspicuous white spots on each segment of the body.. The adult moth is about 3/8 inch a.nd dark brown with white scales on and around the head. For more picture of adults and larvae please see: https://bugguide.net/node/view/432767
Purple carrot-seed moth is a relatively new invasive insect and we do not yet know if it will prove to be a serious pest or an occasional irritation for Iowa gardeners.
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