From: Melanie Filotas IPM Specialist, OMAFRA; Sean Westerveld Ginseng and Medicinal Herb Specialist, OMAFRA; Alex Harris, summer student University of Guelph-OMAFRA

Each summer, herb growers may notice brightly coloured caterpillars eating their parsley, dill and related crops. This is likely the parsleyworm Papilio polyxenes asterius, which is the immature stage of the black swallowtail butterfly. Young parsleyworms are dark brown or black, sometimes with lighter patches but as they mature develop a distinctive colour pattern ranging from light yellow-green to more green in older larvae. Larvae have black and white/pale green bands going horizontally across the body and yellow spots also running across the body, on top or near the black bands

Parsleyworm caterpillar

Adults of this species first appear in mid to late May, feed briefly on the nectar of various flowers, then mate and lay eggs near or on parsley, dill, fennel or other members of the carrot family. The hatching larvae feed constantly as they develop to adulthood, becoming more obvious as they grow. In Ontario, there are 1-2 generations per year, depending on temperature. Typically, larvae of the first generation will be present on crops from June to July, and those of the second generation in late August through early October.  However, we observed pupating parsleyworms as recently as last weekend, suggesting the cool spring and dry summer may have offset their development somewhat.

There are no known thresholds for parsleyworm in Ontario, however low populations can generally be tolerated and control is usually not necessary.  On occasion, this insect can be economically damaging in localized areas of fields if populations become sufficiently high. In small plantings, caterpillars can be hand picked from plants. Parsleyworms are often attracted to stressed plants, so ensure plants have adequate nutrients and water and are protected from other more significant pests.