A showy caterpillar with a taste for herbs
At this time of year, herb growers may be noticing 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.
Parsleyworms are initially, dark brown or black, sometimes with lighter patches and are 2 mm-2 cm long. Older caterpillars have 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 (Figure 1). Fully grown caterpillars will be around 4-5cm long. Adult butterflies are 7-11 cm wide and have mostly black wings with numerous white/yellow and blue spots, with two orange spots and two black tails at the rear end of the wings. Eggs are small, spherical, often white or pale yellow and turn brown when close to hatching.
Figure 1. A late instar parsleyworm caterpillar on dill
The parsleyworm overwinters in the pupal stage. Adults emerge in
mid to late May, feed briefly on the nectar of various flowers,
then mate and lay 200-440 eggs near or on parsley, dill, fennel
or other members of the carrot family. The eggs hatch within 3-7
days, and larvae feed constantly as they develop through five larval
instars. At this time of year the caterpillars are maturing and
should be more obvious on the plants. Mature larvae move to nearby
branches or leaves, form a chrysalis and pupate. Adults emerge in
10-14 days. In Ontario, there are 1-2 generations per year, depending
on temperature. 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
Scouting for parsleyworm is typically not necessary. If chewed plant material is observed as part of normal scouting activities, closely examine plants for presence of caterpillars. In plots with a history of damage, you can also scout for eggs, which may be present as soon as host plants begin growing. Parsleyworm damage is often mistaken for that of other insects.
There are no known thresholds for parsleyworm in Ontario. Low populations can generally be tolerated. The parsley caterpillar is not always a problem in Ontario herbs, but can be economically damaging in localized areas of fields if populations are sufficiently high. Consequently, management is often not necessary for this pest. 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.
For more information: