The Parsley Worm
The parsley worm is a caterpillar that can grow to be up to two inches long. It has a green, segmented body with yellow dots evenly spaced among black stripes. The caterpillar can often be found with orange horns. The parsley worm is actually the larvae of the black swallowtail butterfly.
Although it feeds on many common garden plants of the carrot family, including parsley, celery, dill, and parsnips, the parsley worm is not considered to be highly destructive. It generally only causes real problems in garden beds that are already distressed. Like the tomato hornworm, in reasonably sized gardens it can be controlled by picking it from plants. When doing so, it might be best to use gloves as the caterpillar of the black swallowtail butterfly, when it is aggravated, will push out two orange horns (called an osmeterium) and spray a foul smelling chemical (a terpene).
When they are being excessively destructive, the parsley worm can be controlled using Bt (Bacillus thuringiensis), which is a naturally occurring bacteria. It multiplies in the system of the caterpillar when ingested and kills it. When the larvae dies, it spreads the bacteria to other caterpillars. This treatment is highly effective and safe for other insects, including bees. It is so effective, in fact, that it should not be used anywhere near where butterfly gardens are installed.
The black swallowtail's scientific name is Papilio polyxenes. It begins its life-cycle as an egg which has been laid, usually on its preferred food (members of the carrot family). After taking 3-5 days to hatch, it will feed. In the first instar, or stage, the small caterpillar grows to be about a centimeter and a half long (little more than half an inch). At this point, it is dark black with a white band in the middle. The caterpillar also exhibits spikes. It will go through several more instars culminating in the familiar green black and yellow (as shown in the image above). Finally, the caterpillars will form a chrysalis or cacoon and pupate for 9-11 days (or over winter) into the adult butterfly stage, at which time it will take wing, pollinate flowers, mate and begin the cycle all over again. The parsley worm will go through one to three life cycles, depending on the climate.
The adult black swallowtail butterfly is about 8-11 centimeters wide. Based on its gender it will have a series of white (male) or blue (female) dots near its tail.