My monarchs are eating dill or parsley! What's going on?

What you are looking at is not a monarch, since monarchs only eat milkweed (genus Asclepias). We get this observation and question quite often, because the caterpillars of the black swallowtail butterfly resemble a monarch caterpillar. Black swallowtail larvae feed on a variety of different host plants, but cultivated garden plants like dill, parsley, celery, and fennel are a few on the list. If you come across a caterpillar that you don't know, try identifying the host plant that it is on. This may help you in determining what Lepidoptera (butterfly or moth) species you are dealing with!