Some might consider guests like these to be pests. I only see them as baby Black Swallowtails.
I love them. I plant extra dill, fennel, carrots, and parsley just to entice them into the garden.
The host plant usually survives despite looking quite scruffy. This fennel is actually doing quite well considering the many caterpillars climbing on it.
You can't have the beautiful butterfly without the very hungry caterpillar.