Here's an action shot.........

Those are the caterpillar for the black swallowtail butterfly. Don't hurt them, the butterflies are beautiful!

Haha.. they also love your garden veggies, especially parsley an dills.

Butterflies have an ugly past.

As mentioned, black swallowtail butterfly caterpillars.

They love dill and fennel... also carrots and parsley I think. Lots of people grow fennel just to give these guys a host plant.

I grow rue to give them a host plant. But rue needs to be FAR away from basil, and it needs poor soils and to be kept dry. Any dill, fennel, basil etc, is planted on the other side of the house.

The rue is usually eaten down every year to the nubs by this caterpillar, and grows back like clockwork.

Baths, why far from basil?

I have first instars on my fernleaf dill and fennel. The dill is small, so I'm keeping an eye on it and will move them if they get too destructive.

I was going to pull the fennel. It's a zefo fino that just refuses to bulb up. It's not bolting, but it's not fattening at the base, no matter how much dirt I push against it. So I figure now I'll leave it for these caterpillars, pull it once they're gone. Maybe it'll form a bulb by then, LOL.

Yes it is the caterpillar of the black swallowtail.

Wild carrot (Queen Anne's Lace) and wild parsnip are other alternate host plants and easier to grow than rue. In fact they are weeds.

Unfortunately, . the alternate host plants seem to be just that - alternates. The butterflies apparently much prefer to lay their eggs on carrots and Florence fennel and on cultivated herbs like parsley and dill.

One cultivated host that really seems to draw them in is bronze-leaf fennel. It's also a pretty rank perennial grower once it is established, so you don't have to worry much about the caterpillars causing permanent problems.