Any chance of a photo? It's most certainly not a worm, but a caterpillar, and, if it's not doing a lot of damage, it might even be something you would not want to kill.

People plant butterfly gardens all the time, and then go around massacring the caterpillars that would have turned into butterflies. Go figure!

If a photo is out of the realm of possibility, can you give us a more detailed description - length, width, skin texture, any markings.

About 1/2 inch long, white. I really try not to slaughter the caterpillars, but am suspicious about this one!They are in the flowers, not eating the leaves.

I read on the Perennials forum that they are horsemint caterpillars. Not desirable, I think. On the topic of caterpillars, this year I was delighted to find monarch cats on the fennel I planted years ago for their enjoyment.

Meant to write they are about 1/2 inch long or less, white and skinny. If I could figure out how to post a pic from my iPad, I would!

I suspect capecodder's fennel-loving caterpillars are black or anise swallowtails, not monarchs. I believe monarch caterpillars will ONLY eat milkweed (in fact, can only survive on milkweed), whereas the very, VERY similar looking black swallowtail caterpillars will eat fennel, dill, parsley, Queen AnneâÂÂs lace, and parsnips. Both are wonderful to have, but unless you have milkweed, you will not have monarchs.

Thank you, CT lady. I think you are right. I do have milkweed right next to the fennel but I don't think I've ever had any monarch cats. I'm happy to have swallowtails too.