They are probably Black Swallowtails, since they are eating your carrot tops. The damage they might do to the carrots depends on how much they've eaten and how far along the carrots were. They also eat parsley, dill, fennel, and Queen Ann's Lace. This batch of butterflies will probably make their chrysalis and not emerge until next spring, so they'll need to be kept outside in the cold to prevent them from emerging too early. How many are there and how big are they?

Martha

My wife counted 17 last night. They range in size from maybe 2 inches long to maybe 4 inches or so...all the same thickness. They are black and yellow striped.

a couple stalks of carrots are completely bare now except the stems.

I don't think many would make it if they cocoon in the garden and obviously don't want them destroying my carrots as we are growing organic for the baby's food.

Do you think it's a good idea to put them in an aquarium (out of direct sun of course) outdoors under the deck to keep them protected?

I don't know how to care for them but I could spare a few carrots for them to eat and my wife has an abudance of purple cone flower which they've loved all year that I can put in with it.

They should crawl off any day now to do their thing. If they are that big, I'd just let them be.