"Flight
Two broods; early May and then in late July/August with the adults flying throughout August. A third brood may occur in the southern counties (Ebner, 1970)."

It depends on how cold your area gets. If it is warm in September, they might emerge in August. Some years areas get an additional brood, if the weather allows.

Here is a link that might be useful: Black Swallowtail, Papilio polyxenes

I have 1 now that has yet to hatch. Last year I had one hatch in October.

Thanks for the responses.

Martha

Martha, I had some Black Swallowtail cats pupate last summer in July, and the adults did not eclose until April of this year. I overwintered them in my garage. You just never know with those crazy Swallowtails. They eclose when they are good and ready. Don't give up on them! ;o)