In my experience, Black Swallowtails don't have a good egg-to-food ratio. They lay lots of eggs because, as you saw, predators keep them in check. But they tend to lay more eggs than even the other Swallowtails. It's like they can't judge the size of the plant correctly. FYI, only an average of 1-2 out of every 100 eggs makes it to a butterfly. About the only way to ensure better numbers is to bring them inside and raise them.
