I have been watching them for years...this year they have eaten 99% of the parsley and fennel already. I grow plants for them. Some seem to be going into pupa early, and then emerging in a shorter period of time...5-7 days. Is this because their food supply is so low and when they emerge as butterflies there are plenty of food sources available? What role do light and temperature play?
Good sources should be plentiful this summer, as plants are doing well with all the rain. There are normally 2 generations a season. If they emerge early compared to other years, it may be because they are so robust going into pupation.
However, we are not wildlife experts. We recommend you search online or ask at a butterfly website.