We use pupal when we talk about a pupa: that quiet middle stage in an insect's life. After the young insect has eaten enough, it makes a hard or silk cover and stays still inside. It is not a baby any more, and it is not the flying adult yet. That in-between time, and anything that belongs to it, is pupal. A scientist might say "the pupal stage" or "a pupal case." You will almost never hear this in the street; it lives in science class, nature books, and careful writing. Though actually, people who keep butterflies use it too, because they need a clean word for "while it is in the cover." It is an adjective. The insect itself is a pupa, not "a pupal."