When an insect pupates, it changes into a pupa. The soft, hungry young insect stops eating, finds a safe place, and becomes that still form with a special cover. Later it will come out as the adult. This is a biology word, not kitchen or street talk. Ordinary people often say "it turned into a chrysalis" for a butterfly, or "it's in its cocoon," though actually a cocoon and a pupa are not the same thing: the cocoon is the silk bag some insects spin; the pupa is the insect inside. Pupate is the exact verb for becoming the pupa.

化蛹