Hello!
Pupa hibernates at temperatures around +3 C. Cocoon in the winter should not be much wetter, but must be wetter in the spring and especially in April when just before hatching.
Jevgenij
Pupae of dubernardi usually don't hibernate at all and I don't recommend anybody to do this, if he does not want to lose livestock. Also I don't believe that cocoons survive under -20C! In my personal test they resist something around -8-10C below zero and die, if frost is stronger! Pupae must be in wet conditions all the time, if you don't want lose livestock. The main conditions initiating diapause in this species are light and humidity, but not temperature like ArGeNtUm wrote! He lost his livestock very quickly as I know.
If you have any questions concerning Actias dubernardi, you can contact me.