hi Sounds to me like chilled brood, I get this sometimes in the spring, the colony is expanding rapidly, and I suppose the ratio of bees to brood area is lower than at any time of the year, the cold snap then causes the bees to contract in the hive and the peripheral brood becomes chilled and dies. My bees fly off with these white pupae and deposit them away from the hive. One thing I have noticed is the pupae are always quite well formed. My question is, are the earlier stages more resistant to chilling ? As I have never seen early brood being carried out. Steve