Some sugar water via top feeders, until it gets cold. Then to keep them inside and warm I made sugar boards (see pics) they can eat on all winter long to keep up their energy. Much like Bitcoin, for bees - energy = heat and they detach their wings to vibrate their bodies and generate heat for the hive. They are incredible! 
https://m.primal.net/Lwza.jpg
https://m.primal.net/Lwzd.jpg