Once you get close to the house in the north of the garden, the wasp/mage will approach, start a shot conversation, then attack. Immediately after killing him, the queen bee should approach and give you the key, even if you accidentally killed some bees.
If you can not re-load and going near the house (in front of the stairs) doesn't trigger the cut scenes, you can download the inventory files with the key.
Lost Items - guide and directory of inventory files
