Lindsay, 3 yrs.ago i had a hive similar to yours, it was a strong hive that started to dwindle. The brood all hatched out, then no eggs or larva, i went through the hive twice looking for the queen (non found), so i put another queen in (the 3 hole box kind), checked back three days later found her dead out side of landing board? checked for a queen again (non found)? So i tried putting a frame of eggs in ( to different times) to get them to raise a queen (didn't happen). By this time the hive had dwindled to two frames of bees, so i looked again for a queen, yep, i found her she was about the same sizes as a worker bee, but her pointed tail gave her away. That gives support that a virgin queen can't lay an egg, after thinking about it, i remembered we had a rainy two weeks when she should have been bred when this all started? As for a laying worker knowing if she is laying a non fertile egg or not? i don't think they know, it's just a strong servivle thing in there make up. JMHO. Jack