I have a player that wants to have a bee king as a familiar.
Now I have a problem: What do I have to calculate as Familiar Bonding Level?
Size-15 (the bee king himself) or Size+1 (the complete swarm)?
What would you do?
And if I take the king alone... what happens? He is so small, that the bonding level is negative! What are the consequences?
Definitely just the king! A swarm is a group of creatures; a familiar must be a single individual.
Nothing, really, except that it's so extraordinarily easy to bind that a newly taken apprentice could do it in a foreign aura in a season full of distractions 
Unlike what happened in previous versions of Ars Magica, the low level does not translate into stronger cords etc.
That said, keep in mind that a creature as small as a bee, and with such a low Might, makes for a very, very vulnerable familiar.
Thank you. I wasn´t sure.
I just changed the bee king radically and increased his might. In physical combat he is weak, of course, but at least his bee colony is around him continually.
A bee queen as a familiar is not practical for a simple reason: It will NEVER leave its beehive (with the exception of the mating flight) as it is busy producing offspring all day and all night.
So unless the PC is a beekeeper, and happy to have a stationary familiar that can not possibly assist him in any way, that's really not a good choice for a familiar.
Even then, I sincerely doubt that one could befriend a bee queen, even a magical one. I't will likely have other priorities during its mating flight. Digging it out of the beehive is unlikely to have an endearing effect...
If your player wants a swarm of bees to do its bidding, said player is better of using Cr(Re)An.
Create a swarm where- and whenever you need it, no need to take care of it and no risk of loosing your familiar to a random bird or torchflame.
Just to check, the player wants the ruler of a hive as the insect that is their familiar (as in RoP:M page 66)? Not a person with the "bee king" virtue from The Lion and the Lily (a person with faerie blood that gives them power over bees)? Just checking
The question really runs on how you expect the magic bee of virtue to work, and how different from real-world bees a magic one is. In the real world, the ruler of a hive doesn't leave it, so is ideal for a mage who wants to leave their familiar in the covenant and uses powers in the bond at long distance. It's terrible for someone who wants to carry their familiar around as they travel.
Good discussion! Helpful! Here are some more informations:
The Magus is a Tremere. He had an interesting Twilight experience and since this, insects don´t bite him any more.
Then he visited a forest with some magic and faerie spots. In a village he met a bee colony, got in touch with the insects and looked in awe about this little hard-working eusocial flyers... "a typical Tremere animal", he said... and I had to admit.
In Ars Magica the head of a bee colony is always called a king, not a queen. I think the reason is Plinius, the ancient thinker, who wrote about bee kings. Anyway, I´m talking about the insects, not about a human with the "bee king" virtue.
Now our Tremere is searching for a bee king of virtue to bind as a familiar.
Fury is right, I think. The bee king will never leave his colony (for security, I even gave him a lesser malediction: always followed by his colony). So, if the magus is on a bigger journey, he will be without his familiar (or he has to invent some special spells, that will enable him to do that...we´ll see). But darkwing is right, too: the colony can live at the window of the magi´s tower... maybe even in the sanctum. The bee king could provide help in the lab, the colony can be useful in the case of defense, and can use powers at long distances.
OK, so we have a Tremere apiarist. This is a first, I guess 
He should invest in a couple of bee skeps, just in case he needs to move his familiar some place safe. Or convenient. Or closer to the covenants fields 
No magic required, just a bit of logistics, though magically enhanced logistics are probably a thing for Tremere.
Why do I picture a hovering bee skep directed by a bee king now 
Maybe a Daemon or spirit familiar? If a Daemon familiar goes splat, at least it isn't dead.
Is it repeatedly shouting "Pollinate! Pollinate!!!" in a weird voice?
Why do I picture the beeking with a black helmet and hearing the music of the Death Star played two octaves higher as the flying Bee skep approaches ?
Yikes! 
That could get messy. Hopefully there's a sweeping angel nearby to clean up the trail of pollen left behind while no one is looking. :mrgreen:
Wait, until the bee king casts his circle spells with his pheromones! That will teach you a lesson! 