From my few runs of Qarn I think that Final Sting is triggered both by HP (bees hp going below a certain threshold) and time. I'm not really sure about the inverse function of HP, at least not for the Teratotaur adds. Those can hit for 2k even at full health.

As it was said previously, Final sting can (and should) be stunned. Even so, mark them so your DPS focus fire one of them, then the other one.