"Usurpation swarms" is something that I have never experienced here in the
UK.

Our native bees frequently retain drones well into the autumn as they are
more prone to supersede and therefore need drones on the wing; mating
flights can go into September or even, exceptionally, into early October.
Yellow bees usually throw out their drones around the end of July - earlier
if there is a dearth of forage.

Late swarms are usually new queens on mating flights due to supersedure.

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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