Possible cause?

[Finman]

OK, thank you for all your replies, will re-investigate. Checked my records, they were treated for varroa Nov, Dec to early Jan.

Too late. Right time to treat , when the colony rears its winter bees in September. Then it is necessary. Mites have violated the wintering bees in November. And then the summer bees die.

Winter treatment helps, that mite load does no become too high during next Summer.