Watching the neighbouring hives in the apiary over the past couple of weeks (that sounds far more passive than it actually is) I've been struck by the ongoing collapse of one of the colonies.

All 6 of our colonies have been treated the same way. This one is showing all the signs of collapsing from Varroa. There are large quantities of DWV (deformed wing virus) and K-Wing virus and since they did make attempts to swarm, the brood side of the AS has gone rapidly downhill.

Prior to the AS we'd culled drone brood, using icing sugar, monitored mite drop and uncapped drone brood with no obvious warning signs in terms of mite levels and, of course, they were treated over winter.

All of the other colonies in the apiary are doing fine.

So are there (still) colonies that are more sensitive, if that's the right description, to the viruses carried by Varroa (DWV seems to be the obvious one) than perhaps the mite itself?

Is it a combination of the two? The bees are less hygienic/able to cope with the mites and/or are less immune to the viruses.

Are we just crap beekeepers who missed the obvious signs in 20% of our colonies? (not discounting this one either).