Chris Slade wrote

"Could it be that in the south there are more days of flying weather and
crops
to fly to?  Adult infested bees are then more likely to spend more time in
the field and die there, spending less time rubbing shoulders with
vulnerable
youngsters in the hive than those in less favoured areas."

I think my old friend Chris, and my new friend Medhat are right. I never
needed to worry about THOSE mites either in Southern California or South
Africa. Sometimes (not often) I would see signs of defecation on the hive
(yellowish spots that run slightly), but that was usually associated with
high mite levels. If the mites are under control, then in climates where
the bees can "use the bathroom" and are generally healthy, I've never seen
a problem with Tracheal Mites, and I'd be glad for someone to reply who has
seen a problem under these conditions.