Tracheal mites were common in the U.S. and normally took off in most 
untreated colonies until about the middle of the 2000 decade.  They 
became harder to find after that, but once in a while pockets of 
colonies with high prevalences popped up, particularly in colonies kept 
through long winters.  Curiously, using standard bioassays exposing 
young workers from test colonies at the same time as those from 
resistant and susceptible standard colonies, showed that susceptibility 
was still pretty common in U.S. colonies of various lineages, even at 
the time that colonies in the field showed low infestations.  There has 
been speculation that widespread use of Amitraz has had an effect, 
perhaps in combination with some selection for resistance.  It is still 
possible that we are missing a more arcane explanation.  I have found 
tracheal mites in our hobby colonies in Colorado but not at dangerous 
levels.  And we do not treat at all for varroa since levels stay below 
threshold.  In this case the explanation is probably pretty clear: very 
short seasons and running queens of VSH lineages.

 

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