A few tracheal mite related knowns, backed either by published literature or collected data:

Only workers hours to a day plus post emergence are attractive to adult female tracheal mites seeking a new host after bailing out of the trachea of an older bee.  Vegetable oil makes masks the compounds attracting the questing mites.

Resistant and susceptible young workers are equally attractive to migrating mites, but resistant ones groom the mites off more effectively with their second pair of legs.  Impairing that action makes a resistant worker susceptible.

The genetics of resistance does not appear to be a simple single gene.  Crosses of resistant and susceptible bees are mostly intermediate between the two parents.

In the first few decades after introduction of mites, stocks with more recent European influence (US Buckfast, Yugoslavian, "Russian") were resistant, while all stocks in the US and Hawaii were susceptible.  Susceptible colonies developed very high infestations in the winter, more so the further north.  Resistant stocks kept mite levels very low in all seasons and hardly had peaks of infection during the worst months in the north. 

The situation started changing after two decades of host/parasite association.  At least in some places in was rather abrupt.  For my own research it was easy to find highly infested colonies around Baton Rouge until the early 2000s.  It became a real challenge to find them after that, even searching in areas in the north where they had been extremely problematic. 

The original formulation for Amitraz (Miticur) was labelled for both tracheal and varroa mite control.  Formic acid is also effective against tracheal mites.  The use of both predates the decline of tracheal mites in the early 2000s.  In my view, the enigma remains.

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