" It is not a dominant trait, like aggression in African bees, which tends to move into the population and take hold."

I do not understand the definition of dominant in this context.  Surely you do not mean dominant in a genetic sense.  Dominant (or recessive) in a genetic sense has nothing at all to do with weather the frequency of a gene in a population increases or decreases.

There are several reasons AFB resistance might decrease in frequency in a population unless constantly selected for.  For instance few mutants just have one single effect.  Suppose that the AFB resistant mutant also causes some other phenotypic effects.  Perhaps it results in queens that lay only 90% as many eggs as queens that are wild type at this locus.  Or perhaps it increases swarming.  The bee breeder is going to select for queens that lay the most eggs or queens that produce non swarmy colonies.  The result would be rapid loss of AFB resistance unless the breeder also constantly selects for such resistance.

I think I have read that Africanized drones tend to out compete pure European drones in the mating routine.  If this is really so a few Africanized drones in a breeding area are going to pass on a disproportionate number of their genes, including the genes for aggression.  Those aggressive genes might, or might not, have a thing to do with the drones out competing during mating.  That is, they might be part of the cause for out competing or they might simply be along for the ride.

In both of these cases I have picked simple, easy to understand examples that explain the observation.  Those examples I picked might be right or wrong.  They are simply reasonable guesses.  We even see such effects in humans.  For example, the recessive mutant that causes blue eyes in humans only happened some 8000 years ago and it happened in only one single person.  Yet today about 8% of the human population has blue eyes which means a whole lot of people with brown eyes also carry the recessive blue eye trait.  For this mutant to have spread thru the whole human population in only 8000 years and increased its frequency from one single person 8000 years ago to hundreds of millions of people today it must confer some significant reproductive  benefits to a whole lot of people.  I doubt if those reproductive benefits are only due to blue eyes being more attractive to the opposite sex.  More likely there are other factors in addition to the blue eyes that resulted in the increased frequency of this gene.

Dick

" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken

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