> Exactly!  It therefore follows that if numbers are kept low, by  whatever 
> means, the chances of selection of resistance by mixing genes is  slight; 
> however that still leaves selection for resistance by mutation.

That is one avenue, however I understand that in a population large enough 
with some degree of genetic diversity there are bound to be freaks which 
exhibit unusual characteristics.  Maybe, naturally, they are one in a 
million or fewer.

Given that they are very few in number and perhaps do not pass the 
characteristic on reliably, and additionally that they tend to be less fit, 
as long as normal populations predominate, these freaks are diluted and 
perhaps, probably, outcompeted.

If we repeatedly weed out the 'normal' mites, the frequency of the freaks 
increases as a percentage of the population.  Under the right conditions 
that number can became large, like 99%.

It is a statistical situation. 

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