> Pesticide resistance of a specie is the result of a more or less small
> proportion of the population: Of the billions of individuals that form a
> specie there are always some that are slightly different as a result of
> errors in the replication of their genetic code. As a result there is
> always a small proportion of the bugs that will  survive to the pesticide
> and since they are the only survivors able to reproduce they will
transmit
> this resistance to their descendants and soon populations of the pest
will
> be back to normal or even cause problems worse than before.
 
The major problems exist when you have irresponsible individuals either
placing too low a treatment level, reusing treatments after expiration, or
leaving treatment on after the specified time length.  The reduction in
pesticide levels allows mites that would likely be killed by a correct
level to survive the lower levels of medication.  If the medication is used
properly the likelihood of this happening is reduced although not
eliminated.