Ian,
Resistance can come from two sources: 1) Itīs already there, in some
individuals, and 2) It comes out from mutation (a change in the genetic
code). In general, a chemical like a pirethroid (Flumetrin, fluvalinate)
doesnīt CAUSE such mutations.
Resistant individuals, for some reason, donīt reproduce as rapidly as
non-resistant ones, but if you take out most of the non-resistant, it
will give a tremendous advantage to the resistant individuals to
multiply.
Tere
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