Sorry, but I forgot something important.
Dan Holdworth mentioned a fairly popular theory that says that susceptible
parasites are better fit than resistant parasites (Why, otherwise, the
susceptible population would exist in nature instead of the resistant
population?). A common extension of this theory is to recommed to leave the
resistant parasites alone: since they are less fit, sooner of later they
would be replaced with the susceptible parasites again.
Although this sound right and may even be true in geological times, it does
not work in the time frame of interest to the farmers. There are many examples
that it does not work in real life but the most common might be the resistance
of the domestic fly to the DDT. The flies became resistant in the 1950s
and they are still resistant now, 40 years later.
Omar O. Barriga., DVM, PhD