It appears that varroa mites are not the only insects developing resistance
to pyrethrums (Apistan's fluvalinate is a synthetic pyrethroid). The
February 2000 issue of Consumer Reports magazine on page 49 has an article
head lice resistance to various medications. They say:

"Researchers at the Harvard School of Public Health recently reported that
head lice collected from children in Massachusetts and Idaho are no longer
killed by permethrin, the active ingredient in "Nix," the nation's
best-selling lice shampoo. The researchers didn't test pyrethrum, the active
ingredient in other over-the-counter lice shampoos, but it's likely that
lice are growing resistant to it as well because it's so widely used."