Hi, Al,

        I'm not sure I understand you right, perhaps I've missed part of the
argument. What evidence has been presented (and by whom) to show that
pyrethroid-resistant varroa are not different genetically from susceptible
strains?

In Europe, pyrethroid-susceptible and pyrethroid-resistant varroa mites
exist side by side, within the same population. The resistant individuals
arose from the "original" susceptible population due to high selection
pressure through overuse of pyrethroids. However, not all varroa in Europe
are resistant, there is a cost involved and where the selection pressure is
low, pyrethroid-resistance will not appear.

The resistant individuals have more or less the same gene pool as the
susceptibles but, like all biological material there is some natural
variation. Not all susceptible mites have the ability to become resistant as
they do not posess the correct character make-up. Those individuals
posessing a character for pyrethroid-tolerance at higher doses are selected
for when there is increased selection pressure (presence of pyrethroid).

There is bound to be genetic variability at some level in varroa mites as
with other species. Even with inbreeding there are occasionally mutations.

We looked at the potential resistance mechanisms involved and a paper was
published by three of my team:

 Ref: Elke Hillesheim, Wolfgang Ritter and Denis Bassand (1996). First data
on resistance mechanisms of Varroa jacobsoni (Oud) against tau-fluvalinate.
Experimental & Applied Acarology, 20 (1996) 283-296.

At least in Western Europe, the resistance is in part due to an increased
ability in the mite to detoxify the pyrethroid toxin. More detail is given
in the paper. If anyone on the list is aware of any further work that has
been done on the resistance mechanism(s) please contact me.


Max



Dr Max Watkins

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