Hi Tom,
Varroa mites resistant to Apistan are also resistant to Bayvarol.  In fact, this cross resistance it one of the strongest indicators in the field apart from actual bioassy that you are in fact dealing with resistance not just a bad batch of control strips.  You will need to use something else with a different mechanism of action to control such varroa.  Options include formic acid and other organic acids such as oxalic acid and thymol as well as the coumaphos which you mentioned.  As for where the resistance came from - much speculation of course but likely the varro population that was introduced to UK already had a fair amount of resistance to pyrethoids.  The most likely source of mites of course is the mainland where such resistance seems to be pretty widespread now.

With the use of coumaphos is the resulting contamination of all wax and the accumulation of coumaphos in the wax.  Consider carefully which path to go down as the contamination issue is there and do remember that coumaphos resistance will arrise if you start to use it.  Yes we are using it here in the USA and it is still working well but it is much more toxic to the bees than apistan and does build in the wax so caution is advised.  We really need a control that does not leave accumulating residue in the wax combs.

FWIW

blane



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Blane White
MN Dept of Agriculture
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