UNDER THE MICROSCOPE I BROUGHT TO YOU BY DNATURE
By John Mackay
The genetic mechanisms behind synthetic pyrethroid resistance in varroa have been known for over 10 years – since the first description of DNA mutations in the sodium channels of varroa was made in 2013. Varroa treatments that fall into this class of chemicals include Bayvarol® and Apistan®. Since this time, other mutations have been found to be linked to resistance, but interestingly all mutations are in a very small area of DNA – a so-called ‘hotspot’ of mutations. This area of DNA codes for the ‘pocket’ where the pyrethroids bind to affect the channel. A mutation means no pocket and no pyrethroid binding.
However, little was known about resistance to amitraz treatment products (Apivar® and Apitraz® in New Zealand) until recently. Amitraz resistance was described in various USA locations in 2020 using bioassays (think Pettis tests) coupled with treatment failure reports. The key varroa target for amitraz, an octopamine receptor gene, was described in 2021 and showed why amitraz killed varroa but didn’t harm bees that carry similar receptors. Investigating this target gene for sequence variations showed two different mutations on this target gene – one in varroa found in Europe and one in a different location on the gene of varroa found in the USA. The mutations are known by the location of the mutation – Y215H is the mutation prominent in the USA, and N87S in European varroa.
More recently, a follow-up report reaffirmed Y215H as the cause of resistance to amitraz in the USA when the presence of the mutation was confirmed in nearly three-quarters of all varroa showing resistance when exposed to amitraz chemical in a container/Pettis test.
Bioassays such as those can be difficult to perform and they rely on a high background level of live varroa to be present in the sample. As such, DNA-based assays can be more easily applied, but do have the drawback that they are generally highly targeted to the mutation(s) of interest. That’s not such an issue for the highly targeted mutations of pyrethroid resistance, but may be more important for any amitraz resistance reported here.
To go with our pyrethroid resistance assays, at dnature we have recently developed rapid screening tests for both amitraz mutations, Y215H and N87S, down to the level of a single varroa.
However, the presence of mutations in different areas of the octopamine receptor gene suggests that other mutations may also cause treatment resistance, as compared to the highly localised mutations for pyrethroid resistance. Simply put, identifying amitraz resistance might not be so simple.
Suspect resistance?
We should definitely be looking for it though so, if you suspect resistance, keep the varroa from your alcohol/sugar/CO2 shake and notify the supplier of the treatment. The manufacturers want to know of any suspected resistance issues and are obliged to act on reports. Once you’ve stored the (labelled) samples, an alternative treatment can be immediately applied. The DNA based tests for resistance mutations can be applied to the dead varroa from all monitoring regimes.
Beekeepers complain the manufacturers “don’t do anything about resistance”. Why? Because the suppliers and manufacturers receive very few reports of it!
Report it and remember, keep the varroa from your monitoring that made you suspect resistance in the first place.
John Mackay is a molecular biologist and the technical director of Gisborne-based lab dnature diagnostics and logistics, as well as a hobby beekeeper.
