Are increasing honey bee colony losses attributed to Varroa destructor in New Zealand driven by miticide resistance?

The most devastating pest to honey bees (Apis mellifera) worldwide is the parasitic mite Varroa destructor. The development of miticide-resistant mite populations has been a major driver of colony loss in many countries. We investigated the threat Varroa poses to honey bee populations in New Zealand and tested the effectiveness of the two most popular chemical treatments used by beekeepers. Colony losses reported by New Zealand beekeepers have risen over five consecutive years from 2017 to 2021, as have the proportion of losses attributed to Varroa, with this parasite found to be the main driver of colony loss in 2021. Varroa resistance to miticide treatments flumethrin and amitraz was tested. The concentration of flumethrin required to kill 50% of the mites (LC50) was 156 g/g, 13 times greater than the adjusted LC50 value of 12 g/g observed in a trial also conducted in New Zealand in 2003, thus indicating evidence of developing mite resistance to flumethrin in New Zealand. Molecular analyses searching for mutations in the Varroa genome known to be associated with flumethrin resistance found no evidence of such mutations, suggesting that any extant resistance to flumethrin has evolved independently in New Zealand. No evidence of resistance to amitraz was found, as the LC50 value of 12 g/g was lower than what was observed in the 2003 trial (110 g/g). Further development of integrated pest management, such as gene-silencing RNA interference (RNAi) and selective breeding of Varroa-resistant bees, is needed to effectively manage a parasite that threatens global agriculture.