Hi all

We frequently hear that varroa are unlikely to develop resistance to organic acids, the way they do to conventional miticides. New research reports the presence of bacteria associated with varroa that can metabolize oxalic acid. Natural selection would favor such a symbiotic relationship that would enable mites to survive despite oxalic acid treatments. 



The following quotes are used for review purposes only



> Oxalic acid, legally or brevi manu, is widely used to control phoretic Varroa destructor mites, a major drive of current honey bees’ colony losses. Unsubstantiated by sanctioned research are rumors that in certain instances oxalic acid is losing efficacy, forcing beekeepers to increase the frequency of treatments. This investigation fathoms the hypothesis that V. destructor associates with bacteria capable of degrading oxalic acid. The data show that indeed oxalotrophy, a rare trait among bacteria, is common in bacteria that we isolated from V. destructor mites. This finding may have ramifications in the use of oxalic acid as a control agent.



> Oxalic acid is remarkably effective against varroa (Gregorc et al. 2002). Unsupported by scientific literature are rumors circulated by some beekeepers that oxalic acid is losing efficacy, and that they need to treat more often than they used to in the past. … our data show that indeed, associated with at least the V. destructor that we harvested, there are oxalotrophic bacteria capable of using oxalic acid as a sole carbon source. Therefore the possibility exists that varroa mites can develop resistance to oxalic acid via the acquisition of microflora capable of degrading it.



> It was concluded that the V. destructor mites that we sampled harbor a diverse array of members of the genus Burkholderia and, in agreement with the underlying hypothesis of this investigation, some of them are capable of degrading oxalic acid. Taking these collective considerations lead us to hypothesizethat varroa mites may actively seek association with bacteria capable of degrading oxalic acid in order to shelter from its toxic effect.



Maddaloni, M., & Pascual, D. W. (2015). Isolation of oxalotrophic bacteria associated with Varroa destructor mites. Letters in Applied Microbiology.



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