Since more and more beekeepers are using oxalic acid to treat for mites, it’s important for PCBA members to understand the research behind the treatments so as to use it safely and effectively.  I researched scientific, peer-reviewed journal articles to find out more about oxalic acid treatments for honey bees.  This article is a synthesis by Frank Mortimer of Cornell University and does not contain any opinions.

As any good beekeeper knows, Varroa destructor is the number one threat to Apis Mellifera and is responsible for more deaths of honey bee colonies than any other contributing factor. According to Cornell University, since Varroa mites first entered the United States in 1987, they have become even more deadly to our colonies because the bees no longer can handle mite loads as high as they once could. Varroa is a carrier of multiple bee-killing viruses so it now takes fewer mites to devastate honey bee colonies, which is why monitoring mite levels and developing a yearly treatment calendar is essential to keeping honey bees alive.  Oxalic acid (OA) as a treatment for Varroa is still relatively new in the United States, with the EPA having approved it in March of 2015. As more beekeepers start using OA, more questions arise regarding how to use it effectively as a mite treatment.  Since OA is best used when a colony is broodless, one of the most frequently asked questions is in regard to outside temperature.  When is it too cold to treat? At what temperatures is OA still effective?  Because oxalic acid has been used to combat Varroa mites in Europe for decades, the author reviewed scientific studies and found that a lot of research has been done in European countries where OA has been used for many years. Some of that research specifically mentions temperature in regard to treatments. All the information that I’ve included comes from reviewed journal articles, meaning that the scientific community has deemed that the science behind these studies is accurate and valid.  In the first study, Rademacher and Harz (1) provide data supporting that OA can be sublimated (vaporized) when outside temperatures are greater than 2° Celsius (35° Fahrenheit). The article also states that the dribble method of delivering OA requires an outside temperature that is greater than 3° C (37.4° F).  Their research is a reminder that people in other countries think about temperature differently than we in WA might.  What’s cold in Sweden is certainly different than what’s cold in Egypt, yet Apis m. is native to both regions and can therefore tolerate both versions of what is considered cold.  While it might be hard to imagine dribbling any liquid onto bees when the outside temperature is only 37.4° F, it’s important not to anthropomorphize the honey bees, remembering instead that they are insects and temperature affects them differently than it does humans.   In the second study conducted by Rosenkranz, Aumeier, & Ziegelmann (2), data concluded that OA applied by trickling, spraying, fumigating, or as pure crystals (e.g. vaporized with heat) has an efficacy greater than 90% when colonies are broodless, and less than 60% when there is brood present.  More importantly, it answers our question by stating that “efficacy is independent from temperature.”  In other words, temperature has no impact on how effective OA is at killing mites.  It works no matter what the outside temperature is.  The third study by Charriére & Imdorf (3) simply recommends that beekeepers “carry out treatment at an outdoor temperature of above 0°C (32° Fahrenheit).”  The last study by Radetzki & Bärmann (4) appeared on the Swedish beekeeping site, “Alltom Biodling.”  What I found especially interesting about this study is that it shows the impact that the outside temperature has on the OA treatment’s effectiveness.  Most importantly, the effectiveness didn’t drop off until they treated at -2°C to 0°C (28.4°F to 32°F).  Since most people reading this article are not fluent in Swedish, below is a table from the study where the author added the English words for clarity. 

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In Conclusion, How Cold is Too Cold for Oxalic Acid?  
Based on a review of the scientific literature, treating with OA in any temperature above freezing — 0° Celsius, 32° Fahrenheit — will be an effective Varroa treatment if beekeepers use sublimation (vaporization).  The dribble method is also effective but only at a slightly higher temperature of 37.4°.  The research also supports that OA is most effective when a hive is broodless, which coincides with temperatures in NJ generally dipping down into the low thirties.  Therefore, to ensure its maximum effectiveness, beekeepers should schedule oxalic acid treatments during the winter months when brood levels are at their lowest point, knowing that they need not be as concerned about the outside temperatures as much as with keeping their bees healthy via low mite counts. 

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