I am Dr. Rogan Tokach the honey bee research and extension specialist at North Dakota State University. In this quarterly column, I will provide updates on what we are doing at the NDSU Honey Bee Lab, highlight lab members, or share interesting research findings from the honey bee world that might be relevant to North Dakota beekeepers.
I want to start by providing a brief update on the status of the NDSU Honey Bee Lab. It was great to see a lot of you at the NDBA Tri-State meeting in July. We are still plugging away on our research projects this summer and are in the process of finishing a couple up. Please stay tuned for future newsletters where I will report on some of the results from our various projects. I will also be at both CSBA/AHPA and ABF this year, so I hope to provide an update on results then as well. With that said, let’s get onto the article.
Background
With increases in reports of amitraz resistance (stay tuned for future results on that as I just finished a week of amitraz resistance sampling beekeepers across the state), many beekeepers have turned to using extend-release oxalic acid (OA) as an alternative treatment against Varroa mites. While extended-release OA is rarely capable of significantly dropping high Varroa infestations, they are typically great for keeping infestations low and are often used over summer during honey production with supers on. Whether it is VarroxSan, Canadian Varroa Destructor strips, or homemade OA pads, many North Dakota beekeepers have begun using extended-release OA within their operation. Consequently, the question has arose wondering, what other times of year could extended-release OA be utilized. Here, I would like to highlight the potential drawbacks of using extended-release OA during winter whether in sheds or outside in temperate climates.
What is the publication?
The publication is titled “Managing amitraz resistant Varroa destructor mites in honey bee (Apis mellifera) colonies: effects of winter treatment with extended-release oxalic acid.” It was published open access for anyone to read July of 2026 in Journal of Apicultural Research, and the publication can be read here (LINK). This was one of my chapters in my dissertation with the other authors being Dan Aurell, Frank Rinkevich, and Geoff Williams.
What was the study?
The study was performed in the Southeastern US in the winter of 2022-2023 in collaboration with a commercial beekeeper in the region. The beekeeper was chosen because their colonies had previously been tested for amitraz resistant Varroa populations, and this beekeeper had a history of only applying amitraz-based products for approximately ten years prior to the study. The goal of the study was to determine if treating with an alternative to amitraz when colonies were broodless during winter helped keep Varroa infestations low while incorporating a new mode of action to assist with control of amitraz resistant Varroa populations. In total, 94 single deep colonies were included in the study. Colonies were split evenly into three groups and given one of three treatments. Treatments included Apivar (active ingredient amitraz) with two strips per single deep, Aluen CAP (an Argentinian extended-release OA strip (very similar to VarroxSan)) with three strips per single deep, and a homemade extended-release OA glycerin pad with four SpillTech pads of 1:1 OA and glycerin placed on top of the frames underneath a migratory lid on each single deep. Colonies were assessed in November prior to treatment application, January, 67 days after treatment application, and February, 97 days after treatment application. Assessments included alcohol washes to determine Varroa mite infestation and frames of adult honey bees was recorded.
So, what was the amitraz resistance?
The beginning amitraz resistance level in these colonies was 38.5%. This included 38.5% for the Apivar treatment group, 36.3% for the Aluen CAP treatment group, and 40.7% for the OA Pads treatment group, respectively. This constituted moderate but not extremely high amitraz resistance levels which helped shape results covered in the next section.
Were there differences between the different Varroa control treatment groups?
Yes. Apivar performed the best out of all the treatment groups (Figure 1). It significantly reduced Varroa infestation rates between the first and second inspection. Even with the observed amitraz resistance levels, Apivar still proved to be effective. However, Varroa infestation rate was not completely dropped to zero, likely due to the continued survival of some amitraz resistant mites. Comparatively, Aluen CAP also performed fairly well reducing Varroa infestation rate over 97 days. Meanwhile, the homemade OA Pads were relatively ineffective failing to reduce Varroa infestation rate at all resulting in a significantly higher infestation rate compared to Apivar and Aluen CAP 67 days after treatment in January. One important note though is that beginning Varroa infestation rates were low across the board ranging from 0.55-0.61 mites per hundred bees meaning on average there were only 1-2 mites per alcohol wash. These low Varroa infestation rates at the beginning of the trial make it a little more difficult to determine true impact of the various treatments.
So, Varroa control was marginally impacted, but was there anything else interesting in this study?
Yes, the colony strength results! This is one of the first papers to definitively show the negative impact of using extended-release OA during a cold period. While the Aluen CAP treatment group did reduce Varroa infestation rates, it also coincided with a significant reduction in colony strength in terms of frames of adult honey bees compared to the other treatments (Figure 2). Consequently, colony survival was also lower in the Aluen CAP treatment group with ~25% of colonies dying in that treatment group compared to just one apiece in the other two groups. These results pair nicely with reports from other beekeepers who have used extended-release OA either in temperate California climates or in sheds. Multiple beekeepers have stated that extended-release OA has negatively impacted colony performance if used during these times and has resulted in upwards of 90% colony loss in some situations. Furthermore, these losses are reported from beekeepers both overwintering outdoors in temperate areas and in sheds, so these losses are not solely seen when extended-release OA is used in a shed environment.
There might be a question as to why we did not see as drastic of losses in this study compared to other beekeepers who left extended-release oxalic acid strips in colonies overwinter. While I do not have a definitive answer, I would hypothesize it was likely due to the climate. This study was conducted in a region that may only experiences a brief brood break and rarely has temperatures that require clustering for an extended period. These factors may have limited the time colonies were forced to cluster around strips which creates the negative environment that leads to honey bee death.
Why is extended-release OA killing honey bees in these conditions?
Long story short, honey bees are likely getting a toxic dosage of oxalic acid. I would recommend watching this video by Dr. Zachary Lamas detailing one of his studies on this topic (LINK). In these high moisture environments when colonies are huddled in sheds or in temperate environments, the glycerin in strips absorbs moisture thereby pushing oxalic acid out of the strips leading to a high dosage of oxalic acid being released. Interestingly, we did not see the same negative impacts on colony strength and survival from the homemade OA pads treatment group when pads were placed on top of the frames. However, this does not mean that homemade pads should be used in lieu of strips since beekeepers have also reported higher colony losses with pads or strips placed between brood chambers.
What’s the take home message?
Avoid using extended-release OA in honey bee colonies over winter at least until we know more. Secondly, be sure to remove as much extended-release OA strip remnants as possible as you prepare your colonies for overwinter. Extended-release OA remains a great treatment for colonies before and during honey production. It is the primary treatment currently being used to assist with keeping Varroa infestations in check over the summer. However, research has shown that beekeepers should avoid using extended-release OA alone as a remedy to treat colonies with high Varroa mite infestations or as a treatment during the overwintering period. In the future, I am hoping to do a project with a commercial beekeeper categorizing impact of using extended-release OA while in shed conditions to fully quantify how colonies are impacted.
With that, I want to thank you for reading and stay tuned for the next newsletter where I will highlight results from some of our Lab’s 2026 research!
Rogan Tokach
Assistant Professor
North Dakota State University
(701) 231-5761
rogan.tokach@ndsu.edu
Figure 1. Varroa per 100 adult honey bees by treatment group and experimental day. Capital letters indicate significant differences between treatments within each individual experiment day. Lowercase letters indicate significant differences between days within each treatment.
Figure 2. Frames of adult honey bees by treatment group and experimental day. Capital letters indicate significant differences between treatments within each individual experiment day. Lowercase letters indicate significant differences between days within each treatment.