By Rogan Takach
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. Setting up the lab has gone well, and we have secured funding to conduct multiple applied Varroa mite trials this coming summer in addition to investigating the impacts of sunflowers on nearby apiaries. Beginning in January, we will also welcome our second outstanding graduate student to the program. In the next newsletter, I will provide further insights into our upcoming research studies. Overall though, everyone in the lab is excited to get going on a hopefully productive 2026 field season! With that said, I wanted to take the time this newsletter to highlight some exciting research going on elsewhere that might be of interest to commercial beekeepers.
Background
Varroa mite control is one of the most difficult things for beekeepers to manage throughout the year, and beekeepers are always interested in getting a leg up on Varroa control whenever they can. Unfortunately, most Varroa mite treatments are most effective when limited capped brood is present. While brood breaks present an optimal time for treatments, commercial beekeepers who go to almonds may only see a full brood break coming out of sheds in January or not at all if they venture south from North Dakota in the fall. That is why a recent publication from the Washington State University (WSU) Bee Lab caught my eye. In this study, they used winter storage sheds in the spring, after almonds, to induce a brood break to increase Varroa treatment efficacy. Here, I will highlight their work including the benefits and limitations of this beekeeping strategy.
What is the publication?
The publication is titled “Induced brood breaks by refrigerated bee storage in spring: an effective strategy for Varroa destructor (Mesostigmata: Varroidae) control in honey bee colonies,”. It was published open access for anyone to read October of 2025 in Journal of Insect Science, and the publication can be read here (LINK). Authors on the publication were Briana Price, Riley Reed, Taylor Reams, and Brandon Hopkins.
What was the study?
The study was performed in Twin Fall, ID in 2018. The WSU Bee Lab took 110 double deep colonies that had been performing commercial pollination services for almonds in California and split them up into various treatment groups. Of the 110 colonies, 71 colonies were moved to a refrigerated storage facility at 45°F for 18 days while the remaining 39 colonies were placed in an outdoor apiary. After the 18 days, colonies in the refrigerated storage facility were removed and placed in the outdoor apiary. At this time, all colonies were sampled for Varroa mite infestation with an alcohol wash, and they were given one of three treatments. Treatment groups included oxalic acid (OA) vaporization with 1 g of OA, HopGuard ® II, and “amitraz EC” where amitraz emulsifiable concentrate was mixed with canola oil and allowed to soak into shop towels before application. Colonies were then resampled after 37 days to see how effective the various
treatments were.
So, did the brood break increase Varroa control?
Yes. Varroa infestation rates in colonies that went into refrigerated storage and had a brood break decreased from an average of 4.7 to 1.3 mites per 100 bees. Meanwhile, colonies that were placed in an outdoor apiary and did not have a brood break increased in Varroa infestation rate from an average of 2.9 to 3.8 mites per 100 bees (Figure 1). While likely not extremely surprising, this was comforting to see that inducing a brood break after commercial pollination can assist in increasing Varroa treatment effectiveness.
Were there differences between the different Varroa control treatment groups?
Yes and no. In colonies that went into refrigerated storage and received a brood break, the HopGuard ® II and amitraz EC treatment groups significantly reduced Varroa infestations by approximately 75% or 87% respectively (Figure 2B&C). However, there were no differences in colonies that were in the outdoor apiary for either treatment with Varroa infestations actually slightly increasing when treated with amitraz EC. Meanwhile, OA vaporization failed to reduce Varroa infestations regardless of if the colonies were placed in refrigerated storage or not (Figure 2A). It is important to note that only 1 g of OA was vaporized. This would have been half the label dosage at the time which was 1 g per deep chamber, and the labeled dosage has since increased to 4 g per deep chamber. Dr. Cameron Jack at the University of Florida Honey Bee
Research and Extension Laboratory has evidence that higher dosages of OA vapor have a larger impact on Varroa mites, albeit in single deep colonies. Still, it is interesting to speculate whether a higher dose of OA may have had a larger impact at least in colonies placed in refrigerated storage.
So, what are the drawbacks?
Well for one, you must have access to refrigerated storage facilities after almond pollination. Additionally, this option is unsuitable for operations that go south for queen rearing and splitting after pollination. A further complication lies in resource consumption. This study found no impact on frames of bees per colony. However, colonies coming from almond pollination have begun rearing brood and building up for the year. Putting them directly in refrigerated storage complicates that, especially if you are looking at doing it for an extended period longer than 18 days. In this study, they found a minor impact on colony weight where colonies in refrigerated storage lost 5 kgs over 18 days. In speaking with a couple of commercial beekeepers who have tested this tactic, starvation is a primary concern as colonies use up resources to maintain
functionality during this period. While starvation was not an issue for colonies in this study, it is something to be cognizant of if attempting this method.
What’s the take home message?
Varroa control is a fickle beast, and beekeepers must do what they can to alleviate the constant issue. Utilizing refrigerated storage in the spring after almond pollination presents an interesting opportunity to increase Varroa treatment effectiveness and stay ahead of the game when it comes to seasonal Varroa control. While potentially not possible for all colonies or beekeeping operations, this method presents an additional management technique that beekeepers may be able to implement for a subset of colonies understanding some of the potential limitations regarding resource consumption.
With that, I want to thank you for reading and stay tuned for the April newsletter where I will highlight our Lab’s upcoming research for 2026!
Rogan Tokach
Assistant Professor
North Dakota State University
(701) 231-5761
rogan.tokach@ndsu.edu