Insight from Master Beekeeper Research
By: Brandon Shannon, Greg Rogers, James Rash, & David R. Tarpy
The full peer-reviewed publication is available (not open access) at https://doi.org/10.1080/00218839.2026.26975721. Please email bps0055@auburn.edu to request a full access link to the peer-reviewed article.
Key Findings
-OA-extended treatments were effective in early season, but not late season in North Carolina
-Cutting OA-extended treatments into smaller strips resulted in more consistent control of Varroa
-OA-extended treatments did not have a strong knockdown effect, and would be better used as a preventative (not therapeutic) treatment
Introduction to OA-Extended for Varroa management
In the United States, beekeepers report losing about half of their colonies every year2. Beekeepers report the largest contributor to these colony losses is the Varroa destructor mite and the viral pathogens that it vectors3. Varroa levels must be managed to maintain colony health, but Varroa exhibit resistance to all U.S.-registered synthetic treatments4–6.
Many of the organic treatment options — such as formic acid, oxalic acid, and thymol — are used for short-term treatments. These short-term treatments require multiple applications when brood is present, because reproducing mites are protected by the wax cappings of capped brood cells. To combat this, many beekeepers use an extended-release oxalic acid treatment, hereby called OA-extended, to provide continuous Varroa control beyond the period in which mites are protected by cell cappings.
At the time of this study, OA-extended was only available if beekeepers made their own homemade treatments, off label. But there is now a registered OA-extended treatment available, called VarroxSan®. This study aimed to answer two questions regarding OA-extended treatments:
1. How does timing of OA-extended treatment (Summer vs Fall) affect mite control in the southeastern U.S.?
2. How does cutting homemade OA-extended treatment pads (cutting in 2 halves vs cutting in 4 quarters) affect mite control?
Field Experiments
Experiment 1: Early Season
Two apiaries were established in Buncombe County, NC with 40 and 36 colonies each, with a 1-deep, 1-medium 10-frame Langstroth orientation. Each colony was assigned to one of four treatment groups:
1. Negative control (sham pads containing no OA; 8 colonies)
2. Positive control (formic acid treatment Formic Pro RV following label specifications — one pad followed by a second pad 10 days later; 34 colonies)
3. Homemade OA-extended pads cut into two equal half sections (two of 3.5” x 8”; 15 colonies; Fig 1A)
4. Homemade OA-extended pads cut into four equal lengthwise quarter sections (four of 1.75” x 8”; 19 colonies; Fig 1B)
Treatments were applied for 75 days between May 30th, 2023, and August 14th, 2023. Alcohol washes of 300 bees were performed 1 day prior to treatment application and 1 day after treatment removal.
Following treatment, the colonies with the quarter-cut OA-extended strips were the only group that had significantly lower post-treatment Varroa infestation than the negative control (Table 1). The formic acid and half-cut OA-extended treatments were intermediate, between the negative control and quarter-cut OA-extended.
Using the 3% infestation treatment threshold recommendation that was in place during this study7, only 11% of colonies in the quarter-cut OA-extended treatment group needed additional mite interventions. On the other hand, 75% of negative controls, 61% of formic acid treatments, and 31% of half-cut OA-extended treatments were above the treatment threshold. Note that the 2026 revised treatment thresholds have been decreased to 2% in Summer and Fall8.
Experiment 2: Late Season
Two apiaries were established in Ashe County, NC with 24 and 7 colonies each, with a 1-deep, 1-medium 10-frame Langstroth orientation. Each yard was divided randomly into two treatment groups:
1. Negative control (sham pads containing no OA; n = 15)
2. Homemade OA-extended pads cut into two equal sections (two of 3.5” x 8”; n = 16)
Treatments were applied for 14 days between September 1st and September 15th. Alcohol washes of 300 bees were performed immediately before treatment application, and then 6 days following treatment removal.
Sticky boards, made from Vaseline-coated manila file folders placed under screened bottom boards of each colony (Fig 2), were collected daily for the 14-day treatment period. OA-vaporization treatment was performed as a cleanup treatment once per week in the 3 weeks following treatment, with mite drop monitored in the same way for the 48-hour period following vaporization.
During the 14-day treatment period, treatment colonies had significantly higher mite drops than controls (Table 2). However, there was no difference in mite drop following any of the three OA-vaporization cleanup treatments. Additionally, there was no significant difference in the post-treatment mite infestation (Table 3), suggesting that there was neither a knockdown nor a stabilization effect.
Conclusion
We answered our two questions using combined evidence of these two experiments:
1. How does timing affect OA-extended?
Early-season OA-extended treatment can help beekeepers maintain (but not reduce) mite loads within colonies below treatment thresholds. On the other hand, there was no knockdown or stabilizing effect of OA-extended when used in late-season, even though starting mite levels were similar to the early season.
2. Are OA-extended quarter-cut strips better than half-cut strips? The OA-extended strips cut into 4 quarters did not have significantly different post-treatment Varroa infestation compared to OA-extended strips cut into 2 halves. However, the 4 quarter strips showed a trend for reduced post-treatment mite levels and had three times fewer colonies that were above the treatment threshold compared to the 2 half strips.
It should be stated that off-label use of oxalic acid is not permitted by beekeepers and should be avoided to minimize potential safety hazards to beekeepers. However, this study provides support for Summer use of registered OA-extended products, such as VarroxSan. The current EPA VarroxSan label (March 17, 2026) permits use of VarroxSan while honey supers are on.
Further study will be needed to determine what regions this approach may be helpful, since different climates and timing of the treatment seem to have a significant impact on efficacy, which only underscores the importance of regular monitoring of individual colonies for Varroa populations and following Integrated Pest Management (IPM) principles. This project also demonstrates the strength of collaboration between researchers and the beekeeping community to address timely and critical challenges in the apiculture industry.
About the Authors
Brandon Shannon earned his PhD from The Ohio State University in 2025 and is a postdoctoral fellow at the Auburn University Bee Center researching strategies for Varroa management.
Greg Rogers is a North Carolina Master Craftsman Beekeeper who owns and manages Haw Creek Honey Company in the mountains of Asheville, NC.
Jim Rash is a North Carolina Master Craftsman Beekeeper who manages 47 hives in Ashe County, NC and teaches introductory beekeeping courses at Wilkes Community College.
David Tarpy is a Professor of Applied Ecology and the Extension Apiculturist at NC State researching honey bee biology and behavior to better improve the overall health of queens and their colonies.
References
(1) Tarpy, D. R.; Rogers, G.; Rash, J.; Shannon, B. Timing of Oxalic Acid Extender Pads Is Critical for Varroa Mite Control. J. Apic. Res. 2026, (in press). https://doi.org/10.1080/00218839.2026.2697572.
(2) Giacobino, A.; Steinhauer, N.; Brunner, S.; Garcia-Andersen, N.; Aurell, D.; Bruckner, S.; Rogers, S.; Williams, G. R. Preliminary Results from the 2025-2026 US Beekeeping Survey: Honey Bee Colony Loss and Management, 2026. https://apiaryinspectors.org/US-beekeeping-survey-25-26.
(3) Aurell, D.; Bruckner, S.; Wilson, M.; Steinhauer, N.; Williams, G. R. A National Survey of Managed Honey Bee Colony Losses in the USA: Results from the Bee Informed Partnership for 2020–21 and 2021–22. J. Apic. Res. 2023, 63 (1), 1–14. https://doi.org/10.1080/00218839.2023.2264601.
(4) Rinkevich, F. D. Detection of Amitraz Resistance and Reduced Treatment Efficacy in the Varroa Mite, Varroa Destructor, within Commercial Beekeeping Operations. PLOS ONE 2020, 15 (1), e0227264. https://doi.org/10.1371/journal.pone.0227264.
(5) Pettis, J. S. A Scientific Note on Varroa Destructor Resistance to Coumaphos in the United States. Apidologie 2004, 35 (1), 91–92.
(6) Kamler, M.; Nesvorna, M.; Stara, J.; Erban, T.; Hubert, J. Comparison of Tau-Fluvalinate, Acrinathrin, and Amitraz Effects on Susceptible and Resistant Populations of Varroa Destructor in a Vial Test. Exp. Appl. Acarol. 2016, 69, 1–9.
(7) Honey Bee Health Coalition. Tools for Varroa Management: A Guide to Effective Varroa Sampling & Control (Eighth Edition), 2022. http://honeybeehealthcoalition.org/ (accessed 2026-04-01).
(8) Honey Bee Health Coalition. Tools for Varroa Management, 9th Edition, 2026. https://honeybeehealthcoalition.org/wp-content/uploads/2026/06/HBHC-Guide_Varroa-Mgmt_9thEd.pdf (accessed 2026-06-22).