Efficacy of oxalic acid vaporisation against Varroa mites
Varroa mites (Varroa destructor) are a major threat to honey bee health worldwide. Oxalic acid has become popular among beekeepers as a potential control agent. This study evaluated oxalic acid vaporisation under field conditions.
Study methods
Twenty-four Italian honey bee colonies (Apis mellifera ligustica) in Taiwan were divided into six groups of four. One group was untreated, four received two, four, six or eight oxalic acid applications at two applications per week, and one received flumethrin strips (40 mg per strip). The experimental dose was about 2 g of oxalic acid per hive per application. By day 35, the six- and eight-application groups showed reductions in infestation of 86.78% and 97.65%, respectively, according to Table 2.
Results and discussion
Worker mortality was significantly higher in colonies receiving four, six or eight applications than in untreated controls. Six or eight applications at the experimental dose (2 g per hive) reduced infestation substantially under these field conditions, with an accompanying increase in worker mortality.
Colony standardisation
Italian honey bee colonies (Apis mellifera ligustica) naturally infested at varying levels were used. A week before the trial, the colonies were standardised using the Liebefeld visual estimation method, to approximately 2,500 workers per frame and 50% brood area (Bhusal et al., 2011; Dainat et al., 2020). The method estimates the number of adult workers on each side of a frame and the areas occupied by open brood, capped brood, honey and pollen stores.
Treatments and monitoring
The experiment compared an untreated control, four oxalic acid (OA) groups receiving two, four, six or eight applications, and a flumethrin strip group (40 mg per strip, Table 1). OA was applied twice weekly, on Sunday and Wednesday at 15:00, over periods of 7–28 days. Infestation was assessed twice weekly. The paper gives different weekdays for mite counts in different parts of the methods.
The researchers also monitored whether the queen was present in each trial hive.
Effect of oxalic acid on Varroa mites
At day 35, the percentage reduction in Varroa infestation was significantly greater with eight OA applications than with any other regimen or control (F = 165.84, df = 5, P < 0.01). Figure 3 shows no significant difference between the flumethrin group and the four- or six-application groups. This comparison is between groups.
Compared with baseline, infestation declined significantly by day 35 in all four OA groups and the flumethrin group (each P < 0.01). In untreated controls it rose significantly (P < 0.01). The relative reduction ranged from 35.42% to 97.65% across the OA regimens, whereas controls recorded a 51.77% increase (Table 2).
Bee mortality
By day 35, total worker mortality was significantly higher after four, six or eight OA applications than in controls (F = 14.64, df = 5, P = 0.01), with no significant differences among those three OA groups. Mortality in the flumethrin group differed significantly from those three groups. It did not differ significantly from the two-application or control groups (Fig. 5).
The groups showed three patterns. Mortality was highest with four, six or eight OA applications, intermediate with flumethrin or two OA applications, and consistently lowest in controls. The number of dead workers in controls was about half that in the four-, six- and eight-application groups.
Conclusions
Table 2 reports reductions of 86.78% and 97.65% after six and eight applications. The discussion gives 82% and 98%, so the table values are used here. The precise mode of action remains uncertain. Proposed mechanisms include exposure to airborne OA and contact with crystals. Schneider et al. (2012) suggested that OA may stimulate grooming, which can dislodge mites.
Contact with oxalic acid has been investigated as an explanation for mite mortality (Aliano et al., 2006; Aliano and Ellis, 2008; Papežíková et al., 2017). Wallner and Fries (2003) proposed a role for lower pH. Contact toxicity and changes in bee behaviour may contribute to the observed effect, but the trial did not establish their respective roles or prove that the exposure is harmless to bees.
Final considerations
Oxalic acid vaporisation is among the methods studied for managing Varroa. Under the conditions of this trial, six or eight applications at 2 g per hive twice weekly reduced infestation by day 35. This was an experimental regimen in groups of four colonies. Worker mortality increased with the more frequent regimens. The low cost and availability of OA are practical advantages, but the trial does not establish general superiority over other treatments. Field use must follow authorised product instructions and applicable operator protection requirements.
Further research could compare application methods, timing in relation to brood cycles and combinations with other integrated pest management strategies. The aim is to protect colony health and productivity over time while limiting selection for acaricide resistance.
Published January 2026
Authors:
Pierre Enock Philémon, Lekhnath Kafle - Department of Tropical Agriculture and International Cooperation, National Pingtung University of Science and Technology, Taiwan
Ravindra Chandra Joshi -
Entomology, Philippine Rice Research Institute, Philippines
Full article and author details: JOURNAL OF APICULTURAL SCIENCE VOL. 70 NO. 1 2026
Translated by Davide Simeone
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