Oxalic Acid Varroa Mite Control
Numerous studies have investigated using oxalic acid to control Varroa mites
Clarence Collison
In order to get a potential acaricide registered and legal to use in the beekeeping industry, it takes several years of research to show that it is not hazardous to humans and safe to the environment, does not leave residues within the products of the hive, is not harmful to bees and brood and is effective against the target pest, in this case varroa mites. Besides the safety factors, it has to be determined how it will be most effectively applied to the pest population, what dosages are efficacious and what conditions may limit the effectiveness of the product. One difficulty associated with oxalic acid, was the finding that the acid does not penetrate brood cell cappings. Unfortunately, during active brood rearing in the colony, from ½ to ⅔ of the mites are found in capped brood cells. In addition, it was found that oxalic acid can be detrimental to larvae in open brood cells. Therefore, this treatment is most effective when the colony is broodless.
In the past two decades, the parasitic mite Varroa destructor has become harder to control with synthetic acaricide chemicals due to genetic resistance. Toufailia et al. (2015) determined the efficacy of the natural chemical oxalic acid (OA) in killing phoretic mites on adult worker bees under field conditions in southern England. They compared three OA application methods (trickling, spraying, and sublimation) at three or four (sublimation) doses, using 110 broodless colonies in early January 2013. Treatment efficacy was assessed by extracting mites from samples of c. 270 worker bees collected immediately before and 10 days after treatment. All three methods could give high varroa mortality, c. 93-95%, using 2.25 g OA per colony. However, sublimation was superior as it gave higher mortality at lower doses (.56 or 1.125 g per colony: trickling 20, 57% mortality; spraying 25, 86%; sublimation 81, 97%). Sublimation using 2.25 g of OA also resulted in three and 12 times less worker bee mortality in the 10 days after application than either trickling or spraying, respectively, and lower colony mortality four months later in mid Spring. Colonies treated via sublimation also had greater brood area four months later than colonies treated via trickling, spraying, or control colonies. A second trial in December 2013 treated 89 broodless colonies with 2.25 g OA via sublimation to confirm the previous results. Varroa mortality was 97.6% and 87 (98%) of the colonies survived until Spring. This confirms that applying OA via sublimation in broodless honey bee colonies in Winter is a highly effective way of controlling V. destructor and causes no harm to the colonies.
Laboratory bioassays were performed to characterize the acute contact toxicity of oxalic acid (OA) to varroa mites and their honey bee hosts. Specifically, glass-vial residual bioassays were conducted to determine the lethal concentration of OA for V. destructor, and topical applications of OA in acetone were conducted to determine the lethal dose for honey bees. The results indicate that OA has a low acute toxicity to honey bees and a high acute toxicity to mites. The toxicity data will help guide scientists in delivering optimum dosages of OA to the parasite and its host and will be useful in making treatment recommendations. The data will also facilitate future comparisons of toxicity if mite resistance to OA becomes evident (Aliano et al. 2006).
Jack et al. (2020) tested the efficacy of oxalic acid (OA) vaporization and brood interruption in controlling varroa mites. Sixty experimental colonies were randomly assigned to one of six treatments with 10 colonies per group. The six treatments were: 1) OA applied once, 2) OA applied three times, 3) brood interruption, 4) OA applied once + brood interruption 5) OA applied three times + brood interruption, and 6) no OA or brood interruption. The OA was applied via vaporization, with each application being one gram OA applied through the hive entrance (label rate), on the bottom board. Brood interruption was accomplished by caging a colony’s queen in a queen cage for a period of 24 days. An additional 10 colonies were treated with amitraz (Apivar – positive control). Varroa mite levels were estimated before, during, and after treatment applications using sticky boards left in colonies for three days. Their data suggests that queen caging to achieve brood interruption during the Fall season can negatively impact colony strength and survival. They observed high colony mortality in some treatments, despite diligent colony management to alleviate the side effects of the treatments. Colonies treated with amitraz were healthier and had better survival than those treated with OA vaporization. In conclusion, OA and/or brood interruption did not provide sufficient Varroa control.
Few studies of honey bee colonies exist where varroa mite control is achieved by integrating broodless conditions, through either total brood removal or queen caging, in combination with oxalic acid (OA) applications. Gregorc et al. (2017) observed significant varroa mortality after total brood removal or caging the queens and OA applications in broodless colonies, as well as in colonies with brood that received four consecutive OA applications. In laboratory tests, they recorded higher mortality of caged bees exposed to Apistan® compared to oxalic acid or untreated control bees. However, this mortality is not believed to negatively impact the colony. They therefore recommend combining OA applications with artificial broodless colony conditions achieved either by brood removal or queen caging as an effective management strategy for varroa mites.
Papežiková et al. (2017) studied the effect of oxalic acid on isolated varroa mites and on varroa mites parasitizing caged honey bees treated with oxalic acid per os or topically (by trickling or by sublimation). They also studied the effect of oxalic acid (trickling and sublimation) on individual bees, focusing on their lifespan, midgut morphology and function, and Malpighian tubule morphology. Effect on mites: contact of isolated mites with oxalic acid coated surface (Petri dishes treated by sublimation) significantly decreased mite viability. In an experiment on varroa mites parasitizing caged bees treated with oxalic acid, the strongest acaricidal effect was observed following oral application and the lowest when oxalic acid was applied through sublimation. Effect on bees: oxalic acid applied by sublimation did not decrease bee lifespan over the 21 days of observation contrary to trickling, where a nonsignificant lifespan decrease was observed. Topical application of oxalic acid increased the rate of midgut cell apoptosis, with a stronger statistically significant effect seen in the group treated by trickling. However, neither trickling nor sublimation caused epithelial destruction in the midgut and Malpighian tubules or loss of digestive tract function.
The effects of oxalic acid administered by the trickling method on brood development of honey bee colonies were evaluated (a) by observing the development of marked cells of young (< three days old) and old (> three days old) larvae, and (b) by measuring the area of open brood for several weeks post application. Oxalic acid, dissolved in a 50% sugar solution, with an end concentration of 3% w/ oxalic acid, was applied twice by the trickling method during Summer to 10 colonies. A high percentage of young (12.6% and 9.5%) and old honey bee larvae (10.6% and 5.6%) were removed from their cells after the first and second oxalic acid applications, respectively. The surface of the open brood area was also reduced by 17.5% after the two oxalic acid applications and stayed low for about two months. For the same period of time the open brood area in 10 control colonies increased by 34.5%. The two oxalic acid applications removed 60 ± 12% of varroa mites adhering to adult honey bees, while the natural fall of mites measured in control colonies (for a period of 40 days) was 32 ± 4%. Combining the detrimental effect on brood development with the low relative effectiveness on varroa removal, oxalic acid application by the trickling method when open brood is present is not as safe as has been regarded in the past. Consideration needs to be given to the use of different sugar and oxalic acid concentrations in the treatment solution in order to minimize its adverse effects on open brood (Hatjina and Haristos 2005).
Two oxalic acid treatments were given to five colonies in Autumn and five colonies in Spring. In each treatment, colonies were treated every seven days for four weeks with a 3% sprayed oxalic acid. Another five colonies in each season served as controls and were sprayed only with water. Efficacy of oxalic acid in Autumn was 94% and in Spring was 73%. A long-term study of the colonies for three to four months after the last application of oxalic acid showed a statistically significant negative effect of the acid on brood development. In addition, three queens died in the treated colonies (Higes et al. 1999).
An organic product based on oxalic acid was evaluated for use in Varroa mite control under Spring/Summer climatic conditions in Argentina. The formulation consists of four strips made of cellulose impregnated with a solution based on oxalic acid. Forty-eight beehives were used to assess the product efficacy. Residues of the product were also tested in honey, bees, and wax. Each trial had respective control groups without oxalic treatment. At the beginning of the experiment, four strips of the formulation were applied to the colonies belonging to the treated group. Falling mites were counted after 7, 14, 21, 28, 35 and 42 days. After the last count, the strips were removed and colonies received two flumethrin strips for 45 days. Falling mites were counted throughout this period. Average efficacy of the organic product was 93.1% with low variability. This product is an organic treatment designed for Varroa control during brood presence and represents a good alternative to the synthetic treatments (Maggi et al. 2016).
Since varroa mites are now resistant to synthetic acaricides worldwide, oxalic acid was suggested as an alternative for Varroa control. Oxalic acid is one of the most common natural miticides used against varroosis by spraying and sublimation administration techniques. It is a natural constituent of honey, very active against the Varroa mite, safe to use for beekeepers, and has no residue problems. Nevertheless, some authors have predicted that the risk of developing resistance to oxalic acid in mites is high. The objective of this research was to assess the susceptibility to oxalic acid of a V. destructor population belonging to a commercial apiary where 64 consecutive control treatments with this acid were performed. Bioassays to assess the oxalic acid susceptibility were performed on two mite populations: (1) a ‘focal’ population consisting of mites previously exposed to oxalic acid treatments, and (2) a ‘naïve’ population that was never exposed to this acid, which allows setting a reference in the absence of historical data on our ‘focal’ mites. The results reported here suggest that the Varroa population exposed during eight successive years to oxalic acid treatments remains susceptible to this acid (Maggi et al. 2017).
Numerous studies have investigated using oxalic acid (OA) to control Varroa mites in honey bee colonies. In contrast, techniques for treating package bees with OA have not been investigated. The goal of Aliano and Ellis (2009) was to develop a protocol for using OA to reduce mite infestation in package bees. They made 97 mini packages of Varroa-infested adult bees. Each package contained 1,613 ± 18 bees and 92 ± 3 mites, and represented an experimental unit. They prepared a 2.8% solution of OA by mixing 35 g OA with one liter of sugar water (sugar:water = 1:1; w:w). Eight treatments were assigned to the packages based on previous laboratory bioassays that characterized the acute contact toxicity of OA to mites and bees. They administered the treatments by spraying the OA solution directly on the bees through the mesh screen cage using a pressurized air brush and quantified mite and bee mortality over a 10-day period. Their results support applying an optimum volume of 3.0 ml of a 2.8% OA solution per 1,000 bees to packages for effective mite control with minimal adult bee mortality. The outcome of their research provides beekeepers and package bee shippers guidance for using OA to reduce mite populations in package bees.
References
Aliano, N.P. and M.D. Ellis 2009. Oxalic acid: a prospective tool for reducing Varroa mite populations in package bees. Exp. Appl. Acarol. 48: 303-309.
Aliano, N.P., M.D. Ellis and B.D. Siegfried 2006. Acute contact toxicity of oxalic acid to Varroa destructor (Acari: Varroidae) and their Apis mellifera (Hymenoptera: Apidae) hosts in laboratory bioassays. J. Econ. Entomol. 99: 1579-1582.
Gregorc, A., M. Alburaki, C. Werle, P.R. Knight and J. Adamczyk 2017. Brood removal or queen caging combined with oxalic acid treatment to control varroa mites (Varroa destructor) in honey bee colonies (Apis mellifera). Apidologie 48: 821-832.
Hatjina, F. and L. Haristos 2005. Indirect effects of oxalic acid administered by trickling method on honey bee brood. J. Apic. Res. 44: 172-174.
Higes, M., A. Meana, M. Suárez and J. Llorente 1999. Negative long-term effects on bee colonies treated with oxalic acid against Varroa jacobsoni Oud. Apidologie 30: 289-292.
Jack, C.J., E. van Santen and J.D. Ellis 2020. Evaluating the efficacy of oxalic acid vaporization and brood interruption in controlling the honey bee pest Varroa destructor (Acari: Varroidae). J. Econ. Entomol. 113: 582-588.
Maggi, M., E. Tourn, P. Negri, N. Szawarski, A. Marconi, L. Gallez, S. Medici, S. Ruffinengo, C. Brasesco, L. De Feudis, S. Quintana, D. Sammataro and M. Eguaras 2016. A new formulation of oxalic acid for Varroa destructor control applied in Apis mellifera colonies in the presence of brood. Apidologie 47: 596-605.
Maggi, M.D., N. Damiani, S.R. Ruffinengo, M.C. Brasesco, N. Szawarski, G. Mitton, F. Mariani, D. Sammataro, S. Quintana and M.J. Eguaras 2017. The susceptibility of Varroa destructor against oxalic acid: a study case. Bull. Insectol. 70: 39-44.
Papežiková, I., M. Paliková, S. Kremserová, A. Zachová, H. Peterová, V. Babák and S. Navrátil 2017. Effect of oxalic acid on the mite Varroa destructor and its host on the honey bee Apis mellifera. J. Apic. Res. 56: 400-408.
Toufailia, H.A., L. Scandian and F.L.W. Ratnieks 2015. Towards integrated control of varroa: 2) comparing application methods and doses of oxalic acid on the mortality of phoretic Varroa destructor mites and their honey bee hosts. J. Apic. Res. 54: 108-120.
Clarence Collison is an Emeritus Professor of Entomology and Department Head Emeritus of Entomology and Plant Pathology at Mississippi State University, Mississippi State, MS.
