UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

 WASHINGTON, DC 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION

MEMORANDUM DATE: December 18, 2014 

SUBJECT: Oxalic Acid (PC Code 009601; DP Barcode D497398) 

FROM: Thomas Steeger, Ph.D., Senior Science Advisor 

 Environmental Risk Branch 4 

 Environmental Fate and Effects Division 

THRU: Jean Holmes, Branch Chief 

 Environmental Risk Branch 4 

 Environmental Fate and Effects Division 

TO: Reuben Baris, Risk Manager Reviewer 

 Venus Eagle, Risk Manager 

 Registration Division 

The Environmental Fate and Effects Division (EFED) has completed its review of the environmental fate 

and ecological effect data provided in support of the Section 3 registration of oxalic acid dihydrate (PC 

Code 009601; CAS 6153-56-6) for the in-hive control of the varroa mite (Varroa destructor) in honey bee 

(Apis mellifera) colonies. Data used in evaluating the product were provided by Health Canada’s Pest 

Management Regulatory Agency1

 as part of a work share between regulatory agencies, the Environmental 

Protection Agency’s (EPA) Reregistration Eligibility Document for oxalic acid2

, and open literature. 

Varroa mites have been characterized as the most serious pests in honey bee colonies3 as these mites feed 

on developing bees (larvae and pupae; brood) leading to brood mortality and the reduction of lifespan of 

workers that were parasitized during development4

, and the mite are considered a major cause of 

overwintering colony losses5

. Varroa can affect hypopharyngeal gland development of nurse bees and 

decrease brood/royal jelly production affecting nutrition in developing brood and queens6

. Although no 

data have been submitted to EPA to assess the likelihood of adverse effects on non-target organisms from 

the proposed use of oxalic acid to control Varroa on adult bees (i.e., for control of phoretic mites), EFED 

 

1 Health Canada. 2010. Oxalic Acid Dihydrate Proposed Registration Decision PRD2010-12. 

2

USEPA. 1992. Reregistration Eligibility Document (RED). Oxalic Acid. Office of Chemical Safety and Pollution Prevention formerly the 

Office of Prevention, Pesticides and Toxic Substances (H7508W) December 1992. 

http://www.epa.gov/opp00001/reregistration/REDs/old_reds/oxalic_acid.pdf 3

 USDA. 2013. Report on the National Stakeholders Conference on Honey Bee Health. Naitonal Honey Bee Health Stakeholder Conference 

Steering Committee. Alexandria, VA October 15 – 17, 2012. http://www.usda.gov/documents/ReportHoneyBeeHealth.pdf 4

DeGrandi-Hoffman, G., F. Ahumada, R. Curry, G. Probasco, and L Schantz. 2014. Population growth of Varroa destructor (Acari: Varroidae) 

in commercial honey bee colonies treated with beta plant acids. Exp. Appl. Acarol 64: 171 – 186. 

5 Guzmán-Novoa, E., L. Eccles, Y. Calvete, J. McGowan, P. G. Kelly, A. Correa-Benítez. 2010. Varroa destructor is the main culprit for the 

death and reduced populations of overwintered honey bee (Apis mellifera) colonies in Ontario, Canada. Apidologie 41: 443 – 450. DOI: 

10.1051/apido/2009076 6 Pinto, F d’A, G. K. Souza, M. A. Sanches, and J. E. Serrão. 2011. Parasitic Effects of Varroa destructor (Acari: Varroidae) on hypopharyngeal 

glands of Aftricanized Apis mellifera (Hymenoptera: Apidae). Sociobiology 53(3): 769 – 778. 

THOMAS 

STEEGER

Digitally signed by THOMAS STEEGER 

DN: c=US, o=U.S. Government, 

ou=USEPA, ou=Staff, cn=THOMAS 

STEEGER, dnQualifier=0000012192 

Date: 2014.12.18 13:12:59 -05'00'

Holmes, Jean 

2014.12.18 18:01:40 -05'00'
Page 2 of 6 

believes that the likelihood of non-target exposure is low given that the compound is proposed for use 

within the honey bee colony when brood are not present. While there are data to demonstrate that oxalic 

acid can be acutely toxic to adult bees, the proposed treatment rates have been established based on research 

demonstrating that oxalic acid will provide effective control of mites while minimizing adverse effects to 

adult bees. The available research indicates that in the absence of effective Varroa control measures, there 

is an increased likelihood of adverse effects to both individual bees and the whole colony due to parasitism 

of the mite. Therefore, based on the available information, EFED does not have any evidence that the 

proposed use of oxalic acid will be detrimental to the colony especially given that the use of oxalic acid is 

intended to reduce mite loads that could otherwise be far more detrimental to individual bee and colony 

survival. Given the in-hive use of the product and the absence of exposure to non-target organisms other 

than the honey bee, EFED has determined that the proposed in-hive use of oxalic acid will have no effect 

(NE) on federally listed threatened or endangered species and will not modify their habitat. 

Nature of the Chemical Stressor 

Oxalic acid is a dicarboxylic acid and its conjugate base (oxalate) is used as a chelating agent for metal 

cations7

. While the specific mode of action against V. destructor is unknown, it is hypothesized to 

physically injure the mites via the chemical’s low pH through direct contact with the liquid formulation or 

through a fine coating of small oxalic acid crystals that can result from the vaporization of the oxalic acid8

. 

The chemical occurs naturally as the potassium or calcium salt in the sap of many plants and vegetables, 

notably in those of the Oxalis and Rumex familes9

. Historically it has had a range of non-pesticidal uses 

(e.g., wood bleach); however, it was first registered as a pesticide (disinfectant/sanitizer) in 195710. In 1992, 

oxalic acid was evaluated for reregistration11; at that time, the Agency concluded that the pesticide uses of 

oxalic acid would not result in unreasonable adverse effects to human health or the environment and that 

all registered products containing oxalic acid were eligible for reregistration. However, the uses of oxalic 

acid as a pesticide were cancelled in 1994 for nonpayment of fees. 

In 2005, EPA evaluated oxalic acid for its use as an inert ingredient, i.e., as a calcium chelating hard water 

inhibitor in pesticides used in outdoor gardens and agricultural crops. In that assessment, EPA determined 

that there is a reasonable certainty that no harm to any population subgroup would result from aggregate 

exposure to oxalic acid used as inert ingredients in pesticide formulations12. 

Use Characterization 

According to the draft label, oxalic acid dihydrate (100% active ingredient; a.i.) is proposed for use as a 

varroacide using two methods of application. In one method referred to as the “solution method”, 35 g of 

product is dissolved in 1 L of 1:1 sugar:water (weight:volume) and 5 mL (175 mg a.i.) is then “trickled” on 

to bees “in each occupied bee space” within the brood box. The maximum quantity applied is 50 mL (1,750 

mg a.i.) per colony, whether bees are in nucleus “nuc”, single or multiple brood chambers. The second 

means of application is the “vaporizer method” where 2 g of product are placed in a vaporizer. The draft 

label specifies that the product may only be used in late fall or early spring when little to no brood (eggs, 

larvae, pupae) is present; furthermore, the product may not be used with honey supers are in place. 

 

7

Ibid USEPA. 1992. 8

Ibid Health Canada. 2010. 9

Ibid USEPA. 1992. 10Ibid USEPA. 1992. 11Ibid USEPA. 1992. 12 USEPA. 2005. Inert Reassessment Oxalic Acid. http://www.epa.gov/opprd001/inerts/oxalicacid.pdf 
Page 3 of 6 

The product has been registered in Canada since 2010. The evaluation by PMRA concluded that the 

available scientific information and under the approved conditions of use, oxalic acid had value and did not 

present an unacceptable risk to human health or the environment13. 

Environmental Fate Assessment

Oxalic acid is a naturally occurring compound that transforms rapidly under environmental conditions. It 

occurs as the oxalate ion at environmentally relevant pHs (pka1 1.25, pKa2 4.28); high mobility of the ion in 

soil is expected to occur based on an estimated organic carbon partition coefficient (Koc) value of 5. 

Volatilization from moist surfaces is not likely to be rapid, based on a low Henry’s Law value 1.4x10-10

atm-m3

/mole at 25qC. Oxalic acid will readily dissolve in water because of its high solubility (100 g/L). 

Rapid anaerobic and aerobic biotransformation of oxalic acid in soil and water will occur (HSDB 200314). 

Many plants naturally contain oxalic acid, particularly those belonging to the Oxalis (e.g., sorrels) and 

Rumex (e.g., wild rhubarb) families; several species of mold also produce oxalic acid. Anthropogenic inputs 

of oxalic acid into the environment may be from vehicle exhausts and pulp mill effluents. In soil oxalic 

acid was found at 30 to 6 mg/kg in 0 and 540 cm depths, respectively, at a site in Germany. Lake sediment 

concentrations have been recorded in the range of 0.1 to 0.7 nmol/L of sediment. Oxalic acid was found in 

Los Angeles air samples at concentrations of 6.38 to 2.12 nmol/m2 (HSDB 200315). 

With a bioconcentration factor predicted to be 0.6 (HSDB 200316), oxalic acid is not expected to 

bioaccumulate. 

 

During the use of oxalic acid for control of Varroa mite in beehives, exposure of the chemical to the 

environment is expected to be limited. The product is applied as a liquid or vapour to an affected colony; 

environmental exposure would occur primarily through spillage or leakage during application. Based on 

limited exposure, the chemical’s natural occurrence and the likelihood for relatively rapid transformation 

under environmental conditions, the proposed use of oxalic acid is not expected to pose a significant risk 

to the environment. Therefore, further review of the environmental chemistry, fate, and toxicology of oxalic 

acid was not considered necessary. 

The PMRA has taken into account the federal Toxic Substances Management Policy17 and has followed its 

Regulatory Directive DIR99-0318. It has been determined that this active ingredient and products do not 

meet TSMP Track-1 criteria. 

Although oxalic acid is not currently registered for use as a varroacide in the U.S., historically this has not 

limited the use of other unregistered products (e.g., amitraz) by beekeepers as evidenced by residues of 

unregistered products or their degradates in colonies (e.g., DMPF degradate of amtraz)19. However, these 

 

13Ibid Health Canada. 2010. 14 HSDB Hazardous Substances Data Bank Number 1100. Oxalic Acid. Canadian Centre for Occupational Health and Safety 

15 Ibid HSDB 16 Ibid HSDB 17The federal Toxic Substances Management Policy is available through Environment Canada’s Web Site at : www.ec.gc.ca/toxics 18The PMRA's Strategy for Implementing the Toxic Substances Management Policy, Dir99-03, is available through the Pest Management

Information Service: Phone 1-800-267-6315 within Canada or 1-613-736-3799 outside Canada (long distance charges apply); Fax (613) 

736-3798; E-Mail pminfoserv@hc-sc.gc.ca or through our Web Site at www.hc-sc.gc.ca/pmra-arla 19 Mullin CA, M. Frazier, J. L. Frazier, S. Ashcraft, R. Simonds, D. vanEngelsdorp, and J. S. Pettis. 2010. High Levels of Miticides and 

Agrochemicals in North American Apiaries: Implications for Honey Bee Health. PLoS ONE 5(3): e9754. doi:10.1371/journal.pone.0009754 
Page 4 of 6 

colony monitoring studies have not demonstrated similar residues of oxalic acid. It is uncertain whether 

these monitoring studies included oxalic acid as an analyte; however, the available data do not indicate that 

any potential misuse of the compound is resulting in detectable residues of the parent compound. Oxalic 

acid though is reported to be a natural constituent of honey at concentrations ranging between 1 – 800 

mg/kg, depending on the botanical source of the nectar used to produce the honey20. Concentrations of 

oxalic acid in honey measured after the treatments with oxalic acid ranged from 5 – 289 mg/kg which were 

considered to be within the range of natural background21. 

Ecological Effect Assessment 

Historically, EPA did not require or evaluate ecological effects for oxalic acid because the pesticide was 

only used indoors and the exposure to wildlife was not expected to occur22. The proposed use of oxalic 

acid as a varroacide is confined to the interior of honey bee colonies and the opportunity to affect non-target 

organisms is limited. Therefore, additional ecological effect data have not been required to support the 

proposed registration of oxalic acid as a varroacide. 

Given the proposed use of the compound, non-target animal exposure is likely to be limited to the honey 

bee itself. Although no data have been submitted to assess the effect of oxalic acid on non-target organisms, 

there are data to evaluate potential effects on bees and these data indicate that the compound is practically 

non-toxic to bees on an acute contact exposure basis, i.e., LD50>11 μg/bee. Based on toxicity data contained 

in the ECOTOXicity (ECOTOX23) database, median lethal doses for 50% of the organism tested (i.e., LD50) 

ranged from 1576 μg/bee after 24 hrs to 195 μg/bee24 at 72 hrs of exposure. Data in ECOTOX on the 

toxicity of oxalic acid to Varroa mites reports LD50 values ranging from 0.64 to 1.90 μg/cm2

.

25 Although 

the contact toxicity values are not directly comparable to those for honey bees, these values can be put into 

context by expressing the honey bee toxicity values on the basis of surface area. Assuming that the surface 

area of a worker bee is 1.05 cm2

/bee26, then the reported honey bee LD50 values would range of 186 – 1501 

μg/cm2

 indicating that the mite is several orders of magnitude more sensitive to oxalic acid than the adult 

honey bee. Research by Milani 200127 indicated that the addition of sucrose to oxalic acid made the material 

more hygroscopic and increased its toxicity to mites. 

Based on research conducted by Aliano and Ellis 200828, the application of a 2.8% solution of oxalic acid 

directly to adult bees provided effective mite control with minimal adult bee mortality. In this publication, 

the researchers noted that while the use of oxalic acid is effective for broodless colonies, the compound 

does not kill mites in sealed brood cells. The effectiveness of oxalic acid depends on bee-to-bee contact 

 

20 EMEA. 2003. Committee for Veterinary Medicinal Products Oxalic Acid Summary Report. The European Agency for the Evaluation of 

Medicinal Products Veterinary Medicines and Inspections. EMEA/MRL/891/03-FINAL December 2003. 

http://www.ema.europa.eu/docs/en_GB/document_library/Maximum_Residue_Limits_-_Report/2009/11/WC500015217.pdf 21 Ibid EMEA. 2003. 22Ibid USEPA. 1992. 23 USEPA. 2014. ECOTOXicology database (ECOTOX). Office of Research and Development National Health and Environmental Effects 

Research Laboratory Mid-Continent Ecology Division. http://cfpub.epa.gov/ecotox/ 24Aliano, N.P., M. D. Ellis, 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(5): 1579 – 1582. 25 Milani, N. 2001. Activity of oxalic and citric acids on the mite Varroa destructor in laboratory assays. Apidologie 32: 1`27 – 138. 26 Poquet Y., L. Bodin, M. Tchamitchian, M. Fusellier, B. Giroud, F. Lafay, A. Buleté, S. Tachamitchian, M. Cousin, M. Pélissier, J-L. Brunet, 

and L. P. Belzunces. 2014. A Pragmatic Approach to Assess the Exposure of the Honey Bee (Apis mellifera) When Subjected to Pesticide Spray. 

PLoS ONE 9(11): e113728. doi:10.1371/journal.pone.0113728 

27 Ibid Milani. 2001. 28 Aliano, N.P., and M. D. Ellis. 2008. Oxalic acid: a prospective tool for reducing Varroa mite populations in package bees. Experimental and 

Applied Acrology 48: 303 – 309. 
Page 5 of 6 

and not trophallaxis29 (communal feeding). Health Canada’s proposed registration decision30 on the 

varroacide use of oxalic acid reported that in the absence of poor overwintering conditions in general, the 

proposed application of 3.5% solution to the colony with a maximum of 1,750 mg a.i./colony (50 mL) did 

not result in sufficient adult bee mortality or low survivorship to offset the benefit afforded to the colony in 

terms of mite control. The review also noted that studies examining the efficacy of applications of oxalic 

acid via vaporisation found no adverse effects on bee tolerance, overwintering success or spring colony 

development. 31

A review of the Ecological Incident Information System on December 13, 2010, did not result in any 

incident reports for oxalic acid related to its use or misuse. 

Risk Discussion 

There are limited environmental fate and ecological effect data available with which to evaluate the 

potential for ecological risk. However, the proposed in-hive use of oxalic acid is expected to limit exposure 

of non-target organisms to honey bees and varroa mites within the treated colony. Therefore, a quantitative 

analysis of risk has not been conducted, Environmental fate data indicate that the compound is relatively 

short lived given its susceptibility to both aerobic and anaerobic metabolism, and available chemical residue 

monitoring data from honey bee colonies do not indicate the oxalic acid residues outside of which occur 

naturally in honey. 

Although oxalic acid is classified as practically non-toxic to honey bees on an acute contact exposure basis, 

there is uncertainty as to the extent to which individual bees may be exposed to high acute doses of the 

compound as the compound is trickled into the spaces between frames containing bees and is in turn moved 

through the colony via bee-to-bee contact. Similarly the actual exposure levels for individual bees from 

the vaporization of the compound within the colony is uncertain; however, studies conducted using the 

proposed trickle solution and vaporization methods did not result in an appreciable level of adult bee 

mortality, and longer-term monitoring of these colonies did not report adverse effects on overwintering 

capacity of the colony or in spring development. 

As noted earlier, the Varroa mite has been associated with brood mortality and reductions in worker bee 

lifespan32, increase overwintering colony losses33 and reductions in nurse bee brood/royal jelly production 

affecting nutrition in developing brood and queens34. Research is also showing that varroa mites serve as 

vectors for bee viruses such as deformed wing virus, which has also been identified as a major cause of 

overwintering colony losses35. Synthetic mite controls such as coumaphos and tau fluvalinate have become 

increasingly ineffective due to mite resistance36 and given the mode of action of the organic acids such as 

formic and oxalic acid, there is hope that compounds such as oxalic will serve as an effective means of 

limiting the development of resistance in Varroa mite populations37. 

 

29 Aliano, N. P. and M. D. Ellis. 2008. Bee-to-bee contact drives oxalic acid distribution in honey bee colonies. Apidologie 39: 481 – 487. 30 Ibid Health Canada. 2010. 31 Ibid Health Canada. 2010. 32 Ibid DeGrandi-Hoffman et al. 2014. 33 Ibid Guzmán-Novoa et al. 2010. 34 Ibid Pinto et al. 2011. 35DeGrandi-Hoffman, G., F. Ahumada, R. Curry, G. Probasco, and L Schantz. 2014. Population growth of Varroa destructor (Acari: Varroidae) 

in commercial honey bee colonies treated with beta plant acids. Exp. Appl. Acarol 64: 171 – 186. 36 Ibid. USDA. 2013. 37 Ibid Milani. 2001.
Page 6 of 6 

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the Agency 

evaluates risks to Federally-listed threatened and/or endangered (listed) species from the proposed use of a 

pesticide. Given the proposed in-hive use of oxalic acid to control varroa mite and the limited opportunity 

for exposure of non-target organism, “listed” species are not expected to be affected by the proposed use. 

Therefore, EFED has determined that the proposed use will have no effect (NE) on listed species and will 

not modify habitat associated with listed species.