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Techniques for Using Oxalic Acid to Reduce Varroa Mite Populations in Honey Bee Colonies

Marion Ellis, Associate Professor
Organization: University of Nebraska
Department of Entomology
202 Plant Industries Building
Lincoln, NE 68583-0816
402-472-8696
402-472-4687 (fax)
mellis3@unl.edu

Purpose Statement

To investigate techniques for using oxalic acid to reduce varroa mite populations in honey bee colonies.

Project Duration: 2 years

Funding Request
Budget Category Funding Requested Matching Non-Federal Funds* Matching
Federal Funds
First Year Funding
$23,000
$4,500
0
Second Year Funding
$23,000
$4,500
0
Total Funding
$46,000
$9,000
0

* Non-federal matching funds have been pledged by beekeeping associations in Nebraska, Wisconsin, Iowa and South Dakota.

Executive Summary

Recent studies by Aliano and Ellis (2004a, 2004b), Macedo and Ellis (2002b) and Shephard et al. (2003) have provided beekeepers two techniques for removing varroa mites (Varroa destructor) from adult honey bees that have no environmental risk, that are safe to applicators and that have no risks of contaminating honey or beeswax with harmful residues. Aliano and Ellis (2004a) used powdered sugar to dislodge mites from bees and Shephard et al. (2003) dislodged and killed mites with sucrose ocotnoate. While these techniques are invaluable to beekeepers with small numbers of colonies, they are too labor intensive to be practical for beekeepers who manage large numbers of colonies.

Recently, varroa mite populations have been discovered that are resistant to fluvalinate and coumaphos, the chemicals most frequently used for varroa control by U.S. beekeepers who manage large numbers of colonies (Elzen et al. 1999; Elzen and Westervelt 2002; Macedo et al. 2002a). While fluvalinate and coumaphos have provided welcome relief from varroa injury, they have been shown to negatively affect queens and drones (Sylvester et al. 1999), and residues in hive products are a significant risk to the market for honey and beeswax (Wallner 1999). A strategy for controlling varroa mites that is effective, safe to applicators, low cost and not prone to leave residues in hive products would greatly increase the ability of beekeepers to have strong colonies available for honey production and crop pollination (Ellis 2001).

European studies suggest that oxalic acid is a good candidate for becoming the chemical tool of choice for controlling varroa mites in beehives (Imdorf et al.1997; Arcuelo 2000; Charri?re and Imdorf 2002; Nanetti et al. 2003), and the American Beekeeping Federation has initiated the process of applying for Section 3 registration of oxalic acid for use in the U.S. Their application will include data from Europe and Canada but will need corroborating evidence of efficacy from studies conducted in the U.S.

We propose to evaluate the efficacy of various application techniques for reducing varroa mite populations, to develop extension publications on how to safely and effectively use oxalic acid, and to present workshops on varroa control with oxalic acid when the registration process is complete. Our proposed studies will also determine the LD50 for oxalic acid for both honey bees and varroa mites.

Objectives

  1. Determine the efficacy of fall treatments with oxalic acid for reducing varroa mite populations in honey bee colonies in the U.S.
  2. Determine the efficacy of summer treatments with oxalic acid for reducing varroa mite populations in honey bee colonies in the U.S.
  3. Determine the efficacy of treating package bees with oxalic acid to reduce varroa mite populations in bees destined for sale or shipment.
  4. Investigate how oxalic acid is distributed in honey bee colonies.
  5. Determine the LD50 for adult honey bees and varroa mites treated with oxalic acid.
  6. Publish extension recommendations on how to use oxalic acid safely and effectively in mite management and present workshops and field days to transfer findings to beekeepers.

Rationale

Approach and Methods

  1. Determine the efficacy of fall treatments with oxalic acid for reducing varroa mite populations in honey bee colonies in the U.S. Protocols developed in Europe for treating broodless colonies in early winter will be evaluated for efficacy on varroa populations in the U.S.A. This study will use 120 colonies located at the University of Nebraska’s Agricultural Research and Development Center at Mead, Nebraska, and colonies belonging to a cooperating beekeeper. A single application of a 3.5% solution of oxalic acid dihydrate in a 1:1 sugar syrup will be trickled or sprayed onto honey bee colonies in early winter. Colonies will be monitored for mite mortality, adult bee mortality, queen survival and brood production the following spring. This study will begin in the fall of 2005 and will be completed in the spring of 2006.
  2. Determine the efficacy of summer treatments with oxalic acid for reducing varroa mite populations in honey bee colonies in the U.S. Beekeeping in the U.S. includes a range of environmental conditions and management goals that are not characteristic of European beekeeping including extended brood rearing seasons, migratory movement for pollination, package bee production and the movement of bees from cool climates to warm climates for wintering. We propose to investigate techniques for treating bees when brood is present and to examine the impact of multiple treatments on both varroa mites and their honey bee hosts. Bee safety will be evaluated by quantifying the amount of brood and adult bees both before and after applications are made. These studies will begin in the spring of 2006 and will be completed by the fall of 2006.
  3. Determine the efficacy of treating package bees with oxalic acid to reduce varroa mite populations in bees destined for sale or shipment. We also propose to investigate techniques for ridding package bees of varroa mites. This study will consist of a single application of various dosages to caged bees. Uniform experimental units will be established by shaking bees into a bulk bee cage, letting the bees sit in the bulk bee cage for 12 hours and then scooping 0.8 Kg. of bees into each of 30 cages. These studies will conducted in the spring of 2006 and concluded by the fall of 2006.
  4. Investigate how oxalic acid is distributed in honey bee colonies. Little is known about how oxalic acid is distributed in honey bee colonies. A better understanding of how oxalic acid is distributed in honey bee colonies is critical to designing rational delivery systems. We propose to determine if oxalic acid is distributed via fumigation by separating paired experimental colonies with a double eight-mesh screen placed in the middle of the colonies. We will treat one half by trickling oxalic acid. Following treatment, we will monitor mite fall on both sides of the screen. In a similar experiment we will measure the spread by trophallaxis among bees that are separated by a double eitht-mesh screen that permits trophallaxis but not physical contact. Our third study will measure the spread by physical contact among bees by a similar trial in which the bees are separated by a queen excluder. These studies will be completed by the fall of 2006.
  5. Determine the LD50 for adult honey bees and varroa mites treated with oxalic acid. This objective will be met by first determining the LD50 for oxalic acid applied topically to adult worker bees and the LC50 for varroa mites exposed to vials coated with serial dilutions of oxalic acid. We will use techniques described by Ellis et al. (1997) and Macedo et al. (2002a). Despite the widespread use of oxalic acid in beekeeping, these basic toxicological properties of the compound have not been determined. The results of these studies will help to guide scientists in developing techniques for delivering the optimum dosage to the parasite and its host. These studies will be conducted in the fall of 2005 and will be concluded by the spring of 2006.
  6. Publish extension recommendations on how to use oxalic acid safely and effectively in mite management and present workshops and field days to transfer findings to beekeepers. Timely publication and presentation of results is critical to the success of beekeepers. Currently available chemical tools for reducing varroa populations are no longer reliable (Elzen et al. 1999; Elzen and Westervelt 2002; Macedo et al. 2002a), and beekeepers are struggling to maintain adequate colony populations for honey production and crop pollination. We propose to publish our findings in both scientific and beekeeping publications as soon as they are completed. We also propose to present our results in a timely manner to both scientific and beekeeping audiences. Furthermore, we will cooperate with State Departments of Agriculture and the Environmental Protection Agency to secure registration that will permit beekeepers to use our findings to protect their colonies.

Background Information

The varroa mite (Varroa destructor) is the most serious problem faced by U.S. beekeepers, and varroa injury reduces the number and strength of honey bee colonies that are available for honey production and crop pollination (Fries 1993 and Martin 2001). Recently, varroa mite populations have been discovered that are resistant to fluvalinate and coumaphos, the chemicals most frequently used for varroa control by commercial beekeepers in the U.S. (Elzen et al. 1999; Elzen and Westervelt 2002; Macedo et al. 2002). While fluvalinate and coumaphos have provided welcome relief from varroa injury, they have been shown to negatively affect queens and drones (Ellis et al. 1997; Sylvester et al. 1999), and residues in hive products are a significant threat to the market for honey and beeswax (Wallner 1999). A strategy for controlling varroa mites that is effective, safe to applicators, low cost and not prone to leave residues in hive products would greatly increase the ability of beekeepers to have strong colonies available for honey production and crop pollination (Ellis 2001).

European studies suggest that oxalic acid is a good candidate for becoming the chemical tool of choice for controlling varroa mites in beehives (Imdorf et al.1997; Arcuelo 2000; Charri?re and Imdorf 2002; Nanetti et al. 2003). Initial studies conducted in Nebraska in the fall of 2004 also support the efficacy of oxalic acid treatments. We were able to reduce mite populations by spraying and trickling oxalic acid by 92 and 87%, respectively, when colonies were treated in November of 2004 (Aliano and Ellis, In prep.). In a comprehensive review of the experiences of various European investigators, Charri?re and Imdorf (2002) reported that trickling or spraying oxalic acid provided 93-99% control when a single application was made in the fall or early winter to broodless colonies.

Oxalic acid is a natural constituent of honey and many vegetables, and no significant residues have been found in hive products in Europe (Del Nozal et al. 2000; Bernardini and Gardi 2001). It is approved as a varroa treatment in three western European countries (Switzerland, Austria and Finland), but due to its low cost, availability, and efficacy, it is reported to be widely used by beekeepers throughout Europe (Charri?re and Imdorf 2002). In December of 2004, the European Agency for the Evaluation of Medicinal Products (EMEA) established a Maximum Residue Limit (MRL) for oxalic acid in honey. Now each individual country can apply for national approval of oxalic acid or products containing it to control Varroa destructor in honey bee colonies (Rademacher and Imdorf 2004).

European studies have concentrated on single applications to broodless colonies in early winter. Few investigators have examined the efficacy of multiple treatments when brood is present, and there are no recommendations as to how frequently colonies would need to be treated when brood is present. Likewise, few studies have been done to ascertain if multiple treatments will injure colonies and to determine if a withdrawal period is necessary to avoid residues in hive products. U.S. beekeeping involves extensive migratory movement, package and queen bee production, and movement of bees between climatic zones. While protocols developed in Europe are useful, different approaches may be necessary in the U.S.A.

Bee breeding programs have shown progress, but they have not produced a stand-alone product that beekeepers can reliably use to avoid colony injury due to mites. Since the reproductive biology of the honey bee includes open mating, out crossing and multiple matings, it is unlikely that bee breeding will provide a stand-alone solution. However, stock selection may increase the usefulness of treatments that are less than 99% effective, such as oxalic acid. It appears that for the foreseeable future, beekeepers will need a technique to reduce mite populations to complement the improved stocks that are being developed by bee breeders.

Little is known about the mode of action of oxalic acid and how it is distributed in honey bee colonies. Investigations that establish a mode of action and distribution route could improve the efficacy and consistency of oxalic acid treatments. They may also improve the bee safety of treatments. The persistence of mite toxicity and the effect on mites in brood cells also need further examination. Some adult bee mortality has been reported for overdosed colonies and for repeated applications to the same colony. This issue should be revisited, and any colony weakening from overdosing and repeated applications should be quantified.

Before seeking approval for use in the U.S.A., efficacy and bee safety need to be determined for a variety of colony conditions and application techniques. Although oxalic acid may be eligible for GRAS (generally regarded as safe) registration, the studies included in this proposal will contribute to the registration process. Most importantly, these studies will provide research-based guidance to beekeepers for using oxalic acid to reduce mite populations without weakening colonies or contaminating hive products.

Resources

Human resources will include:

  1. Marion Ellis, Associate Professor, University of Nebraska, Department of Entomology. Dr. Ellis will direct the project, supervise student and support staff, write research and extension publications and present workshops for technology transfer.
  2. Nick Aliano, Graduate Student, University of Nebraska, Department of Entomology. Mr. Aliano will conduct the field and laboratory studies.
  3. Bill McCormick, Apiculture Technician, University of Nebraska, Department of Entomology. Mr. McCormick will maintain the honey bee colonies required for this study and assist with field studies.
  4. Mark Johnson, Beekeeper, Lincoln, Nebraska. Mr. Johnson will provide a portion of the colonies needed for this study and empty hive equipment for the package bee study.

Apiculture Laboratory Resources

Apiculture laboratory resources include 120 honey bee colonies, trucks, and beekeeping equipment needed to conduct these studies

Beekeeping Association Support

Beekeeping Associations in Nebraska, Iowa, Wisconsin, Kansas and Missouri will contribute funding and support for the Mid-West Master Beekeeping Workshop. This annual workshop attracts beekeepers from across the North Central Region and will be used to disseminate the results of the studies included in this proposal.

Measures and Outcomes

The goal of this proposal is to reduce the use of chemicals in beehives that are no longer effective and that are at risk for contaminating hive products. The proposed studies will develop techniques that rely on oxalic acid, a natural product that is commonly found in fruits and vegetables, to reduce varroa populations. The outcome will be measured by surveying beekeepers to determine changes in varroa control practices following the completion and publication of these studies. The quality and value of the studies included in this proposal will also be measured by the publication of results in appropriate scientific journals.

Outreach

Extension publications will be prepared to describe techniques for safely and effectively using oxalic acid to reduce varroa mite populations in honey bee colonies. Results and recommendations will be distributed by printed publications, the University of Nebraska’s Bees and Beekeeping Web Site (http://entomology.unl.edu), and by regional and national presentations at beekeeping meetings.

Sustainability

The expected outcomes from this proposal will be applicable to anywhere honey bees are kept. Varroa mites are a global problem and the techniques investigated in this proposals will have global value. Although the mode of action of oxalic acid is unclear, there is no evidence that bees have developed resistance to it in Europe where it has been used for over ten years. In combination with mite tolerant stocks that are currently being developed, oxalic acid treatments have the potential to provide an invaluable tool for beekeepers who implement an integrated pest management strategy for reducing varroa-induced losses.

Literature Cited

Timetable

Major Participants

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