Testing Oxalic Acid treatment against Ascetic Acid treatment against Varroa destructor in South Western British Columbia
Principle Investigator - Paul van Westendorp, Provincial Apiculturist, BC Ministry of Agriculture and Lands
Technician - Jaquie Bunse, Apiary Inspector, BC Ministry of Agriculture and Lands
Background
The arrival of the Varroa mite in British Columbia in 1990 from the United States changed beekeeping practices as dramatically in this coastal rainforest environment as surely as anywhere else in the world. In areas of the province where the mites were first found, beekeepers had at least some warning of impending doom. In adjoining areas, beekeepers relied on pre-mite cues to the health of their bees.
The initial impact of Varroa occurred when BC beekeepers first noticed the collapse of strong, seemingly healthy colonies of 40 to 50,000 bees just after the end of the honey flow in August. Many colony populations dwindled as adult bees deserted through absconding and migrating to other colonies in the vicinity, taking their mite loads with them and infecting new colonies.
By spring, many apiaries had suffered losses of 75 to 80%, with remaining colonies severely weakened. The British Columbia Ministry of Agriculture and its staff attempted to contain the mite infestations by developing quarantine districts, but as in other parts of the world, bees and mites refuse to adhere to Government regulations and the entire province showed signs of infestation within a few years. Even Vancouver Island, too far for bees to fly from the mainland and quarantined to prevent the importation of live bees, eventually became infested.
Beekeepers learned to adapt their management to deal with Varroa more effectively, and although the numbers of beekeepers registered in the province dropped from 2377 in 1998 to 2101 in 2005, the number of colonies rose from 39,742 in 1998 to 44,645 in 2005.
The first miticide used to treat the colonies with great success was Apistan, a synthetic pyrethroid marketed by Wellmark in North America. The continuous use of this product for over ten years led to the development of Apistan-resistant mites. Another mite control product brought onto the market in recent years was coumaphos, an organophosphate marketed in Canada by Bayer under the tradename CheckMite+. However, the Varroa mite became resistant to coumaphos even faster than to Apistan, and beekeepers had little choice but adopt Integrated Pest Management strategies.
Integrated Pest Management (IPM) involves the timely application of chemical, mechanical and cultural management practices that collectively control Varroa at below the economic threshold level. The IPM program involved the judicious application of organic acids including Formic Acid, available to Canadian beekeepers since the early 1990s, and Oxalic Acid. Essential oil products containing thymol have not yet been registered for use in Canada, but these products may become available in Canada in the next few years.
The organic acids remain very attractive as potential mite control products because of the following characteristics:
In this study, domestic formulation of Acetic Acid (~vinegar) was tested for its mite control efficacy in comparison to the proven efficacy of Oxalic acid.
Project Design
In 2003, the BC Ministry of Agriculture and Lands carried out a small study involving 29 honeybee colonies with known varroa mite levels. A monitoring test of 24-hr mite drop onto a sticky board trap produced between 4 mites and 240 mites between all the colonies. The hives were then randomly divided into 4 groups, comprised of 2 Oxalic acid treatment groups of 10 colonies each, a third group of 6 colonies receiving Acetic acid treatment and a fourth group of 3 controls.
Each group was treated once except the control colonies. Mite drop was measured 7 days and 14 days after treatment to assess mite drop over time.
Oxalic acid
Oxalic Acid Solution Preparation:
Exactly 15 g of oxalic acid crystals, in dihydrate form, supplied by Xenex Laboratories Inc, Coquitlam, BC (and commonly available in BC drug stores) were dissolved in a lukewarm solution of 200 g of granulated household sugar in 200 ml of tap water. The solution required vigorous shaking to dissolve crystals. The solution was used within one week of preparation.
Treatment
Oxalic acid treatment was applied in two different methods:
Group #1
· 10 colonies were treated with liquid oxalic acid in a 1-1 sugar syrup applied by a drip method once.
· Using a syringe, 35 ml oxalic acid solution was applied equally to spaces between frames occupied by bees in each super containing bees.
Group #2
· 10 colonies were treated with liquid oxalic acid in a 1-1 sugar syrup applied as a fog at the entrance of each colony for approximately 15 seconds.
· The acid solution fog was delivered with a propane-powered garden fogger once.
· 15 seconds of fogging equalled approximately 35 ml of acid solution.
Acetic acid
Acetic Acid Solution Preparation:
100 ml of acetic acid (domestic formulation at 5%/volume) were mixed with 100 g of granulated sugar.
Treatment
Group #3
· 6 colonies were treated with acetic acid solution using the drip delivery method.
· Using a syringe, 35 ml of solution was applied equally to the spaces between frames occupied by bees in each super containing bees.
Controls
Group #4
· 3 colonies did not receive any treatment and were used as controls.
Results & Discussion
Mite Count
| Group | 7 days | 14 days |
| #1 | 34.4 | 127.7 |
| #2 | 45.1 | 50.6 |
| #3 | 51.7 | 5.0 |
| #4 Controlled | 42.3 | 27.3 |
Group #1 - Colony #24 produced 1,280 mites and subsequently died. This information was not included in the numbers.
Group #2 - Colony #23 succumbed to mites during the treatment period.
Group #3 - Varroa mite drop from these colonies was counted on sticky boards as in Group #1 and Group #2. The average mite drop after 7 days was 51.66 mites and 5 mites after 14 days.
Group #4 - Varroa mite drop in the control group averaged 42.3 mites after 7 days, and 27.3 after 14 days. Colonies # 28 and #30 succumbed to mites in the weeks following the treatment period.
We considered a possible reason for the increase in mite fall after 14 days the result of the residual effect of the treatment when the last of the fall bee brood emerged during the treatment period.
2 of the Oxalic acid drip-treated colonies became queenless, one succumbed to varroa mites and one colony died of starvation between December 9 and March 24, 2004. 1 colony treated with Ascetic acid succumbed to starvation during the same period.
Conclusions
Oxalic acid treatments have been successfully used in Scandinavian countries for many years. The product has only been recently introduced in Canada using the dribble and fog methods. According to the findings of this study, Oxalic acid applied to honeybee colonies with these application methods provided minimal mite controls.
In comparison to the oxalic acid treatments and controls, the acetic acid application provided the best mite control. The attraction of acetic acid is its readily availability, low cost, ease of application and little concern about residues or contamination.
We recommend further studies be conducted on a larger scale by comparing mite control efficacy of acetic acid with oxalic acid crystal sublimation applications.