Varroa Mite Treatment Options
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Original article published in BeesCene, April 2022
By Nuria Morfin
Spring 2022: concerns about honey bee mortality and high Varroa destructor levels
At different paces, beekeeping season is arriving in our different regions of BC. Some beekeepers are still looking at hives covered on snow and looking for signs of activity (like dead bees in front of the hive or signs of cleansing flights). Others are already opening their hives (10°C) or inspecting frames (>15°C). Spring is a critical time to inspect for brood diseases, like American Foulbrood, and to start monitoring for Varroa destructor. Varroa destructor is an ectoparasite that feeds on the hemolymph and fat body of bees, and acts as vector for a number of viruses like Deformed wing virus A and B. The parasitosis caused by V. destructor is not easy to control; the best way to treat honey bee colonies against varroosis is by implementing an Integrated Pest Management (IPM). An IPM has different components, including a) incorporating a resistant stock against V. destructor, b) monitoring mites every 4-6 weeks, c) using cultural and mechanical methods to disrupt the biological cycle of the mite, and d) apply treatments (acaricides/miticides).
There are two types of acaricides based on the origin of the active ingredient: non-synthetic acaricides and synthetic acaricides.
Non-synthetic acaricides
1.1 Essential oils (Thymovar®, Api Life VAR®, and Hop Guard II®)
Essential oils are volatile compounds extracted from plants. A number of essential oils have shown to be effective against V. destructor, and some are available as commercial products like thymol and menthol (Table 1). The products are formulated in different matrices, including impregnated wafers, tablets, and strips.
Mode of action: thymol and menthol likely kill V. destructor by binding to GABA or octopamine receptors, but we need more studies to understand their acaricidal effect. Essential oils should be applied using the commercial products, which contain the dose necessary to kill the mites.
Advantages: low risk of V. destructor mites developing resistance, low cost, and low risk to human and environmental health.
Disadvantages: variable efficacy, their delivery depends on outside temperature to evaporate and reach adequate concentrations inside the hive, and they have low efficacy against mites inside capped cells.
1.2 Beta acids (Hop Guard II®)
Beta acids are compounds derived from hop plants, only one product is commercially available in combination with essential oils (Table 1).
Mode of action: their mode of action is not fully understood, but they have shown to have repellent effect on some arachnids (e.g. two-spotted spider mites) and to be effective for the control of varroa mites. Beta acids and essential oils should be applied using the commercial product, which contain the dose necessary to kill the mites.
Advantages: low toxicity to honey bees and mammals, and the product is relatively easy to apply (strips).
Disadvantages: their efficacy is variable and depends on outside temperature (reported efficacy from 43-88%).
Table 1. Non-synthetic acaricides registered to treat for V. destructor parasitism in Canada.
1.3 Organic acids
There are two organic acids used for varroa control: oxalic acid and formic acid.
1.3.1 Oxalic acid. Oxalic acid comes in crystals, and it can be applied by tricking a solution of oxalic acid (dissolved in sugar syrup) or using an electric evaporator (Table 2).
Mode of action: its acidic nature seems to be responsible for mite mortality upon contact, but their mode of action is not fully understood.
Advantages.: if used properly, it has a >90% efficacy, and there is no evidence of resistance of the mites to oxalic acid.
Disadvantages: it is not effective for mites inside capped cells, and it can have negative impacts on brood development.
1.3.2 Formic acid. Formic acid can be applied as a gel or as a liquid solution (Table 2).
Mode of action: the mode of action is not well understood, but it appears to have an effect on energy metabolism by inhibiting electron transport in the mitochondria.
Advantages: it is effective at killing mites during the dispersal phase (when the mites are attached to adult workers or drones) and mites inside capped cells. Formic Pro® and MAQS® are the only treatments that can be used during honey flow.
Disadvantages: its efficacy is variable (35-90%); it depends on outside temperature and the amount of brood. It can cause high mortality if the outside temperature is high.
When using oxalic and formic acid, make sure to wear Personal Protective Equipment. Ask for help if you are not familiar with the use of organic acid treatments.
Table 2. Organic acids registered to treat for V. destructor parasitism in Canada.
2. Synthetic acaricides
The three synthetic acaricides registered to use in Canada are Apivar® (amitraz), Apistan® (tau-fluvalinate), and Bayvarol® (flumethrin) (Table 3). These products have been in the market since the 90’s and were very effective to control mite levels (>99% efficacy). There is growing concern that synthetic acaricides are showing a reduced efficacy to control mites through the development of resistance. To confirm suspected low efficacy an appropriate bioassay, like Apiarium or Pettis test, should be done. Additionally, to prevent the development of resistance by the mites to the synthetic acaricides and to prevent environmental pollution, it’s important to follow the instructions of the products and dispose of the plastic strips adequately when the treatment is completed.
Mode of action: The active ingredients of these synthetic acaricides are neurotoxic (tau-fluvalinate and flumethrin act by inhibiting gated sodium channels, and amitraz by interacting with octopamine receptors in the mite’s central nervous system).
Advantages: easy to apply and relatively low cost.
Disadvantages: decrease efficacy has been reported for some of the products in parts of North America, they can leave residues in the hive and the environment, and they are toxic to mammals.
Table 3. Synthetic acaricides registered to treat for V. destructor parasitism in Canada.
Main recommendations when treating for mites:
1. Don’t rely only on treatments, use an Integrated Pest Management (IPM) strategy to control mites by:
-Monitoring mites every 4-6 weeks.
-Monitor mite levels before and after mite treatment (check if the treatment worked!).
-Using cultural and mechanical methods to control V. destructor population growth.
-Incorporate a resistant stock (honey bees bred for V. destructor resistant traits).
2. Apply registered products in Canada and consider:
-Mite levels; if mite levels are 3% or higher in the fall, the colony is at high risk of not surviving the winter.
-Consider outside temperature.
-Consider if there is capped brood present.
-Consider if the product can be applied during honey flow.
3. Follow the label’s instructions.
4. Follow the Ministry of Agriculture and Food recommendations.
5. Use Personal Protective Equipment.
6. Ask for help if you don’t have experience, especially with organic acids.
For more information on BC-TTP
Social media: @bc_ttp
Acknowledgements: We gratefully acknowledge the financial support of the Province of British Columbia through the Ministry of Agriculture and Food. We thank Paul van Westendorp for reviewing the note. Also, thanks to all the beekeepers that are supporting our projects and collaborating with BC-TTP.
References.
British Columbia Ministry of Agriculture and Food. Bees. https://www2.gov.bc.ca/gov/content/industry/agriculture-seafood/animals-and-crops/animal-production/bees [Accessed April 13, 2022]
Jack, C. J., & Ellis, J. D. (2021). Integrated Pest Management Control of Varroa destructor (Acari: Varroidae), the Most Damaging Pest of (Apis mellifera L.(Hymenoptera: Apidae)) Colonies. Journal of Insect Science, 21(5), 6.
Pest Management Regulatory Agency (PMRA). Search Labels. https://www.canada.ca/en/health-canada/services/consumer-product-safety/pesticides-pest-management/registrants-applicants/tools/pesticide-label-search.html [Accessed April 13, 2022]
van der Steen, J., & Vejsnæs, F. (2021). Varroa Control: A Brief Overview of Available Methods. Bee World, 98(2), 50-56.