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  • Writer's pictureSebastian Owen

Varroa – A Warning from Experience in the Northern Hemisphere

In the third and final article of his series, Sebastian Owen, commercial manager of Vita Bee Health, discusses the development of resistance in varroa mites and explains what beekeepers can do to help ward off such problems and enable treatments to continue to be effective for long periods.

The varroa mite continues to spring surprises in the northern and southern hemispheres as weather patterns fluctuate and colony treatment regimes make an impact. It’s quite a battle against the parasite as it exploits chinks in the defensive armour that beekeepers provide.

Gaining successful varroa ‘drops’ is only attainable long-term with an integrated pest management plan, just take it from northern hemisphere beekeepers’ experiences, warns Vita Bee Health’s Sebastian Owen.

Since beekeepers in the northern hemisphere have had to deal with varroa for much longer than in the southern hemisphere, their experience shows what could be in store for New Zealand beekeepers unless preventative measures are taken immediately. Latest news from the northern hemisphere about colony losses are disturbing. Preliminary figures have been released in the USA indicating 2022/3 losses approaching 50%.1 In Britain, the British Beekeepers Association has launched a survey following reports of serious unexpected and unexplained losses by experienced beekeepers.

Resistance Research

Ground-breaking work on varroa mites’ resistance to treatments is being undertaken in France by Apinov, honey bee specialist based in La Rochelle with strong university connections. Apinov has been monitoring mites that have become resistant to different active ingredients and believe that it has developed as a result of repeated and possibly incorrect use of treatments. The multi-organisational research team has noticed an increase in resistant mites to different active ingredients that have gone from being mildly resistant to highly resistant in just five years, meaning that certain treatments are currently ineffective in particular areas.

Unfortunately, different mutations may be responsible for resistance to the same treatments in other parts of the world, so it is vitally important to do everything to inhibit resistance appearing in the first place.

The Risks of DIY Treatments

As a result of the growing resistance, many French beekeepers have turned to other treatments, particularly oxalic acid. As yet there is no research into the development of resistance to oxalic acid. However, it is generally believed that beekeepers are buying raw materials and applying them – an illegal practice in many countries. Apinov hears regular reports of colony losses even after twice-yearly treatments that include oxalic acid treatment mid-winter.

As New Zealanders experience the effects of poor seasonal weather and beekeepers have faced an economic downturn, treating for varroa mites with the most effective products may not be deemed a top priority. However, poor or no treatment to control varroa populations seem already to be taking an enormous toll and adding to beekeeper and honey bee misfortunes.

Beekeepers should monitor mite populations to determine when to treat, and use treatments with different modes of action to avoid, or at least delay, the development of resistant mites, according to Sebastian Owen.

A recent survey has shown that in New Zealand 13.5% (98,000) of all colonies healthy at the start of last winter were lost because of either wasp attacks, problems with queen bees, varroa mite infestations or suspected starvation.3

Reports also suggest that many beekeepers are using low-cost DIY oxalic acid varroa treatments. While oxalic acid can be an effective varroa control treatment, it is essential that it is supplied in the correct dosage and following manufacturers’ instructions precisely. As Rusty Burlew has pointed out it’s a toxic product (“the dose makes the poison”) and requires very careful planning before use because it kills only phoretic mites (not those in capped brood cells) and to be most effective application should be at a time when a colony is broodless.4

As the French experience indicates, treatment without following protocols is also likely to lead to resistance issues, although rigorous scientific resistance studies of this unregulated treatment have yet to be undertaken. Nonetheless, the sub-lethal effects of casual treatment regimes are already well known.5

The way forward to inhibit resistance

For New Zealand beekeepers, permitted treatments to control varroa include fluvalinate (Apistan), flumethrin (Bayvarol), amitraz (Apivar, Apitraz), formic acid (FormicPro), thymol and other essential oils (Apiguard, ApiLifeVar, ApiBioxal).

A CO2 test is a gentle way to monitor varroa loadings on bees.

However, alternating treatments is essential to inhibit the development of resistance. Treatments permitted for use in organic farming in the European Union, are increasingly popular in the northern hemisphere and are an ideal component of an integrated pest management programme (IPM).

Resistance develops not just to the active ingredients of a treatment. Mite behaviour can help develop a form of resistance and can be expressed in mutations, so applying different active ingredients in the same way (by strips or by fumigation, for example) can lead to mutations which can lead to resistance to different active ingredients. Therefore, leaving products inside a hive is especially bad because not only does the dosage of active ingredient fall and long-time exposure can lead to resistance, but also the very presence of strips for a long period can help develop resistance to products containing any active ingredient by changing mite behaviours.

As New Zealand beekeepers have found, the presence of varroa radically alters some beekeeping practices. However, readers can take advantage of lessons learned in the northern hemisphere where varroa has been present for much longer. The key message is to deploy IPM techniques: monitor mite populations to determine when to treat and use treatments with different modes of action to avoid, or at least delay, the development of resistant mites.


2 Almecija, G, Poirot, B and Ventalon, M. (2023) Summary of Varroa Resistance to Acaricides. Apinov, Lagord, France


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