The varroa mite presents the largest threat to bees today. By feeding on bees and pupae, spreading viruses, and lowering reproductive output, the varroa mite is decimating bee populations around the globe. For more than thirty years, beekeepers have turned to miticides to solve this problem. If miticides are an effective weapon in the fight against varroa, why are the mites more threatening than ever?
One of the things that attracts us to beekeeping is the connection to nature. While no beekeeper wants to treat their bees with potentially harmful or unnecessary substances, chemical varroa treatments, or ‘miticides,’ are commonly viewed as a necessary evil. The one thing that beekeepers can agree on when it comes to miticides, is that they are generally good at killing varroa. On each treatment, some chemicals have been advertised to kill up to 95% of a varroa infestation. These numbers are impressive and have fuelled a multi-million euro industry for decades.
In the 2018 study ‘Toxicity of Selected Acaricides to Honey Bees (Apis mellifera) and Varroa (Varroa destructor Anderson and Trueman) and Their Use in Controlling Varroa within Honey Bee Colonies’, the efficacy of 5 common miticides was tested in a laboratory over a 48 hour period, followed by a field environment test over a two month period. The field test results displayed variable levels of efficacy, showing an actual relative varroa mortality rate of 86%, 84%, 79%, and 64% for Apiguard®, Apistan®, Apivar®, and HopGuard® respectively (2018).
Studies such as this reflect the general sentiment of the beekeeping community: Yes, miticides can be a useful tool against varroa.
But there is a difficult question that needs to be answered. If miticides are as effective as we are led to believe, why are honey bee populations in the worst shape they have ever been in? While it is important to recognize that the varroa mite is not the sole cause of bee population losses (Mono-cultural practices, habitat loss and pesticide use also play a role), this parasite is generally considered the primary reason for global honey bee losses . And still, the reliance upon miticides often goes unchallenged. Could it be that the issue of varroa is a little more complex?
The (not so natural) world of miticides
Though miticides aim to produce the same outcome, their active chemical ingredients are widely varied. As such, it is difficult to judge their impact in the same light. Chemical varroa treatments are primarily broken into two groups- hard and soft. Hard chemicals are synthetic in nature such as amitraz (Apivar), coumaphos (Perizin), bromopropylate (Folbex VA) and tau-fluvalinate (Apistan), while soft are derived from natural sources such as formic acid (MAQS, Mitegone), oxalic acid (ApiLiveVar), thymol (Apiguard) and hop beta acids (Hop Guard). But don’t let the name trick you. These are indeed active chemicals and must be used with utmost caution. Each miticide has its own list of positive and negative attributes, and beekeepers rely on a rotating collection of them to solve their varroa pains.
In the aforementioned study ‘Toxicity of Selected Acaricides to Honey Bees (Apis mellifera) and Varroa (Varroa destructor Anderson and Trueman) and Their Use in Controlling Varroa within Honey Bee Colonies’ the two month period used to determine miticide effectiveness certainly shows that miticides can kill mites. However, is it a long enough period of time to represent the shifting seasonal dynamics that effect varroa infestation in a hive environment?
The same study claimed the following:
«Their highest efficiencies were recorded at 6 h post-treatment, except for coumaphos and thymol, which exhibited longer and more consistent activity.’
‘It was important to note that, even after 48 h of exposure, none of acaricides eradicated all of the Varroa mites on the caged honey bees, despite the naturally high mortality among the mites.»
Varroa infestation is an ongoing issue within every colony and needs to be continuously managed. Simply put, mite infestations return quickly.
There are several reasons for this such as mite migratory behaviours (Phoretic stage), as well as their biology and reproductive cycles. As bees get in contact with neighbouring hives for robbing purposes, varroa spread between locations by attaching themselves to the raiding bees. While a beekeeper may have a really good grip on varroa in their hives, if a neighbouring colony is in a poor state of health, the mites quickly return and multiply.
Varroa reproduce in capped brood cells. A mite finds an uncapped cell and lays up to 6 eggs within 60 hours of the cell being capped. Mites take from 5 to 8 days to develop and then mate further amongst themselves. Once the bee hatches, the mite offspring leave the cell to continue the migration and reproduction cycles. Just like a virus, the more varroa present, the faster the infestation can compound. Many chemical treatments such as oxalic acid, thymol and hops beta acid are not able to penetrate the wax of the capped brood comb. As such, they can only be effectively applied during brood less periods to destroy mites in the Phoretic stage. If administered during periods where brood is present, these treatments cannot effectively destroy the varroa already reproducing in the colony.
Depending on the method of administration, a treatment can be a one-time application, such as dripping, or can be expressed from a ‘tray’ or ‘strip’ left in the hive for a set time period. Treatment cycles usually happen 1-2 times per year, due to the negative and potentially harmful side effects. Even with more frequent or longer treatments, colonies are left unprotected for extended periods of time- providing opportunity for varroa to multiply. The length and repetition of treatment is indeed significant in determining effectiveness, and the ability of mite populations to bounce back quickly means that miticides simply have not been able to keep up.
In an effort to encourage useful varroa treatment practices, the study ‘Efficacy assessment of soft and hard acaricides against Varroa destructor mite infesting honey bee (Apis mellifera) colonies, through sugar roll method’ tested various miticides at the recommended dosages and applications over a period of 35 days. Each had been able to reduce the amount of varroa at the beginning, however, before the end of the experiment, all treatments except for formic acid had failed at keeping mite levels below the economic threshold levels. (6 Varroa mites per 300 adult bees during low population season (dearth period) and 9 Varroa mites per 300 bees during high population season. It must also be noted that formic acid required re-application every two days throughout the test period.
«Application of one strip flumethrin, one strip fluvalinate and oxalic acid per 10 frame hive can keep the mite population below economic threshold level up to 14 days. Application of two strips of fluvalinate and flumethrin were found effective up to 28 and 38 days, respectively. Amitraz treatment proved effective for up to 21 days. Application of 10 mL formic acid (70%) on alternate days remained effective throughout the period of application.»
Big in Europe
One of the most relied upon miticides in Europe is formic acid. Available in several forms such as strips, gels and liquids, this acid is released as a vapour in the hive. Viewed as a powerful miticide, formic acid requires precise temperatures, humidity conditions and hive environments to function. Dr Randy Oliver of scientificbeekeeping.com states the following:
«In temperate regions, you’ll generally have two temperature windows when you can apply either product—one before the honey flow, and one after. The shorter your summer, the more narrow your windows will be.»
As such, it is not practical in certain conditions or climates, and time periods must be calculated precisely. As well as being toxic for humans, it is not recommended for weak colonies due to the heavy toll it takes on bee mortality.
Dr Oliver continues:
«Colonies weaker than six frames of bees can suffer substantial brood loss, and even colony mortality, since there aren’t enough bees to adequately control the ventilation of the vapor.»
The toxicity of formic acid supports its status as a powerful miticide; However, its diva-like requirements also result in high inconsistency.
The 2019 study ‘Acaricide efficacy and honey bee toxicity of three new formic acid-based products to control Varroa destructor’, tested three formic acid products Nassenheider Professional® , MAQS® and Varterminator® . A dispenser and two gel based products, all three were created in an attempt to reduce the dangers to beekeepers and bees and minimize the variance of liquid acid distribution. The 32-day test displayed the following results:
«The mean acaricide efficacy of Nassenheider Professional® was 73.2%, while MAQS® and Varterminator® showed a mean efficacy of 49.3% and 81.2 %, respectively.»
Even in these more stable forms, the high variance of efficacy is clear.
The Swiss Federal Council body Agroscope sums up the situation with the following statement:
«Formic acid not only destroys varroa mites on the adult bees, but also those developing in the sealed brood cells. Despite this, formic acid alone is often not enough to sufficiently reduce the numbers of this parasite. A more comprehensive approach to control must therefore be implemented.»
A matter of time
The 2017 study ‘Influence of Varroa Mite (Varroa destructor) Management Practices on Insecticide Sensitivity in the Honey Bee (Apis mellifera)’ examined the effectiveness of miticides, in particular amitraz, on protecting bee colonies from varroa infestation. While it acknowledged the effectiveness of amitraz as a chemical used for killing mites, the study noted the shortcomings of the treatment due to the application periods and the ability of mite populations to rapidly replenish.
«The gap between amitraz treatments (to mimic a honey harvest) allowed for the mite population to rebound dramatically. For practical purposes concerning managing mites with chemical means, the beekeeper may have to balance taking a honey crop with colony survival. The current Apivar® label limits application to 2 treatments annually with a 56 days maximum treatment interval. Amendment of the Apivar® label to allow uninterrupted year-round treatment would very likely improve product effectiveness in the short term. However, a constant treatment regime would also increase selection pressure for amitraz resistance in Varroa mites. The loss of effective amitraz treatments to control Varroa mites is a disconcerting prospect due to the low rate of product development to specifically and effectively control Varroa mites .»
It is also important to note that Amitraz cannot legally be used in Switzerland.
The frequency of treatment cycles as a critical factor for a long-term solution was presented in the two year study ‘Field-Level Sublethal Effects of Approved Bee Hive Chemicals on Honey Bees (Apis mellifera L)’, which called into question all of the miticides in the field trial.
«Mite levels in colonies treated with Apistan or Check Mite+ were not different from levels in non-treated controls. Experimental chemicals significantly decreased 3-day brood survivorship and increased construction of queen supercedure cells compared to non-treated control.»
The paper concluded with the following excerpt:
«Given the poor performance of the miticides at reducing mites and their inconsistent effects on the host, these results defend the use of bee health management practices that minimize use of exotic hive chemicals.»
At what cost
Over extended periods of time, the efficacy of miticides can clearly be inconsistentand even counter productive While miticides may be able to kill mites in a single treatment cycle, the jury is still out on whether they offer a reliable long-term remedy. Even with this dubious result, there are some significant costs.
Miticides containing formic acid, oxalic acid, coumophos, hop acids and fluvalinate have all been directly linked to worker bee mortality, brood loss and even queen loss. Fluvalinate, coumaphos, and amitraz have additionally seen mites develop a resistance – effectively creating a time bomb for future bee generations. To worsen matters further, studies have suggested that miticides such as Tau-fluvalinate can themselves act as stressors for bees and increase susceptibility to virus’s such as Deformed Wing Virus. This shouldn’t be too surprising, we are dealing with pesticides after all.
The ‘only solution’ to these negativities and shortcomings has been the practice of cycling between different miticides. Apiculturists have come to accept that some miticides are simply not as effective, inconsistent, easy to misuse, toxic for bees and brood, and lead to future generations of resistant mites. And none can be used continuously throughout the season to really solve the problem of varroa. It is no wonder then, that the bee populations are in trouble.
None of this information is new to beekeepers. The real answer to question ‘why do beekeepers still rely on miticides’ is that until recently, there hasn’t been a better option. But with the complex calendar of cycling and dosages, varied rates of efficacy and heavy price to pay, there is no sustainable solution when it comes to miticides.
Toxicity of Selected Acaricides to Honey Bees (Apis mellifera) and Varroa (Varroa destructor Anderson and Trueman) and Their Use in Controlling Varroa within Honey Bee Colonies.
Gregorc A, Alburaki M, Sampson B, Knight PR, Adamczyk J.
Influence of Varroa Mite (Varroa destructor) Management Practices on Insecticide Sensitivity in the Honey Bee (Apis mellifera), Frank D. Rinkevich, Robert G. Danka, Kristen Bartlett-Healy, 2017.
Field-Level Sublethal Effects of Approved Bee Hive Chemicals on Honey Bees (Apis mellifera L) Jennifer A. Berry1 , W. Michael Hood2 , Ste´phane Pietravalle3 , Keith S. Delaplane1 , 2013.
Methods to control varroa mites-an integrated pest-management approach, ROBYN UNDERWOOD, PHD, MARGARITA LÓPEZ-URIBE, PH.D., 2019
Fate of Dermally Applied Miticides Fluvalinate and Amitraz Within Honey Bee (Hymenoptera: Apidae) Bodies, Neil Kirk Hillier, Elisabeth H Frost. 2013.
Acaricide Treatment Affects Viral Dynamics in Varroa destructor-Infested Honey Bee Colonies via both Host Physiology and Mite Control, Barbara Locke,, Eva Forsgren, Ingemar Fries, Joachim R de Miranda. 2012.
Fertility and reproductive rate of Varroa mite, Varroa destructor, in native and exotic honeybee, Apis mellifera L., colonies under Saudi Arabia conditions
Saudi J. Biol. Sci., 24 (2017), pp. 992-995
Efficacy assessment of soft and hard acaricides against Varroa destructor mite infesting honey bee (Apis mellifera) colonies, through sugar roll method
ZulNorain Sajid Muhammad AsifAziz ImranBodlah Rashid MehmoodRana
Hamed A.Ghramh Khalid AliKhan
Acaricide efficacy and honey bee toxicity of three new formic acid-based products to control Varroa destructor, Marco Pietropaoli & Giovanni Formato (2019), Journal of Apicultural Research, 58:5, 824-830, DOI:
Formic Acid, Agroscope, 2020.