The dark side of miticides

«Unfortunately, the use of in-hive acaricides comes at a price, as they can produce sublethal effects that are difficult to quantify, but may ultimately be as damaging as the mites they are used to treat.»

(O’Neal, Brewster Bloomquist, Anderson)

The varroa mite has been a major threat to the European honeybee for decades. As with many other contributors to our food systems, we have depended upon chemicals to save the day. But as Colony Collapse Disorder takes an increasing toll on our pollinators, isn’t it time to re-evaluate the benefits and costs of miticides?

There is no shortage of miticides available to the modern beekeeper, each with its own chemical make-up, and distinct way of killing mites. Here lies the complication- as not all miticides have the same negative effects on bees, comparing them by the same criteria simply isn’t possible. To deal with this ‘apples vs oranges’ situation, beekeepers must take a more macro view on the costs and benefits, and generalise about certain miticides being ‘good’ or ‘bad’, ‘better’ or ‘worse’. And in the end, isn’t any solution better than no solution at all, even if it isn’t perfect? Most beekeepers agree that miticides are not ideal, but it is a compromise we have been willing to take, as long as the colony survives. This argument seems logical. The bees are facing challenging times, and desperate times call for desperate measures. But could it be that in our dire search for a solution, we have overlooked the damage caused by miticides; that well-intentioned beekeepers are in fact contributing to the problem?

If miticides are the answer to the devastating effects of varroa, why are honeybee populations in the worst state they have been in in decades? It should be acknowledged that the scientific community do not believe varroa to be the sole cause of colony collapse. Pesticides, mono-cultural practices and habitat loss also play a significant role. How then do miticides complicate this cluttered group of stressors? In this article we delve into the research, and explore some of the often overlooked and frightening attributes of miticides.

Miticides are directly linked to bee fatalities

Let’s begin with the most obvious point. It shouldn’t cause too much of a shock to learn that miticides are in fact toxic. But we are at war with varroa, right? For beekeepers, the question then becomes, ‘what is an acceptable mortality rate for bees and brood’. The type of miticide, hive strength, environment and infestation should all be taken into consideration when calculating dosages. However, despite the best efforts of beekeepers, these shifting variables mean that generalised treatment plans lead to mixed and often dangerous results.

The 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’, examined the efficacies of 5 of the most common commercially available acaricides in order to control varroa. The research showed that, while all were indeed toxic for bees, two of the most commonly used miticides- Tau-fluvalinate and coumaphos, proved to be particularly lethal to colonies-even when applied in recommended dosages.

«The CheckMite® 1 dose also induced a high bee mortality at the level of 59.82% ± 11.86, in comparison with the CheckMite® ½ dose, which induced a 4.18% (±1.8) bee mortality. The HopGuard® 1 dose simultaneously induced a 100% Varroa mortality and a 93.68% ± 4.92 bee mortality. On the other hand, the HopGuard® ½ dose induced a 95.79% (±1.9) Varroa mortality and a 23.41% (±6.1) bee mortality.»

(Gregorc, Alburaki, Sampson, Knight, Adamczyk) .

The mortality of bees goes beyond the immediate losses during treatment. Tau-fluvalinate and coumaphos are responsible for negatively impacting bee nutrition, immunity and physical development, as described in the study ‘In-Hive Acaricides Alter Biochemical and Morphological Indicators of Honey Bee Nutrition, Immunity, and Development’. This ultimately effects the colonies long term susceptibility to viruses and other pesticides, ability to forage and brood rearing.

«The results described here demonstrate that exposure to tau-fluvalinate and coumaphos has an impact on 1) macronutrient indicators of bee nutrition by reducing protein and carbohydrate levels, 2) a marker of social immunity, by increasing glucose oxidase activity, and 3) morphological indicators of growth and development, by altering body weight, head width, and wing length.»

(Reeves,  O’Neal,  Fell,  Brewster,  Anderson)

The distinction between natural and synthetic chemicals positions natural or ‘soft’ miticides as a more friendly option. In many parts of Europe, the most heavily used miticides are based on formic acid (MAQS, Mitegone). The danger to bees, queens and brood, however, is its sensitive and volatile nature combined with its strength.

Biologist Dr.Randy Oliver of states:

«Formic vapor (which smells similar to vinegar) is a corrosive irritant that can be toxic to the applicator, and to adult bees and brood at high concentrations. This safety issue is without a doubt the biggest concern.»

Dr. Randy Oliver

Due to its small molecule size, formic acid evaporates at room temperature, allowing it to be used as an in-hive fumigant. While many new products have made the application less volatile, variables such as humidity and temperature remain beyond the control of the beekeeper.

«Formic acid is temperature dependent and can cause damage to the colony if used at ambient temperatures higher than 85F (29°C) because it can increase brood mortality and the potential for queen loss. When used below 50F (10°C), formic acid results in low efficacy.»

(Underwood, Lopez-Uribe)

Colony size is another significant factor in the lethality of formic acid, as the bees themselves are needed to circulate the toxic vapours. In small colonies, less than six frames, formic acid can lead heavy brood loss and colony collapse as there are not enough bees to control the vapour ventilation, as Dr. Randy Oliver states.

In an effort to find the optimal dosage and exposure periods for formic acid treatments over winter , the study ‘Effect of concentration and exposure time on treatment efficacy against Varroa mites (Acari: Varroidae) during indoor winter fumigation of honeybees (Hymenoptera: Apidae) with formic acid’ looked at the effects of different dosages over different lengths of time. The research discovered that both milder and extreme treatments lead to higher bee mortality, while a long term-low dosage treatment, was significantly less effective at killing mites.

«Both short-term high-concentration and medium-term medium-concentration fumigation with formic acid killed varroa mites, with averages of 93 and 83% mortality, respectively, but both treatments also were associated with an increase in mortality of worker bees, queen bees, or both. Long-term low-concentration fumigation had lower efficacy (60% varroa mite mortality), but it did not increase worker or queen bee mortality.»

(Underwood, Currie)

Oxalic acid is another soft chemical gaining popularity. However, it too is directly linked to mortality of bees and brood. In Penn State Universitiys’ guide ‘Methods to control varroa mites’, researchers outline that oxalic acid can cause harm by:

«crystalizing in the midgut of larvae, increasing larval mortality, and reducing brood area. Overuse of this treatment can also decrease the activity and longevity of workers.»

(Underwood, Lopez-Uribe)

Miticides can enhance honeybee susceptibility to viruses

The term ‘Out of the frying pan, into the fire’ has never been more fitting. As we fight so hard to rid one stressor, the introduction of miticides can inadvertently reduce the bees defence against others.

In the report ‘Acaricide Treatment Affects Viral Dynamics in Varroa destructor-Infested Honey Bee Colonies via both Host Physiology and Mite Control’, researchers looked at the link between Tau-fluvalinate (Apistan) and Deformed Wing Virus. The paper concluded that the increase in DWV titers in treated adult bees and pupae, suggested an increased susceptibility to Deformed Wing Virus, due to the direct effects of the active ingredient Tau-Fluvalinate on honey bee physiology and /or immune responses. (Locke, Forsgren, Fries, de Miranda)

It is not just Tau-Fluvalinate that weakens the immune systems of bees. There is also a documented synergistic effect between Amitraz (Apivar) and viruses. The study ‘ Amitraz and its metabolite modulate honey bee cardiac function and tolerance to viral infection’. Investigated the physiological and immunological effects of amitraz and its primary metabolite in honey bees. The results demonstrated the negative effects of amitraz on bee viral susceptibility and cardiac function.

«Using flock house virus as a model for viral infection, this study found that exposure to a formamidine acaricide may have a negative impact on the ability of honey bees to tolerate viral infection. Furthermore, this work has demonstrated that amitraz and its metabolite significantly alter honey bee cardiac function, most likely through interaction with octopamine receptors.»

(O’Neal, Brewster Bloomquist, Anderson)

Miticides contaminate bee resources

Some miticides, particularly fat-soluble hard chemicals, such as bromopropylate (Folbex VA Neu), coumaphos (Perizin, Asuntol) and fluvalinate (Apistan, Klartan, Mavrik) leave residues in honey, propolis and wax. This not only creates a dangerous environment for the bees that are chronically exposed to this toxic cocktail, but also contaminates the materials they require for energy, nutrition, food storage and brood rearing. Wax is of particular importance here, as it is the least renewable resource in the hive. As such contaminant levels are at their most concentrated in wax, causing ‘toxic-house’ syndrome.

The world’s largest study of miticide and pesticide residues in bee colonies collected samples from beekeepers across 23 U.S states, one Canadian province and several agricultural cropping systems during the 2007–08 growing seasons.  The research discovered the following:

«Almost all wax and pollen samples (98.4%) contained two or more pesticide residues, of which greater than 83% were fluvalinate and coumaphos.»

(Mullin , Frazier, Frazier, Ashcraft, Simonds, vanEngelsdorp, Pettis

The researchers additionally stated that:

 «Chronic exposures to high levels of these persistent neurotoxicants elicits both acute and sublethal reductions in honey bee fitness, especially queens , and they can interact synergistically on bee mortality.»

(Mullin , Frazier, Frazier, Ashcraft, Simonds, vanEngelsdorp, Pettis)

Another cause for concern, is the compounded stress caused by the synergistic effects of agricultural pesticides and miticides working in unison. Just as with in-hive miticides, crop pesticides also amass in bee products. The aforementioned study made note of one such example:

«More concerning is that the combined effect of coumaphos and trace amounts of an unrelated, but common crop insecticide, the neonicotinoid imidacloprid reduces food uptake by approximately 33%.»

(Mullin , Frazier, Frazier, Ashcraft, Simonds, vanEngelsdorp, Pettis)

Oh, and by the way, varroa are gaining resistance

Just in case you needed another reason to rethink miticide use, resistance to fluvalinate, coumaphos , and amitraz have been clearly documented since the 1990’s.

The decreased sensitivity of mites to these miticides has resulted in beekeepers relying on increasing levels of the chemicals. This in turn results in higher concentrations within the hive.

The study ‘Toxicity of Selected Acaricides to Honey Bees and Varroa and Their Use in Controlling Varroa within Honey Bee Colonies’ suggests the following:

«An over-reliance on these products, without adequate rotation, is associated with increasing incidences of Varroa resistance, particularly to the pyrethroids.»

(Gregorc, Alburaki, Sampson, Knight, Adamczyk)

For the die hard miticide fan, it is a battle that cannot be won.

A changing landscape

It is unlikely that miticides are going to completely vanish anytime soon. Varroa after all is still a devastating reality. But the toll on bee populations is itself incredibly heavy. Varroa is not the only stressor. Pesticides, habitat loss and monoculture also impact bee populations. Miticides amplify the effect of each of these by lowering bee immune systems and nutrition, weakening their physical development, poisoning their hive environments, and lowering their ability to reproduce.

The difference between now and 10 years ago, is that now there are good alternatives. After forty years of use, and at best ‘questionable’ results, it is time to re-evaluate our dependence on miticides and really consider, is it worth it?


Amitraz and its metabolite modulate honey bee cardiac function and tolerance to viral infection. Scott T. O’Neal, Carlyle C. Brewster,Jeffrey R. Bloomquist, Troy D. Anderson, Journal of Invertebrate Pathology, 2017:

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., 2018:

In-Hive Acaricides Alter Biochemical and Morphological Indicators of Honey Bee Nutrition, Immunity, and Development. Alison M Reeves, Scott T O’Neal, Richard D Fell, Carlyle C Brewster, Troy D Anderson, Journal of Insect Science, 2018:

IPM 7 The Arsenal: “Natural” Treatments – Part 1. Dr Randy Oliver,

IPM 7 The Arsenal: “Natural” Treatments – Part 1

Methods to control varroa mites-an integrated pest-management approach. ROBYN UNDERWOOD, PHD, MARGARITA LÓPEZ-URIBE, PH.D., 2019:

Effect of concentration and exposure time on treatment efficacy against Varroa mites (Acari: Varroidae) during indoor winter fumigation of honey bees (Hymenoptera: Apidae) with formic acid. Robyn M Underwood, Robert W Currie, 2005:

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:

High Levels of Miticides and Agrochemicals in North American Apiaries: Implications for Honey Bee Health. Christopher A. Mullin , Maryann Frazier, James L. Frazier, Sara Ashcraft, Roger Simonds, Dennis vanEngelsdorp, Jeffery S. Pettis, 2010:

Vatorex, Grant Morgan
28 July, 2020
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Vortrag von Dr. Wolfgang Ritter
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