Fit against Varroa - Fit for winter

A dangerous parasite poses a threat to honey bee health

In Europe and North America, honey bees are productive livestock which require intensive care by humans to stay healthy. The biggest threat beekeepers have to manage is the Varroa destructor mite. This parasite weakens the bees and transmits viruses that cause deadly diseases. Bee experts see Varroa as the main reason for increased honey bee colony losses, especially over the winter season. Bee experts are working on new approaches to control Varroa and thus improve the overall health of honey bees.

Fit against <em>Varroa</em> - Fit for winter


// Varroa destructor literally means “destructive mite.” In fact, it is the most deadly pest of the Western honey bee.

// The parasitic mite, Varroa destructor, originated in Asia and has been introduced to most of the world through human movement of honey bees.

// Unlike the Asian honey bee, the Western honey bee has not yet developed sufficient mechanisms to protect itself against the parasite.

// Varroa infests adult honey bees and their brood and, thus, weakens the entire bee colony. Additionally, Varroa transmits deadly viruses to honey bees.

// Varroa is the main cause for regionally increased overwintering honey bee colony losses in Europe and North America. Another key contributing factor is starvation.

// Bayer is actively involved and committed, worldwide, to monitor the Varroa and to develop strategies to control the mite to improve the health of honey bee colonies.

Ernst Caspari

Ernst Caspari is a member of the Leverkusen beekeeper association in Germany and has many years of beekeeping experience, having started beekeeping as a teenager. Today, he still manages 20 honey bee colonies.

Unlike honey bees in many parts of the world, Ernst Caspari’s bees remain calm when he removes their honeycomb from the beehive. He does not need to wear gloves or a net-like veil to protect his face. “These bees are calm – a direct result of quality breeding,” the experienced beekeeper and breeder explains. 86-year-old Caspari, who keeps 20 honey bee hives, is a member of the beekeeper association of Leverkusen, Germany.

As a former Bayer scientist in the area of organic chemistry, Caspari continues to be in close contact with research colleagues and bee experts at the Bayer Bee Care Center located nearby, in Monheim. Bayer has been actively engaged in protecting and improving bee health for 30 years now and Caspari benefits greatly from this intensive exchange of expertise.

His enthusiasm for honey bees has not changed at all over the years – his tasks as a beekeeper, on the other hand, have changed tremendously: fighting bee pests is now much more a focus than it was when he started out as a beekeeper. The main threat is the Varroa mite. The parasite infests honey bee colonies and crawls into the bees’ brood cells – right before the worker bees cap these cells. Initially, the adult female mite that enters the cell will hide under the bee larva. Once the cell is capped and the larva has consumed all of its brood food and pupates, the parasite starts feeding on the newly formed bee pupa. Female mites can also live on adult honey bees, especially during broodless periods such as winter: they penetrate the chitin armor with their mouthparts and suck the hemolymph, a bees’ blood-like fluid.


Development of Varroa mites in a honeycomb cell

Varroa Scheme
  1. Queen bee lays eggs

  2. Worker bee feeding larva

  3. Varroa mite enters cell with larva inside

  4. Reproduction: The female mite starts to lay eggs. The first egg is not fertilized and develops into a male.

  5. Larval growth to final stage

  6. Worker bee closes cell with wax

  7. Pupation phase

  8. From the next 4-5 fertilized eggs female mites will develop

  9. Young bee hatches from the cell with adult Varroa mite and 1-2 mature daughter mites.


In addition to the parasite itself, there is also a more severe danger for honey bees: secondary infections. Like a tick, the Varroa mite transmits viral diseases. “The mites weaken the bees’ immune system, making honey bees more susceptible to diseases,” says Peter Trodtfeld, bee expert and beekeeper at the Bayer Bee Care Center. “Moreover, the mite transmits viruses directly into their hemolymph, so normally less harmful viruses can become deadly to bees.” Varroa is a vector for Deformed-Wing Virus (DWV), for example. If bee pupae are infected with this virus, their wings do not develop correctly. Consequently, the hatching bee is not able to fly and survive. To spread from hive to hive, the Varroa mite depends on honey bees to transport it. This happens when bees are searching for food, as they may frequently come into contact with bees from other colonies, even those located several kilometers away. In this way, the mite can rapidly spread.

Keeping Varroa infestation in honey bee colonies as low as possible is a particularly important task for beekeepers especially in Europe and North America, where the mite can cause high losses. As such, one of Caspari’s tasks is to assess the extent of the Varroa infestation in his own honey bee colonies. To do this, he counts the number of mites that have fallen through an open grid onto a sticky board on the bottom of the hive. “From the number of dead Varroa, we can extrapolate the number of live mites in the bee colony,” he explains. Regular diagnosis is critical for the appropriate timing, frequency and choice of treatments to control Varroa.

One virus the mite transmits is Deformed Wing Virus (DWV). This can affect both brood and adult bees.

One virus the mite transmits is Deformed Wing Virus (DWV). This can affect both brood and adult bees.

Symptoms may not be visible until infected bee pupae hatch with deformed wings (see above). As these bees cannot fly, their life span is shorter.

Symptoms may not be visible until infected bee pupae hatch with deformed wings (see above). As these bees cannot fly, their life span is shorter.

Photo Credit: copyright Rothamsted Research Ltd.


The Varroa mite is native to Asia. The parasite Varroa jacobsoni was discovered on the Indonesian island of Java 100 years ago and at that time it was associated only with the Asian honey bee (Apis cerana). Presumably, this bee had been living alongside the mite for thousands or millions of years in a host-parasite equilibrium. When hives of the Western honey bee (Apis mellifera) were introduced to East Asia, it was not foreseen that, through direct contact with the native Asian honey bee, the newly introduced Western honey bee colonies would also become infested. Due to the speed with which this occurred, the Western honey bee did not have time to build up sufficient mechanisms to protect itself against the parasite. “Without man’s help, an infested Western honey bee colony will normally die within a few years,” Trodtfeld explains. Unfortunately, the detrimental longer-term effects of the Varroa mite on Western honey bee health was discovered too late to prevent infested colonies from being returned to Europe, from where the invasive parasite has continued to spread since the 1970s.

Peter Trodtfeld, bee expert and beekeeper at the Bayer Bee Care Center in Germany

Each colony requires intensive care. Especially, the control of pests and diseases to ensure the bees stay healthy as they are then more likely to survive the winter. Peter Trodtfeld, bee expert and beekeeper at the Bayer Bee Care Center in Germany, sees good nutrition and timely Varroa treatment as the most important elements in preparing a honey bee colony for winter.

In the following years, based on genetic analyses, researchers discovered that the mite that was carried to Europe is a different species than Varroa jacobsoni. That new species is today known as Varroa destructor. Nowadays, this mite populates most regions of the world: China, Russia and other Asian countries have confirmed finding the parasite in addition to Europe, Africa, North and South America, and New Zealand. In almost all areas of North America and Europe, the mite is present in virtually every honey bee colony. This is why there are fewer feral honey bee colonies than there were before Varroa in North America and almost no feral honey bee colonies in Europe any more. So far, Australia is the only continent to have been spared from a lasting invasion, primarily through intensive inspections and strict quarantine controls. Although Varroa is prevalent worldwide, it does not show the same impact on bee colony health in each region because of the different behaviors of the bees. In Africa and parts of South America, Varroa does not appear to be a key factor influencing honey bee health because the bees show a distinct tendency to swarm, leaving a part of the colony behind, along with the brood combs which contain a large number of the mites. Similarly, in South-East Asia, no major problems are known so far. That said, in many other countries beyond Europe and North America, specific information about the current situation regarding the prevalence and impact of Varroa is still missing.

To control the mite, there are synthetic products, so-called Varroacides, as well as organic acids, such as 15-percent oxalic acid and 60-percent formic acid which can be used: “I do the first treatment with formic acid immediately after the last harvest of honey,” Caspari says. The liquid compound vaporizes and disperses throughout the hive in gaseous form. “The vapor bath even penetrates the sealed brood cells, killing the mites feeding in there,” he explains. Temperature plays a decisive role in the effectiveness of the treatment: it influences how fast the formic acid vaporizes. So, products that require specific outside temperature and environmental conditions are only suitable under certain climatic conditions and as such can only be used in certain regions of the world. Beekeepers can also use synthetic Varroacides which were developed and registered specifically to control the parasitic mites. Some of the synthetic active ingredients, such as amitraz, coumaphos or flumethrin, are embedded into a plastic strip and hung between the honeycombs in a hive. “The active ingredient particles are picked up in the bee’s hair and are passed onto other bees inside the hive through their social interaction. In this way, the female mites outside the brood combs and on the bees are also exposed to the substance and are killed,” Trodtfeld says.

Dr Tjeerd Blacquière Senior Scientist at Plant Research International of Wageningen University, The Netherlands

Dr Tjeerd Blacquière
Senior Scientist at Plant Research International of Wageningen University, The Netherlands

Photo Credit: Willem Boot

A Varroa population can double every three to four weeks during the breeding season. It can grow from 50 mites up to around 3,200 mites from the beginning of February to the end of August.

However, a single Varroa treatment at the end of a bee season is not sufficient to protect the winter bee colony from these parasites. With the correct treatment earlier in the year, in summer or early fall, beekeepers can reduce the Varroa numbers but not completely eradicate Varroa from a honey bee colony. “A few mites always remain and will begin to reproduce again in spring,” Caspari says.
A Varroa population can double every three to four weeks during the breeding season. It can grow from 50 mites up to around 3,200 mites from the beginning of February to the end of August. The health and vitality of winter bees that developed during a high mite infestation in autumn are severely compromised. As a result “The Varroa treatments have to be timely as we are unable to subsequently treat or cure diseases that may have been transmitted by the parasite previously,” Trodtfeld explains.

In addition, “Many environmental stressors in late summer can impair the development of vital winter bees: bee diseases and parasites such as the gut parasite Nosema, lack of forage or adverse weather effects may reduce the quantity and quality of the winter bee populations”, explains Dr Tjeerd Blacquière, Senior Scientist at Plant Research International of Wageningen University in The Netherlands.

Dick Rogers

Dick Rogers
Principal Scientist and Entomologist at Bayer Bee Care Center in North America

“Since its arrival in the USA, the Varroa mite has been a tremendous challenge that has to be overcome – with the help of intensified monitoring and improved hive management. To prevent resistance problems, it is essential that beekeepers do not overuse effective products and rotate product use each year.”


"The most serious pest is the Varroa mite together with the viruses it spreads."

Dr Tjeerd Blacquière

Ultimately, the lower the Varroa infestation at the end of a bee season – and the stronger the honey bee colony – the greater the chances are that the colony may survive the winter. “Honey bee colony losses of up to 10 percent are normal during the winter,” Caspari explains. The increased presence of Varroa mites has caused increased honey bee colony losses during winter in recent years, particularly in many European regions and in North America. Caspari has witnessed this himself, since mites first infested his honey bees in 1984: “Initially, 15 dead mites falling into the sticky board during a Varroa treatment in late summer were considered a lot. Today, I often count hundreds of mites,” he states.

Worldwide, the number of managed honey bee colonies has increased about 65 percent over the last 50 years. However, increased honey bee colony losses during winter mean beekeepers have to compensate by replacing their lost colonies in the spring. “The Bee Informed Partnership” (BIP) – supported by the United States Department of Agriculture (USDA) – reports preliminary results of up to 28 percent of US honey bee colonies lost during winter 2015-16. “This is above the loss rate of 15 percent, considered acceptable in the USA. Moreover, we did our own survey and talked to many beekeepers, who recorded losses up to 100 percent,” says Dick Rogers, Principal Scientist and Entomologist at Bayer Bee Care Center in North America. Rogers sees the Varroa mite as the main cause of honey bee colony losses in North America: “Virtually all colonies in the USA are infested to some extent,” he states. “A survey we conducted in 2015 showed that 78 percent of the checked honey bee colonies had more than three mites per 100 bees. Extrapolated to a colony of 40,000 bees, this means a minimum of over one thousand mites per colony – an alarming number.”


Professor Dr Robin Moritz and Alexis Beaurepaire

Professor Dr Robin Moritz (right) and Alexis Beaurepaire (left)


Genes: Going back to basics

Dr Robin Moritz is a professor of molecular ecology at Martin Luther University, located in Halle-Wittenberg, Germany. Biologist, Alexis Beaurepaire, has been conducting research at the university since 2011, writing his dissertation on the host-parasite interaction of the Asian honey bee and the Varroa mite. Since 2014, Bayer has been supporting his study.


What is the background of your dissertation?

Beaurepaire: If we want to control Varroa destructor efficiently, first we have to understand its biology. For a while now, scientists and beekeepers have noticed that Varroa mites are becoming resistant to insecticide products with the same active substance, if the mites are exposed to them continuously. We want to find out how these resistances develop. Our approach is to first learn more about the development of mite populations through genetic research.

Prof. Moritz: It is interesting to note that, at the beginning of their lifecycle in the brood cells of bees, mites depend on inbreeding to be able to reproduce. Normally, this is bad for the genetic diversity of populations. Varroa, however, seems to benefit from it.

In what way?

Beaurepaire: During my year-long investigation of the dynamics within a Varroa population, I used genetic markers and observed higher genetic diversity than expected assuming inbreeding alone and also a rapid evolution. This may provide an explanation for Varroa’s fast development of insecticide resistance. My goal now is to examine mite reproduction more closely.

How is your research connected to the Varroa Gate technology?

Beaurepaire: The Varroa Gate helps kill the mites that have already entered the beehive – and targets others as they enter. The Varroa Gate technology is very elegant. It is, however, rather useless if the mites become resistant to the active agent within a short period of time. Therefore, it is important to rotate different active ingredients and treatment options systematically.

How does the collaboration with Bayer work?

Prof. Moritz: Our collaboration with Bayer started in April 2014. Bayer Animal Health is testing the Varroa Gate through large field studies, which are under the supervision of Professor Dr Nikolaus Koeniger and Dr Gudrun Koeniger at the University of Halle (in Germany). They collect mites which have and don’t have resistance. Alexis Beaurepaire then identifies the type of genome for these mites and examines if, by the end of the season, the seasonal population dynamic of both types of mites is influenced by inbreeding as well as genetic recombination – in other words, by mixing genetic material.

How can we combat resistant mites?

Beaurepaire: By timing the treatments with the natural recombination phase of the mite. When the amount of bee brood in the colony is low towards the end of the season and if there was no treatment before, the few resistant mites in the colony have to mate with susceptible ones to produce susceptible offspring. Hence there will be many more susceptible mites after this phase which can then be killed efficiently by the Varroacide.


Honey bees


Helping honey bees help themselves: breeding Varroa-resistant bee populations would be a future solution for the mite problem.

Honey bees with a behavioral trait called Varroa-sensitive hygiene (VSH) can detect a Varroa mite in a closed brood cell. These bees pull out the infested pupae and remove it, thus, also removing the Varroa and slowing its proliferation in the colony.

Researchers are trying to strengthen this defensive capability through selective breeding among European bees, to create long-lasting protection against the parasites. Research and breeding programs have had some success and there is potential to significantly improve the health of honey bees in the future – however more research is required.


For Europe, Dr Blacquière, sees a more positive situation: “In most European countries, including The Netherlands, Germany and Austria, bee losses were very low last winter,” he says. “Only some beekeepers report high losses, which I would relate to a late or missing Varroa treatment.” The research network COLOSS (Prevention of honey bee COlony LOSSes) published a preliminary report on the winter losses in many European countries in July 2016. In most countries, the honey bee colony losses were low with the European average around 12 percent. Dr Klemens Krieger, bee health expert at Bayer, believes that the mild winter of 2015-16 could still have long-term effects: “The warm temperatures may mean the honey bees continued breeding for a longer time span. This also means that the Varroa mites could reproduce inside the bees’ brood cells, throughout almost the entire year. As a result, their numbers will have increased significantly in the bee colony, making them even harder to control.”

In cooperation with external research partners, Bayer has been working continuously to improve honey bee health and find strategies to effectively control the Varroa mite. Bayer researchers have developed a plastic strip containing an acaricidal active substance, which is fitted over the entrance to the beehive and designed to prevent mite infestation. It is planned to bring the product to market in 2017 for use by beekeepers as part of their integrated Varroa management programs. Bayer has developed this technology in cooperation with Professor Dr Nikolaus Koeniger and Dr Gudrun Koeniger at the University of Halle in Germany.

As a general rule, beekeepers need to vary their treatment strategy: “If beekeepers continuously treat the Varroa mite with the same Varroacide or another with the same mode of action, there is a risk that the parasite will develop resistance to that substance,” explains Trodtfeld. The development of such resistances has already been reported for all available synthetic Varroacides. Through the rotation of varying substances which work in different ways to control the mites, a fast development of resistance can mostly be prevented and existing resistances can be counteracted.

To test how far pyrethroid resistance was occurring, Bayer asked beekeepers in Germany to send in samples of Varroa mites. In a molecular biological test the mites were analyzed for resistance. “Luckily, we have found very few of these pyrethroid resistant mites in Germany,” Trodtfeld reports. “This is a favorable tendency,” he says. Resistance monitoring has recently been extended to areas in Hungary, where no pyrethroid resistant mites were found. In cooperation with the University of Valencia and Rothamsted Research, Bayer scientists are developing a test to show if a mite is resistant against pyrethroids. They also aim to develop a similar test for other Varroacides. “Our aim is to develop a fast, easy to handle and inexpensive method to detect all kinds of resistances”, explains Krieger. In this way, beekeepers can immediately clarify, if resistant mites appear in their bee colony and choose the most effective treatment option.


Infestation 5–10 times higher

To reproduce, a female Varroa mite enters a bee brood cell shortly before it is capped. Mites infest drone brood 5 – 10 times more often than worker brood because drones need 2 – 3 days longer to develop into bees. The extra days allow more daughter mites to develop before the drone bees emerge from the brood cell.


Bayer has already discovered resistant mite populations in studies in the USA: “In some regions, only a few products available for beekeepers remain effective against Varroa,” Bayer scientist Rogers says, summarizing the previous findings of an ongoing study. Therefore, the US Bee Care Team promotes the development of more effective Varroa management which includes a search for more effective treatments and management strategies. Among many other projects and initiatives, the Bayer Bee Care Center in North America provided funding for a Healthy Hives 2020 Initiative (HH2020) in 2015. The program’s goal is funding external projects that seek to better understand honey bee health in the United States and make measurable improvement by 2020.

Rogers is confident that by taking the right measures, Varroa can be controlled more effectively: “Beekeepers can minimize the infestation if they learn to take the right precautions at the right time,” he says. The level of a Varroa infestation in bee colonies may vary significantly from one year to another, which is a good reason for Rogers to intensify monitoring activities: “Within our Smart Hive Program, we work with early warning and data collection systems that remotely monitor colony conditions. Hive monitoring sensors in and near the hive include acoustics, brood area temperature, relative hive humidity, hive weight, and environmental weather conditions. With the sensor data we are able to assess the condition of a bee colony from any internet connected computer or mobile device,” he says. “And in conjunction with systematic in-hive observations we can more fully understand colony health status and provide for the needs of the bees in a precise and timely way.”

Beekeeper Caspari is pleased with his Varroa diagnosis: the infestation in the bee colony is currently low. He will, however, keep an eye on the parasitic threat throughout the entire bee season. “We have to take measures early enough to keep the honey bees healthy. Only strong colonies have a chance to survive the following winter and to rebuild a strong colony in the spring,” Caspari says. Bees are not only important to beekeepers worldwide who earn a living from honey, hive products, and pollination services, but these insects ensure the pollination of many crops and, thus, are an essential part of sustainable agriculture.


Udo Gensowski


“The Varroa mite is the main topic of conversation among beekeepers.”

Udo Gensowski is the chairman of the non-profit bee breeding association Bechen e.V. located near Bergisch Gladbach, Germany. The association was established in 1930 and has currently more than 160 members. At monthly meetings, experienced beekeepers share their expertise with younger beekeepers. One important topic: the treatment of Varroa.


How much of an impact does Varroa destructor have on beekeeping?

The Varroa mite has changed our job enormously over the last few years. It is no longer advisable – without training – to keep bees simply as a hobby. The spread of this parasite requires us to know more than we used to. We have to monitor more carefully and have to use new methods – for example, removing the drone brood. The male brood is particularly prone to mite infestation. Therefore, the removal of the drone brood reduces the reproduction of Varroa.

In your opinion, how dangerous is this parasite?

The Varroa infestation determines how well a bee colony survives winter, particularly because the mites also infect honey bees with secondary diseases that are potentially fatal. There are other bee diseases such as Nosema or American Foulbrood – however, the most prevalent issue in Germany is the Varroa problem.

What do beekeepers say about Varroa?

The Varroa mite is really the main topic of conversation among beekeepers. Every year we see heavily infested bee colonies – and others with very little infestation. The way the beekeeper handles the problem determines the extent of infestation. In my opinion, you need to be consistent and precise and be able to interpret even the smallest sign of a problem. If there is still increased bee breeding in the autumn, for example, this can be a sign of a bee colony’s struggle with increased Varroa infestation. With treatment, timing is crucial. Substances such as formic acid and oxalic acid are harmful to bees if applied incorrectly. Our workshops help with the correct application. My recommendation to every young beekeeper is to seek support from an association.

What are other factors that impact honey bee health?

It is important for honeybees to find sufficient nourishment. As a beekeeper, you have to consider different factors every year – like the weather and nearby flowering plants. Even if there are a lot of flowers in the surrounding areas, it does not mean automatically that there will be enough nectar. Many blossoms do not produce nectar at external temperatures below 20 degrees Celsius. I always tell young beekeepers: monitor your bee colonies and nature closely.




Understanding Honey Bee Health

The impact of different factors

Dr. Geoff Williams, from the Institute of Bee Health, Bern University talks about the impact of different factors on honey bee health, explains how the data gathered by COLOSS is helping improve situation worldwide ...


The Western honey bee’s biggest enemy: Varroa Destructor

The mite is a huge threat to the health of the bees as it transmits dangerous diseases. Bayer expert Dr Klemens Krieger knows the mite well and understands the impact this pest can have on honey bee colonies. In the video he explains how Bayer aims to support beekeepers to protect the bees.


Varroa destructor: reasons for bee decline

A continous challenge for research and beekeeping. Results of a field study conducted by the Frankfurt University Bee Research Institute, Oberursel.



Additional information

This is how honey bees with a behavioral trait called Varroa-sensitive hygiene (VSH) are “Getting Rid of Mites”.

In “Combating Varroa, we look at efficient mite control for honey bees, in the field.

Interested in knowing more about Varroa? Our brochure "The Varroa mite – Varroa Destructor, a deadly and dangerous bee parasite” is packed with useful information.

Complete Story

Fit against Varroa - Fit for winter A dangerous parasite poses a threat to honey bee health
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