Varroosis: a parasitic infestation of honey bees

What is Varroosis ?

The infestation of honey bee colonies by the parasitic mite Varroa jacobsoni is known as varroosis (pronounced varro-osis. This mite, generally referred to simply as varroa, is specific to honey bees. the natural host of varroa is the Asian honey bee, Apis cerana , which has its own natural defences against the mite. Through movement of colonies, varroa has spread to our own honey bee, Apis mellifera, which has no such defences. If left untreated, infested colonies in the UK will eventually die.

Since the 1970’s varroa has spread throughout Europe, North and South America and North Africa. it was first found in Western Europe in 1977, in Germany. The first outbreak in Britain was discovered in April 1992 in Torbay, Devon. At the time of writing, it has been found to have become widespread throughout most of the counties of England. Many cases have also been found in Wales and one case in Scotland. Within Europe, currently only Northern Ireland and Eire appear to remain free from varroa.

Beekeeper responsibility

Varroa cannot be eradicated and will eventually spread to every apiary in the UK as it has done in all other infested countries. However, regular and suitable husbandry and medication can reduce varroa infestation to harmless levels. All beekeepers must monitor their colonies regularly, even if they believe their area is still free from varroosis, and when necessary take steps to control the mites. Doing nothing is not an option, either before or after varroa arrives. Beekeepers who leave colonies untreated will lose them and cause serious problems for those who are trying to deal with the parasite, because neglected colonies will become a source of infestation for treated and managed colonies in the locality. In the future, successful beekeeping in the UK will depend on the techniques of varroa monitoring and control becoming part of routine bee husbandry.

Key strategies for effective varroa control

  1. Regularly monitor the infestation level in your colonies. This will warn you if the mite population is building up to a damaging level faster than expected, or if your control methods are not proving effective.Don’t just treat and leave it to chance.
  2. Don’t assume that a single chemical treatment or a single biotechnical method will always provide sufficient control. Practise integrated control of varroa, using a combination of biotechnical methods and varroacides, each working in different ways and at different times of the year. This will give you the most effective control.
  3. Use only UK or EU registered varroacides. These are proven in their efficacy and safety to bees, the user and consumers of bee products. Follow strictly the manufacturer’s instructions. Misuse may risk leaving increased residues in the hive and promote the development of resistant strains of mite.
  4. Try to co-ordinate treatment programmes with other beekeepers in the area, to reduce mite re-invasion problems. Your local beekeepers’ association will be able to help organise this.
  5. Never leave infested colonies unmanaged. They will eventually be killed by varroa and in the meantime they will re-infest the colonies of other local beekeepers who are trying to control infestation. If you can’t manage your bees, you should consider selling or giving them to others who can.
  6. Be prepared to be flexible and adaptable and change your control programme as circumstances dictate.

Varroa legislation in the UK

Following the discovery of varroa in the UK, in order to slow the spread of the mite the Ministry of Agriculture, Fisheries and Food (MAFF) imposed a ban on the movement of bees and hive frames containing comb from a Statutory Infected Area (SIA) covering parts of England to other areas of Great Britain except under licence. Subsequently, the SIA, now administered jointly by MAFF and WOAD, has been extended to include other areas where further cases of the disease have been detected. At the time of writing,  the SIA includes all counties of Wales and England with the exception of Greater Manchester, Lancashire, Cumbria, Tyne and Wear and Northumberland.

Beekeepers who wish to move bees should seek details of current restrictions. these can be obtained from the National Bee unit, or, outside England and Wales, from the appropriate National Agricultural Department Office (the addresses of which are listed at the end of this leaflet).

Varroosis is a notifiable disease under the Bee Diseases Control Order 1982. All new suspected cases in England and Wales must be reported to the Central Science Laboratory National Bee Unit (NBU). Samples of mites should be sent for confirmation to the NBU, together with the following information: beekeeper’s name and address, apiary name, O.S. map reference and number of colonies affected.

Varroa biology

Varroa is an external parasitic mite that lives exclusively on honey bees, feeding from their blood (haemolymph). To breed, the adult female mite enters a brood cell shortly before the cell is capped, where she remains in the brood food until the cell is sealed. She then feeds on the developing bee larva. Mating between mite offspring (brother and sister) takes place within the cell. The male mite cannot survive once the bee emerges from the cell; consequently all mature Varroa mites visible within the hive are female.

Mites, if given a choice, prefer to breed in drone brood, yet they are also well suited to infest worker cells of the European honey bee. In winter, when brood rearing is restricted, mites over-winter solely on the bodies of the adult bees within the winter cluster, remaining there until brood rearing commences the following spring.

Mite life-span

During the summer, female Varroa mites may live for 2-3 months. However, during the winter or broodless periods, they can live much longer, feeding on adult bees. Mites cannot survive without bees — for instance on combs or equipment — for more than about two days.

How varroa spreads

Varroa mites depend on adult bees for transport, through the natural processes of robbing and drifting. Although adult drones do transfer mites, the majority of mites are transferred by workers (see mite invasion section). The principal way in which mites migrate, however, is when the beekeeper moves mites over long distances, often unaware of the mites’ presence.

Harmful effects of varroosis

Effects on infested bees – Individual bees infested by varroa during their development normally survive, but suffer a degree of damage depending on how many mites have infested them. Infested bees may have a shorter life than uninfested bees. Severe colony infestation may cause some brood to die in the cells and emerging adults may have deformed wings and abdomens. The reasons for this are not fully understood; however, it is thought that much of the harm results from the presence of viruses transmitted between bees by the mites. Such viruses may be naturally present in many colonies at very low levels without causing any obvious disease. However, in heavily varroa-infested colonies they are sometimes present at much higher levels, and capable of injuring of killing the bees they infect.

Effects on colonies – The harm caused to infested colonies depends largely on the level of infestation. In the early stages, the low numbers of mites normally will have no obvious effect on the colony and infestation will easily go unnoticed by the beekeeper. In time, the mite population will increase until it ultimately reaches a level that the colony can no longer tolerate. At this stage a colony will appear to lose its social organisation and disband – a process referred to as colony collapse. The size of the mite population that is needed to cause collapse varies greatly between colonies. However, typically it is several thousand.

Colony collapse

Severely infested colonies may show no signs of harm until late summer and may well produce very good honey yields. However, examination of pupae at this stage would show a high proportion of both drone and worker brood to be infested, often with multiple mites per cell. Such levels of infestation prevent the replacement of the ageing adult bee population with healthy young bees. Colony collapse can then occur very quickly, frequently within two to four weeks. Many colonies may collapse during August and September, while others may collapse the following spring.

The signs of colony collapse are:

  • A sudden decrease in the adult bee population.
  • Very few dead bees within the hive.
  • Various abnormalities of the brood, often including bald brood, poor brood pattern, neglected and dead sealed brood which may be discoloured brown and partially removed from the cells. Be sure to exclude the possibility of these signs being caused by foul brood infection.
  • Close examination will reveal numerous Varroa mites on remaining bees and in brood cells.

Mite invasion

The crucial feature of the spread of varroa is the movement of mites, frequently in large numbers, from infested colonies into other colonies in the vicinity, spread by the movement of bees between apiaries. This process plays a key role in the increase in the level of infestation. Such mite invasion can occur at any time of the year, but is normally most acute in late summer when heavily infested colonies in the area are collapsing.

As colonies collapse, they are robbed by other colonies from within the apiary or from nearby apiaries. It seems that bees from a collapsing colony may abscond from their own hive and return to the hive of the robbing bees. In this way, very many mites can be transferred to the robbing colonies and the influx of mites causes the mite population to increase far faster than would otherwise have occurred.

Experience in the UK so far has shown that in areas of high colony density, with many heavily infested colonies left untreated, the rate of mite invasion has been extremely high and mite populations have built up to critical levels in less than one season, sometimes in only a few months. It is essential to regularly monitor infestation so that sudden increase can be detected and action taken to control it before colonies collapse.

Detecting varroa in your colonies

Varroa is probably present, undetected, in many places in the UK, outside the area of known infestation. Varroa infestation may not show obvious signs until the infestation is very severe, by which time it may be too late to save the colonies. Beekeepers must never assume that their colonies are varroa free, but must regularly check for the presence of mites.

There are several simple methods that can be used by beekeepers to detect infestation. In each case it will be easier to detect a well-established infestation with many mites than an early infestation with very few mites. Female Varroa mites can be easily recognised by their flattened oval shape (1.6 x 1.1mm) and reddish-brown colour. (Figure 1.). The bee-louse, Braula coeca, which is commonly seen harmlessly living on adult bees may be confused with Varroa but can be easily distinguished by its more rounded shape and the presence of legs on both sides of the body. (Figure 7. Braula coeca on a worker bee).

Drone brood sampling

  • Select an area of sealed drone brood at an advanced stage (ideally pink-eye or purple-eye stage), as this will not disintegrate on removal.
  • Slide the prongs of a honey-uncapping fork under the domed cappings parallel to the comb surface and lift out the pupae in a single scooping motion (Figure 8.)
  • Examine the drone pupae for Varroa mites – these are easily seen against the pale drone bodies. Repeat until at least 100 cells have been examined.

Detecting natural mite mortality (‘mite drop’)

  • Place the colony on a varroa-floor, consisting of a framed rectangular metal mesh panel (3 – 5mm mesh) through which colony debris can fall but bees cannot pass, to remove it.
  • Below this place a collecting drawer which can be accessed to remove debris without disturbing the colony.
  • At regular intervals remove the debris without disturbing the colony.
  • If the amount of debris is large, as for instance after winter, mites may be very difficult to detect. It can be helpful to mix the debris with methylated spirit in a large container. Wax and propolis particles will sink to the bottom, whereas many of the dead mites will float to the surface. Alternatively, samples can be sent to the National Bee Unit laboratory for specialist examination.

Using a proprietary varroa treatment for detection

Many of the proprietary treatments for varroa can also be used for detection and can be very sensitive, capable of detecting very small numbers of mites in a colony.

  • Arrange a method of collecting floor debris, either with a mesh varroa-floor as described above, or (as a temporary measure) a card sticky plastic insert covered by 3 – 5 mm mesh covering the existing hive floor, preventing the bees from removing the debris.
  • Apply the varroa treatment in the way described in its label instructions. Follow these instructions closely.
  • Examine for mites after 24 hours. If none are seen check again after several days.

Monitoring mite levels in colonies

Once varroa has been found in an apiary, the beekeeper must regularly estimate the level of infestation in the colonies. This is because the rate at which the infestation will increase depends on many factors and will differ between areas and from year to year. This makes it difficult to anticipate in advance how long it will take before the infestation reaches a level where colonies are likely to suffer harm.

By monitoring their colonies, beekeepers can see how the infestation is developing and can use this information to help decide what method of control would be most appropriate and when it should be applied. For instance, if it is found that colonies are becoming heavily infested in spring or early summer, this provides early warning that some form of varroa control will be required before the autumn treatment.

The monitoring methods outlined below are based on the detection methods previously described, but here use the number of mites detected to indicate the infestation level. It is advisable to monitor, if not every colony in the apiary, then at least a representative proportion as the level of infestation may vary greatly between colonies; monitoring only one colony may give results that are unrepresentative of other colonies.

At present it is unfortunately not possible to use a simple rule-of-thumb to determine accurately the varroa population within a colony, or to predict when a colony will collapse. This is partly because the complex biology of varroa infestation is not yet fully understood; and partly because biological and geographical variation means that figures that apply in one region may not always apply in another. A MAFF research project, partly funded by the BBKA,  aims to provide beekeepers with a monitoring method, intended to allow more accurate assessment of the mite population of colonies in the UK.

The experience of beekeepers in Europe, who have been managing colonies with varroa for many years, suggests that the best policy for beekeepers is to become experienced in monitoring mite numbers in their own region at different stages of the year and to learn to relate this information to the degree of infestation and subsequent performance of their colonies.

Assessing the proportion of infested drone pupae

  • At regular intervals, remove about 100 drone pupae with an uncapping fork as previously described.
  • Estimate the proportion of pupae that have Varroa mites on them.
  • As a rough guide, colonies with less than 5% drone brood infestation can be considered lightly infested, whereas those with 25% and above are severely infested and in danger of suffering collapse.
  • At times when there is no drone brood, it is possible to examine worker pupae in the same way. However, as mites have a preference for drone brood, a smaller proportion will normally be infested and this must be taken into account when assessing the level of infestation.

Monitoring natural mite-mortality

  • Maintain the colony on a mesh varroa-floor so that colony debris can be collected and removed as previously described.
  • At intervals of about 7-10 days examine the debris and carefully count the number of dead Varroa mites on the floor tray. Convert this to a daily mite-drop figure by dividing by the number of days since the last measurement.
  • The number of mites dying from natural causes in a colony is closely linked to the size of the varroa population. However, other factors within the colony also have a significant effect, in particular the amount of emerging brood present. At times when a large amount of brood is emerging, such as in late summer, this results in a correspondingly large daily mite drop.
  • As a very rough guide, very lightly infested colonies may have mite-drop in late summer of less than one per day, whereas very heavily infested colonies at this time might have a mite drop of 50 or more per day.
  • Note that during the summer it is essential to remove floor debris from the hive regularly to prevent severe infestation with wax moths.

Examining bees and brood for mites and signs of damage

  • During routine colony inspections, carefully observe adult bees for attached mites. Look for bees with deformed wings or abdomens
  • Examine combs of brood for dead or deformed pupae. Make sure you don’t overlook the possibility of such signs being caused by foul brood infection.
  • Experience in the UK suggests that once mites are clearly visible on combs of bees and in particular once damage to brook caused by varroa starts to occur, then it is very likely that colony collapse will occur before the end of the season if the infestation is not promptly controlled.


Varroacides and biotechnical control methods

Varroa control methods can be divided into two main categories: those using mechanical methods or beekeeping management methods to remove mites from the colony or otherwise limit their numbers (often referred to as ‘biotechnical methods’) and those using chemical substances to kill the mites (often referred to as ‘varroacides’ or ‘chemical treatments’). In practice the best control may result from combining a variety of different approaches (referred to as ‘integrated control’).

Biotechnical methods

A variety of biotechnical control methods are used in Europe, particularly by smaller scale beekeepers who want to minimise use of chemical treatments. Most methods work by trapping mites in combs of brood which are then removed from the colony, or by causing mites to drop off adult bees by mechanical means. Biotechnical methods are most effective in areas where colonies have low levels of infestation; in heavily infested areas they will not normally provide sufficient control without being supplemented with chemical treatments. Despite this, they may slow the rate of growth of the mite population and so reduce the frequency with which chemical treatments need to be used. Methods based on comb trapping can be particularly effective, but depend on a high degree of beekeeping skill and are labour-intensive to apply.

Chemical control of varroa

Table (3) shows a selection of the chemical treatments commonly used against varroa in the UK and elsewhere in Europe. Most of these are based on chemicals that have been previously developed as pesticides to control mite pests of crops and livestock. For use as varroacides, they are typically used in minute quantities, at a level lethal to Varroa mites, but harmless to bees. Other treatments use commonly available or naturally occurring chemicals that have been found to have some varroacidal activity. It is important that beekeepers understand the legal status of the treatments that might be considered for use.

Licensed and unlicensed chemical treatments for varroosis

Chemical treatments for varroa can be divided into four categories:

UK licensed products
Products sold in the UK for the treatment of varroosis must be licensed for this purpose under the Medicines Act 1968. Licenses are only given to veterinary medicines for which the efficacy, safety to bees, safety to users and safety to consumers of bee products is proven. At the time of writing the only licensed product is Bayvarol, manufactured by Bayer and available from beekeeping suppliers.

EU licensed products
Products that are licensed as veterinary medicines for use against varroa by an appropriate authority of another EU Member State may also be imported for personal use by a beekeeper, but not for resale or distribution. Such treatments have been proven in safety and efficacy to a similar standard as those in the UK.

Unlicensed treatments
Several chemical substances exist which have been used by beekeepers against varroa. In most cases, no formal testing of efficacy or safety has taken place and there is a danger therefore that they might be ineffective, harmful to bees or the user, or leave harmful residues in bee products. It is not legal to distribute these to others for use as a treatment for varroa.

Illegal treatments
In some countries, agricultural chemicals intended for other purposes are used in home-made treatments. In the UK such use is illegal and must be very strongly discouraged, as these chemicals may be hazardous to the user, leave serious residues and promote mite resistance to related licensed compounds.

Treatment timing

The main period for treatment of varroa is most frequently after the removal of the honey crop and before colonies are prepared for winter. Colony size will be decreasing at this point, but infestation level will normally be continuing to increase. In severely infested colonies it is essential that treatment is applied early enough to protect the last few brood cycles which produce the young bees that enable the colony to survive the winter. The timing of treatment may also be dictated by the choice of treatment method: those depending on evaporation of the active ingredient may require warm conditions to be effective, while those requiring broodless conditions may have to be applied very late in the year when brood rearing has ceased.

In a mild winter, there may be a great deal of bee activity and, as a consequence, mite invasion during the winter months. In these circumstances, it is possible that a treatment in early spring might be justified if monitoring reveals infestation to be sufficiently serious.

During summer, while supers are on the hives, it is not permitted to use the majority of chemical treatments in order to prevent contamination of honey. There are exceptions to this rule (e.g. Bayvarol, Apistan), which can be used at any time provided that the label instructions are strictly observed. Summer treatment may be necessary where very severe infestation demands emergency treatment.

Coordinated treatment

Treatment with a proprietary varroacide will generally be extremely effective in reducing the number of mites in a colony. Following treatment, however, more mites can be expected to invade the treated colony, originating from untreated infested colonies in the locality. In extreme cases, this can cause colonies to become heavily infested again shortly after treatment finishes, making a second treatment necessary. Mite invasion problems can be reduced if all colonies in the locality are treated at the same time.

  • Always treat all the colonies in an apiary at the same time.
  • Try to co-ordinate with other local beekeepers to treat at the same time. Your local beekeepers’ association will be able to help organise this.
  • Avoid having untreated colonies in the vicinity of treated apiaries.

Chemical residues in bee products

Licensed varroacides do not leave detectable residues in honey provided they are used as directed by the manufacturers. Some treatments have been shown to leave residues in beeswax, although these are typically very small ( a few parts per million) and are not necessarily harmful. Beekeepers should minimise potential residues by observing the following simple rules:

  • Never treat immediately before or during a honey-flow, or while supers are on the hive, unless this is specifically permitted on the label directions of a licensed product.
  • Do not use more than the prescribed dose.
  • Do not apply the treatment for longer than directed.
  • Always follow strictly the label directions supplied with all licensed products

Mite resistance to treatments

Many mite species, including Varroa, are able to develop resistance to pesticides. This can occur because individual mites differ in their susceptibility to a given substance. If a population of mites is exposed to a varroacide dose that only kills the more susceptible mites (for example, by using too small a dose), then only resistant mites will survive to reproduce. Over many mite generations this can lead to the development of a resistant population.

Mites resistant to one varroacide are likely also to be resistant to other closely related substances. Varroa resistance to fluvalinate (Apistan) is already known to have developed in Italy, and has since spread to Switzerland and France. Such mites have been shown to be resistant to other pyrethoids such as flumethrin (Bayvarol).

Beekeepers can delay the development of resistance by observing the following rules:

  • Always use the full dose of a varroacide, as directed on the label instructions.
  • Always remove used varroacide strips at the end of the treatment.
  • Do not attempt to re-use strips, as these will not release a full dose of active ingredient.
  • Where possible alternate treatments using unrelated products

Integrated control

Rather than depending on one chemical treatment or biotechnical method alone, it will generally be most effective to use two or more in combination,as part of an integrated control strategy. For example, a biotechnical method such as drone-brood trapping might be used during the summer months, followed by a chemical treatment in August and if necessary a second treatment with another product the following spring.

Such an approach has several benefits:

  • Control at several points of the year makes it harder for the mite population to reach a level that harms the colony.
  • Use of biotechnical methods slows mite population increase and so can help reduce the frequency with which chemical controls need to be used.
  • Use of two or more unrelated chemical controls will help delay the onset of mite resistance to chemical treatments.
  • There is no single integrated programme that, is ideal for all beekeepers, as infestation levels, mite invasion pressure, climate and beekeeping practices within the UK vary greatly. Beekeepers should be prepared to be flexible and adaptable as they seek a combination of methods that suit their particular circumstances.

Experience in the UK

The experience of many beekeepers in the UK over the past few years has been that, for one or two years following initial infestation, the level of infestation increased relatively slowly, and for many beekeepers an annual treatment alone provided sufficient control.

However, it has been widely found that after about two seasons the level of infestation increased much more rapidly than previously, with the result that mite population levels were dangerously high by the end of summer. This has been most acute where the presence of many heavily infested colonies in the locality has led to particularly serious mite invasion problems. In such circumstances, colonies treated in the autumn may have significant infestation by the following spring and may require treatment.

In extreme cases colonies treated in the spring may become so infested as to be in danger of collapse by the end of summer if no varroa control is employed in this period.

It is essential to employ some form of monitoring so that increase in the level of infestation can be detected before irrevocable harm has occurred, and an appropriate form of control can be applied

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