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Ulster Beekeepers’ Association
Varroa management guidance notes 2009

See also The Federation of Irish Beekeepers' Associations Guidelines on Varroa Destructor Guidelines on Varroa Destructor - Integrated Pest Management.

Summary
Integrated Pest Management (IPM) is the best basis for a sustainable control programme.

All colonies should be fitted with open mesh floors (OMF) with an inspection tray to monitor mite natural drop rate.

In Northern Ireland conditions in 2009, UBKA recommends the following regimen:

• Frequent monitoring using natural mite drop and drone brood inspection
• Oxalic Acid in December if necessary
• Sacrificial drone brood from May to July if necessary
• Either thymol every year during July/August
• or if necessary, thymol and synthetic pyrethroids in alternate years

           (If there is a suspicion of pyrethroid resistant mites then
            neither Apistan® nor Bayvarol® should be used).

As a simple rule-of-thumb, if the natural mite fall is greater than 5 per day, or the proportion of infested drone brood pupae is greater than 5%, then treatment should begin as soon as possible. The only exception to this simple rule is that natural mite fall in December should not be greater than 1 mite per day.

The appearance of pyrethroid resistant mites in Northern Ireland now seems inevitable. We need to be ready to react immediately to them by routinely testing for their presence.

The UBKA will continue to update its advice to members from time to time as new and better information becomes available.

A brief history
Varroa destructor was at first misclassified as Varroa jacobsoni, a non-lethal parasite of the East Asian honey bee, Apis cerana, but is now known to be a separate species. It is a problem spread by the international movement of bees by beekeepers. Because the Western honey bee, Apis mellifera, has not yet had time to evolve resistance to the mite, Varroa destructor has become the major killer of honey bees throughout the world, mainly due to the way it interacts with other bee diseases.  It was first identified as a parasite of honey bees in Western Europe in the 1970s, was first discovered in GB in 1992, in Republic of Ireland in 1998 and in Northern Ireland in 2002. Now, in 2009, probably every apiary in Northern Ireland has some degree of Varroa infestation – even though clinical symptoms are not yet evident in a few.  

Varroa has become, globally, the number one beekeeping management issue because of the difficulties with its control and complicated interactions of Varroa infestation with other diseases.

Northern Ireland is quite a few years behind many other countries in the development of the complications due to Varroa. We should therefore be able to learn from the experiences of others and predict more accurately the path that the condition will take here, and take appropriate action. That was the thinking behind the themes of the Annual Beekeepers’ Conference at Greenmount in 2007 and 2008. These guidance notes incorporate both the information provided at those conferences and other recent information.

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Biology of the Varroa mite
To control Varroa we need to understand as much as possible about it and the manner that it uses to parasitize honey bees. It is not the purpose of these notes to do this in detail so the reader should refer either to the excellent booklet “Managing Varroa” 2005, published by DEFRA (also available on their website at https://secure.csl.gov.uk/beebase/pdfs/managing_varroa.pdf), or to “Parasites of the Honeybee” 2007, by Mary Coffey, published by Teagasc, Dept Agric, Fish. & Food.

In summary, the adult female mite enters a brood cell just before it is capped. The mother mite has evolved to prefer drone brood but will accept worker brood. After about 3 days she begins to lay eggs at about 30 hour intervals. All the mites feed on the haemolymph (blood) of the bee pupa. The first egg develops as a male. He then fertilises the females that develop in about 6 or 7 days from the other eggs. Only the mature young females (plus the original mother mite) leave the cell with the emerging bee – the rest die.

From the above summary it can be seen that there is time only for the development typically of about 1 or 2 young females in a worker cell, while the additional 2 days for a capped drone cell enables the development of typically 3 young females in a drone cell. This is why the mites have evolved to prefer drone brood – drone cells give them a reproductive advantage. Of course, all these figures do vary a bit and if more than one mother mite had entered the cell before capping these figures could be higher.

Adult mites prefer to hitch a ride on young bees and can move from bee to bee, feeding on their haemolymph. They cause a lot of damage to both the adult bees and especially to the pupae. In summer the adult mites live 2 to 3 months but much longer in winter.

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Varroa acts in combination with other disease agents to increase colony losses
The greatest threat from Varroa arises because of the way it interacts with other disease agents. It is known to act as a vector for several viruses – it accelerates the spread of viruses throughout the colony which then succumbs to virus infection. Several viruses that are often present without obvious symptoms suddenly become lethal when combined with high levels of Varroa.

Deformed wing virus (DWV), Kashmir bee virus (KBV), slow paralysis virus (SPV) and acute bee paralysis virus (ABPV) are all transmitted by Varroa. Indeed, one of the common indicators of heavy Varroa infestation is the large numbers of adult bees with typical DWV abnormalities of shrunken abdomens and shrivelled wings.

Varroa, in combination with Nosema has also been implicated in colony collapse. The species of Nosema (Nosema cerana) that originally came (like Varroa) from the Eastern honey bee seems to be more virulent than the species that has been most common in Ireland (Nosema apis). In 2008, Nosema cerana was discovered in Co. Dublin and it seems likely that it will spread rapidly throughout Ireland – if it is not already widespread.

Varroa infestation normally kills only a minority of pupae in the cells, but feeding mites severely weaken the pupae so that the emerging bee is lighter, has a shorter life and workers damaged in this way may not be very effective as providers of brood food for rearing succeeding generations. Varroa-damaged drones may result in poor mating of queens. Experience shows that Varroa on its own will not normally cause colony collapse in the year of first infestation, but the addition of mites from drifting adult bees from dying colonies can significantly hasten colony collapse.

This relatively slow build up can cause complacency in the unwary beekeeper. If a very effective treatment has been given in year one then the colony may come through year two in an apparently healthy state, but without further treatment, the probability of collapse in year three is high.

Monitoring mite infestation level
All Varroa control measures have some detrimental effects – some much more than others. Therefore it is important to know when the level of Varroa infestation requires treatment rather than treating regardless of need.

Northern Ireland beekeepers should assess Varroa levels at least in March, June, August, October and December. The recommended method of doing this is to measure natural mite fall by use of an open mesh floor (OMF) with a sampling tray underneath. Place a clean insert on the tray and measure the mite drop over about 2 weeks. The average mite drop should be no more than about 5 mites per day, except in December when it should be no more than 1 mite per day. Please note that the measurement will be invalid if any treatment is taking place at the same time.

Levels can also be estimated in June and August by uncapping drone brood at the pink-eyed stage and estimating the % of brood cells infested, although this method is less accurate than the natural mite drop method. Using an uncapping fork, withdraw pink-eyed pupae from about 100 cells of the correct age. There should be no more than about 5% of the pupae with mites on them.

If either of the above methods indicates that Varroa levels are too high then the colony should be treated appropriately (see below). A late summer treatment is usually necessary and particularly important because if colonies are to reach spring in good condition it is essential that bees born in early autumn should be free of Varroa while pupating in their cells. If they have been parasitized, they will have insufficient body reserves to last the winter and to feed brood in early spring.
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Recommended methods of controlling levels of Varroa infestation
There are currently four main types of Varroa control that have been sufficiently well researched to be included in a coordinated control programme in Northern Ireland:
            Flumethrin & tau-fluvalinate (synthetic pyrethroids)
            Oxalic acid (an organic acid)
            Thymol (an essential oil) also  Varroa Treatment with Thymol by Peter Edwards of Stratford-upon-Avon & District Beekeepers' Association
            Sacrificial drone brood
These four should be used in combination with Open Mesh Floors (OMFs) as described in paragraph 5, above. Their use is described more fully in paragraphs 7, 8 & 9 below.

Ulster Beekeepers’ Association is aware of many other treatments that are being used in the context of Varroa management. Some of these treatments leave unacceptable residues. Some are not appropriate for Northern Ireland conditions and many are not adequately researched to ensure consistent effectiveness. We would be doing Northern Ireland beekeepers no favours if we recommended treatments that led to unnecessary colony losses after a few years of use. The situation will be kept under review and changes made to our recommendations only when there is sufficient evidence to justify them.

Synthetic pyrethroids; flumethrin is marketed as Bayvarol® while tau-fluvalinate is marketed as Apistan®. For practical beekeeping purposes the two products may be thought of as alternative options. They are in the form of plastic strips that are hung between the frames of the broodnest for 6 weeks when supers are not in place either in early spring or in autumn. They should be used strictly in accordance with the manufacturer’s instructions. The cost of a 6-week treatment is about £3.00 to £4.00 per colony.

These were very effective products but pyrethroid-resistant Varroa are now an imminent threat. Therefore they should be reserved for occasional use only in combination with other means of control. If Varroa resistance is suspected at an apiary they should not be used at all – see paragraphs 11 & 12 below.
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Oxalic acid; This is a naturally occurring organic acid present at low levels in many foods. However in the concentration necessary to control Varroa it is hazardous to beekeepers. It will kill brood in normal use and will damage or kill adult bees if wrongly applied. Therefore it should only be used in the brood-free period (normally December) and never more than once per season.

There are three properly researched methods of application of oxalic acid - by dribbling, by spraying and by sublimation. UBKA recommends the dribbling method, whereby 5ml of solution from a syringe is dribbled evenly along each seam of bees between the frames. A nucleus colony that has 4 seams of bees in December will therefore get a dose of 20ml while a colony with 7 seams will get a dose of 35ml. Overdosing may kill bees and even worse, kill the queen.
Ready-made solutions are commercially available, but they deteriorate quickly and should not be used if they are showing any brown or yellow colouration. They cost about £1.00 to £2.00 per colony.
Alternatively, make up a sugar solution by dissolving 1 kg sugar in 1 litre water. Then add 4.5g oxalic acid dihydrate crystals to each 100ml of the warm (not hot) sugar solution and stir thoroughly until dissolved. This will give approximately a 3.2% (w/v) solution of oxalic acid solution (N.B. oxalic acid dihydrate contains 71% oxalic acid) at a cost of about £0.07 per colony. Accurate weighing is very important. A solution that is too strong will kill the bees and one that is too weak will not be effective.
Beekeepers should use gloves and glasses and be careful not to allow any oxalic acid to contact their skin. Also, take particular care to prevent children accessing it because the poisonous clear final solution probably tastes sweet!
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Thymol; This is often referred to as an essential oil. There are several essential oils that have been researched but thymol (from the Thyme plant) is the only one that UBKA currently recommends. The main problem with thymol is that it is most effective at ambient temperatures above 15°C and in the cool Northern Ireland climate higher temperatures can be infrequent.  It is a warm-weather treatment but it can “taint” the honey crop if used when supers are in place. Under our conditions, the best time to apply thymol is therefore immediately after removing the supers in late July or early August.

The most convenient method of applying thymol is dissolved in a gel. This is currently marketed as Apiguard®. The product is packed in a small tray that is placed on the frames directly above the brood nest. A second tray is added after 2 weeks. The cost of the total treatment (2 trays) is £2.50 to £5.00 per colony.

A cheaper alternative is to purchase thymol crystals and place 8g for a full sized colony (about 2 heaped teaspoons) in a tray such as an old honey-jar lid. This must to be replenished regularly each week for 4 weeks. The problem with this method is that it requires accuracy by the beekeeper in measuring out the dose. Too much and the bees may be forced out of the hive – too little and the treatment will lack effectiveness.  But the total cost of the 4 treatments is only about £1.00 to £1.80 per colony.

In either case it is important to place the thymol directly above the brood–nest to take advantage of the heat. Enough bee-space, to allow free circulation of the thymol and to give the bees free access to the tray, should be provided below the crown board by using a shallow eke or inverted Millar-type feeder and simple insulation will help to maintain a high temperature.  Open mesh floors should be blanked off with trays during treatment to reduce ventilation.

In cool Northern Irish conditions Thymol may not always be sufficiently effective as an annual treatment on its own but it is certainly a valuable Varroa treatment if used in combination with others. Users should monitor mite fall in December to check the efficacy of the August treatment. The natural mite drop should not be greater than 1 mite per day in December.

Thymol is also active against both Tracheal mite and chalkbrood.
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Sacrificial drone brood; The mother Varroa mite prefers to enter drone brood cells rather than worker brood cells to lay her eggs. We can use this information to control the numbers of mites. During the period from May to July, inclusive, a shallow frame with comb or foundation is inserted into the brood nest, towards the back. The bees will draw out drone comb below the bottom bar and mites will be attracted to the developing drone brood. When the drone comb is sealed (after about 2 -3 weeks) it can be cut off and discarded. The shallow comb can then be returned to the brood nest to repeat the cycle.

Drone brood trapping, as it is sometimes known, will reduce the number of flying drones from colonies treated in this way. This may be an advantage when the colony is not suitable for fathering future generations, but equally it may make effective mating of queens less reliable. A generous supply of healthy drones is very important in queen rearing.

Obviously, if the drone comb is not removed within 4 weeks and drones are permitted to emerge, this procedure is like setting up a Varroa mite nursery and is a recipe for increasing the mite load. So timing is vital.
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Integrated Pest Management (IPM).
Integrated Pest Management has long been normal practice in both plant and animal husbandry. As applied to Varroa management, it means controlling, by a variety of methods, the infestation at a level that will not do significant harm to the bees. It is the approach recommended by UBKA. In practice this means keeping the natural mite drop below 5 mites per day (1 mite per day in December) and the proportion of drone pupae infested below 5%, as explained in paragraph 5 above.

The key features of IPM are:
            Monitoring the level of infestation and treating accordingly.           
            Treatments that are applied at different times of the year
            Using a variety of different treatments to prevent the development of resistance
            A flexible approach that can be varied to reflect infestation levels

In Northern Ireland conditions in 2009, UBKA recommends the following regimen:

• Frequent monitoring using natural mite drop and drone brood inspection
• Oxalic Acid in December, if required
• Sacrificial drone brood from May to July
• Either thymol during every July/August

                        Or, if really necessary, in alternate years
      synthetic pyrethroids during August/September
(If there is a suspicion of pyrethroid resistant mites (see paragraph 12 below) then neither Apistan® nor Bayvarol® should be used).
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Get prepared for pyrethroid resistant mites
There are two theories about the spread of pyrethroid resistant mites. The first is that regular treatment with pyrethroids is a kind of breeding selection process – the very few naturally slightly resistant mites survive (helped by improper use of pyrethroids) and interbreed while susceptible mites are killed. Over a period of years the population of mites gradually changes to consist mainly of very resistant mites. The other theory is that a single mite mutated to become resistant and then multiplied in the presence of pyrethroids (which killed their competitor mites). The resistant mites were then geographically spread by beekeepers through movement of bees.

It seems very likely that resistant mites will appear in Northern Ireland soon because they are widespread in mainland Europe. They were first discovered in Devon in 2001 and are now common throughout most of GB. They are not yet believed to be anywhere in Ireland but we need to prepare ourselves. Once they arrive, both Bayvarol® and Apistan® will be much less effective and we will be dependent on the alternative treatments.

Unless pyrethroid resistant mites are identified through deliberate testing, the first indication of their presence will be the collapse of colonies after pyrethroid treatments fail to control them. Therefore Northern Ireland beekeepers need to test routinely for resistance from now onwards.

• A simple way of doing this is to check the natural mite fall following normal treatment with pyrethroids. Wait for two weeks after completing the normal treatment and then insert a tray under the OMF for a further two weeks. If more than 28 mites (i.e. two mites per day) drop then the situation requires further investigation.
• A more rigorous test is the NBU field resistance test which is described on page 26 of DEFRA’s “Managing Varroa” (see also their website https://secure.csl.gov.uk/beebase/pdfs/managing_varroa.pdf)

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These guidelines are produced by UBKA in good faith for the benefit of members of affiliated associations. However, no liability will be accepted for loss or injury resulting from their use.

                                                                                                UBKA, August 2009