Tag Archives: oxalic acid

Get dribbling

There has been a prolonged spell of cold weather in Eastern Scotland. Temperatures have rarely risen above 5°C, with hard frosts overnight. However, a warm front moved in on Tuesday night and the last few days have been significantly warmer. The lack of activity at the hive entrances and a quick peek under the insulation through the perspex crownboards (where fitted) indicated the bees were all tightly clustered during the cold spell. Furthermore, the absence of debris on the removable Varroa monitoring trays fitted to many of the open mesh floors, suggested that little or no brood was being reared.

Ridiculous to the sublime

Ridiculous to the sublime

Varroa counts

Varroa trays ...

Varroa trays …

There was another clue that the colonies are likely broodless. I had been recording the natural Varroa drop of a few colonies over the last month. I did this by simply counting Varroa at each visit, calculated on a mites/day basis. Although generally low (and very low in a few colonies), it had been steadily increasing. This is a good indication there were more phoretic mites in the colony … again, presumably due to the absence of suitable brood for them to parasitise.

It’s worth noting that the natural mite drop is a notoriously unreliable method of accurately determining mite levels in a colony. For example, it’s dependent upon the amount of sealed brood in the colony. With no sealed brood all mites must be phoretic. In contrast, with limitless sealed brood 80-90% of the mites are within cells. However, although estimates from mite drop are not hugely accurate, they are a lot better than doing nothing. The National Bee Unit has published a Varroa calculator. This allows you to use a combination of the mite drop per day, the time of year, length of season and level of drone brood to predict the total numbers of mites in the colony. For some inexplicable reason this asks for the level of drone brood in December … with 0% not being an available option  🙁

Time to treat

With little or no brood in the colonies, now is a perfect time to treat with an oxalic acid-containing preparation to hammer down the remaining mite population. I’ve previously discussed the importance of this midwinter treatment (see Two treatments … a double whammy). In many ways it’s preparation for the season ahead, rather than for the protection of the bees already present in the colony. The lower the mite levels are at the beginning of the season, the longer it will take for the mite population to reach dangerously high levels.

BEEHAVE ...

BEEHAVE …

You can model these events using BEEHAVE. This is an interesting in silico model of a beehive. With mite numbers of ~10 at the beginning of the year, maximum levels reached are low to mid-hundreds by late summer, reducing to a couple of hundred the following winter. This assumes no intervening treatment and runs the model using all the default settings. In contrast, using the same parameters but starting the year with ~100 mites, levels peak at between 3000 and 4000 mites, returning to about 1800 in December.

Remember that the National Bee Unit recommends mite levels should not exceed 1000 or there is a risk of “significant adverse effects on the colony”. Therefore, the midwinter treatment is an important preparation for the year ahead, delaying the point at which these dangerously high mite levels are achieved.

Have your hives got less than 100 mites in them now?

Remember also that, with no sealed brood, midwinter is also the ideal time to expose as many mites as possible to the treatment. With the exception of prolonged treatment with hard chemicals like Apistan or Apivar, it’s probably the only time you’ll achieve greater than 95% reduction in mite numbers. With little or no brood present there’s nowhere for the mites to hide.

Dribbling or vaporisation?

An oxalic acid-containing treatment is recommended in midwinter. This can be delivered by dribbling or sublimation (vaporisation). Under optimal conditions, efficacy of the two methods is broadly similar (90%+) though there is some evidence that dribbled oxalic acid is slightly detrimental to colonies (when compared with sublimation, but not when compared to doing nothing).

Sublimox in use

Sublimox in use …

Api-Bioxal is the VMD-approved oxalic acid-containing treatment. If used for dribbling be aware that the suggested concentration on the side of the packet is higher than conventionally used in the UK. It’s also worth noting that it’s not available pre-mixed so has to be made up from powder. In this regard it’s a less useful product than the pre-mixed oxalic acid solution that Thorne’s (and possibly other suppliers) sold each winter. The one- or two-hive beekeeper needs to weigh out very small amounts accurately, or get together with others to make a large batch. Hardly what I’d call progress. Furthermore, the inclusion of glucose and powdered silica (as an anti-caking agent) in Api-Bioxal means it leaves a caramelised mess if used for vaporisation. Although a scouring pad and elbow grease will get rid of this mess, it’s another example of how the “approved” commercial product is actually less good – and no more effective – than the oxalic acid dihydrate that beekeepers have been using for 20 years or more.

Notwithstanding these negative comments, Api-Bioxal works well and is less expensive (per treatment) than most of the other VMD-approved Varroa treatments.

Don’t delay, get out and get dribbling …

The forecast for the next 7-10 days is for significantly warmer temperatures. This means that the queen – if she was having a break from egg-laying – will start laying again. There will be open brood by this weekend and sealed brood in your colonies by about the 15th of December. Dribbled oxalic acid is detrimental to – and may kill – open brood so if this is your preferred method of treatment then don’t delay. If you sublimate you’ve got a few days leeway, but don’t delay any longer than that.

Here are a couple of old videos showing trickling (dribbling) oxalic acid onto a large and small colony in the middle of winter. The Trickle bottle from Thorne’s makes administering the treatment very quick and easy.

Of course, sublimation using an active vaporiser like a Sublimox is even faster and doesn’t involve opening the colony. Here’s an example showing treatment of a recently hived swarm in midsummer … I could have removed the Sublimox after about 30 seconds.

The Daily Mail may be predicting the coldest winter since the last ice age (so perhaps there will be another broodless period§) but I wouldn’t rely on them to influence something as important as the midwinter treatment for reducing Varroa levels.


Here’s a perfect example of the problems encountered by the ‘topical blogger’. I wanted to write about midwinter Varroa treatment in the middle of winter, at a time when others – particular new beekeepers – should be treating their own colonies. Typically these treatments are made in late December or early January. However, the long-range (10 day) forecast in late November suggested the second week of December might be suitable. Some of this was therefore written in very late November, the Varroa drop comments added once I’d completed counting around the 4th to the 6th, and the post finished off the following day once I’d treated my own colonies.

This assumes that the queen started laying on the 7th, the first full day with elevated temperatures.

§ I didn’t open any colonies to confirm they were broodless. I was happy enough to take the clues from the increased mite drop on the Varroa trays and the absence of debris indicating uncapping of brood cells. However, I was told by friends that other colonies they opened on the 7th were broodless.

 

Out, damned mites

Sublimox vaporiser

Sublimox vaporiser …

Today was very mild, slightly damp and breezy after a prolonged cold spell (at least here in Scotland). The long, cold spell means that colonies are broodless. Now is an ideal time to apply your midwinter Varroa treatment. Don’t wait until the Christmas holidays, don’t wait until the weekend after next … colonies will probably have sealed brood again by then. For maximum effect treat while the colony is broodless and decimate the phoretic mite population.

I treated all my colonies late this afternoon and evening. I finished the last using a headtorch for illumination and tidied up under bright moonlight. The bees looked good and it was great to be doing some beekeeping again, if only briefly.


 A longer post justifying why the colonies were considered broodless and why it is so important to treat when they are broodless will appear this Friday.

The rather weak title is a variant of Shakespeare’s “Out, damned spot” from the play Macbeth. The words are spoken by the sleepwalking Lady Macbeth who is going insane with guilt after her husband killed Duncan (the King of Scotland). The spot refers to Duncan’s blood. Mites on the Varroa tray look like tiny spots of blood …

 

Those pesky mites

DWV symptoms

DWV symptoms

If you haven’t yet treated your colonies to reduce Varroa levels before the winter arrives it may well be too late. High Varroa levels are known to result in the transmission of virulent strains of deformed wing virus (DWV). These replicate to very high levels and reduce the lifespan of bees. If this happens to the ‘winter bees’ raised in late summer/early autumn there’s a significant chance that the colony will die during the winter.

Mite levels in most of my colonies have been very low this year. Partly due to thorough Varroa management in the 2015/16 winter (the only thing I can take credit for), partly due to the relative sparsity of beekeepers in Fife, partly due to the late Spring and consequent slow build-up of colonies and partly due to an extended mid-season brood break when requeening. Most colonies yielded only a small number of mites (<50) during and after a 3 x 5 day treatment regime (to be discussed in detail in a later post) by sublimation.

Infested arrivals

The low mite drop definitely wasn’t due to operator error or vaporiser malfunction. At the same time I treated a swarm that had moved into a bait hive in early June …

Out, damn'd mite ...

Out, damn’d mite …

This is ~20% of the Varroa tray. Have a guess at the number of mites in this view only. Click on the image to read the full legend which includes the mite count.

The image above was taken on the 18th of September, a day or two after starting the second round of 3 x 5 day treatments. The colony really was riddled. When a colony swarms 35% of the mites in the colony leave with the swarm (or, in this case, arrives with it). For this reason the swarm was treated for mites shortly after it arrived in June. It did have a reasonably high mite load but subsequently built up very quickly and didn’t experience the mid-season brood break my other colonies benefitted from.

The colony now has an acceptable mite drop (<1 per day). Similar colonies are still rearing brood – I’ve not checked this one, but they are bringing in some pollen from somewhere – so there’s a possibility the majority of the remaining mites are tucked away in sealed cells. I’ll keep a close eye on this colony through the next few weeks and will be treating again midwinter to further reduce the parasite burden.

Treat ’em right

If you are treating this late in the season make sure you use a miticide that is appropriate for the conditions. Apiguard (a thymol-containing treatment) is almost certainly unsuitable unless you’re living in southern France as it needs a temperature of 15°C to be effective. MAQS has a recommended temperature minimum of 10°C which may be achievable.

Hard chemicals such as Apivar and Apistan can be used at lower temperatures but there’s little point in treating with Apistan unless you’re certain all your mites are sensitive. They almost certainly are not as Apistan/Bayvarol resistance is very widespread in the UK mite population. Just because you get an increased mite drop in the presence of Apistan does not mean treatment has been effective. Perhaps all you’ve done is killed the sensitive mites in the population, leaving the remainder untroubled. This is what’s known as a bad idea … both for your bees next season and for your neighbours.


 I’m posting this now due to the large number of searches for, and visits to, pages on use of Apiguard or other Varroa treatments. These are currently running second to ‘fondant‘ in one form or another.

Varroa control in the bee shed

The last colonies to be treated for Varroa this late summer (early autumn?) are those in the bee shed. These have had consistently low levels of mites all season … levels were so low that we uncapped two full frames of drone brood (individually) from one of them in June without finding a single mite.

Nevertheless, because …

  • mite levels can rise dramatically from low levels if not tackled – for example, see the modelled expansion of the Varroa population.
  • reduced queen laying at this time of year means mites have fewer pupae to target resulting in elevated infestation levels in the critical winter bees (and why this is important). In recent sampling of pupae we’ve seen an increase in the number of mites in capped in cells which we assume is due to this.
  • we need to keep these colonies with the lowest practical mite levels.

… they were treated anyway. I’m reasonably confident that sublimated oxalic acid (which is the active ingredient in Api-Bioxal) does little or no harm to the colony, and am sure that the mite reduction is always beneficial. I’d therefore prefer to treat than regret not treating at a later stage in the winter or early next season.

Expose the bees to the vapour … not the beekeeper

There’s nothing fundamentally different about treating colonies in the bee shed than those outside. Using a Sublimox vaporiser is very straightforward. However, two points need a little more care than normal.

The first is the sealing of the colony. To be effective the vapour must be evenly spread throughout the hive. Because of the ‘tunnel-like’ entrances there are more potential gaps from which the vapour can escape. I therefore do my best to push the hive tightly against the entrance tunnel after sealing the latter with a block of foam. The floors on these hives were built by Pete Little and have a commendably leakproof Varroa tray, making them ideal for sealing the open mesh floor. As an aside, don’t try squirting the vapour in from the entrance … direct inspection through the Perspex crownboard suggests that (at least in my setup) the vapour only poorly permeates the hive if administered like this. Been there, done that. The goal is to get the oxalic acid crystals spread evenly and thoroughly throughout the hive, ensuring maximum exposure to the mites, and maximising the duration of activity against,

The second point relates to the ‘leakiness’ of the hive and the fact that it’s in an enclosed space (the shed). There’s therefore no chance of standing upwind and allowing escaping vapour to drift away safely. Operator protection is particularly important as the shed is liable to fill with oxalic acid vapour. Eye protection and a suitable particle mask rated for acid particulates are essential. It’s a case of “lighting the blue touch paper and retiring to a safe distance”. With a Sublimox you can simply invert the machine – into the ‘delivery’ mode – and leave it hanging out of a hole through the sidewall of the floor (see photo above right). There’s a couple of seconds before sublimation starts which you can use to step out into the fresh air, only returning once the vapour has cleared.

Finally, if you run your vaporiser off a generator it should also be left outside the shed. Don’t gas the bees when you’re gassing the bees 😉


Plus a recalcitrant swarm that’s on it’s second round of treatment due to the stubbornly high mite levels. Grrrr.

Same time, next year

About this time last year a swarm arrived in a bait hive in my back garden in Fife. Almost exactly one year later a different bait hive in the same spot was occupied by another swarm … or, possibly, a very good-sized cast.

The bait hive was being investigated by scout bees for a few days but on 6th, which was a very warm day here in Fife, the numbers increased markedly from a couple of dozen to a hundred or more. On my return from work on the following day the swarm was in residence. My neighbour reported seeing a ‘huge swarm arriving’ at about 11am.

Foundationless frames and bait hives

The hive contained a single old, dark brood frame and about five foundationless frames, together with a cotton bud dipped in lemongrass oil. I’ve previously described why I think foundationless frames are so convenient for bait hives – they provide the bees with guides to build new comb without taking up significant space in the box. It’s worth remembering that the scout bees are seeking out a sheltered, south facing, bee-smelling (ideally), empty space of about 40 litres volume i.e. about the same as a single National brood box. Foundationless frames take up little space, but mean that an arriving swarm can start building new comb immediately … and they do.

I posted a photo last week of a swarm from the bee shed that had clustered because the queen was clipped and so unable to fly. I dealt with the swarm within a couple of hours of it settling. Once cleared, the wall of the bee shed was dotted with small crescents of wax as the bees had already started to build new comb. In the bait hive, when checked on the evening of the 8th (less than 48 hours after the bees arrived) they were well on their way to drawing out the first three foundationless frames, with the first of these being half full of nectar, presumably from the dregs available in the nearby OSR fields.

Mite treatment be needed?

Almost certainly … and there’s no better time. When swarms leave a hive they take with them up to 35% of the Varroa population as phoretic mites. A large swarm from a heavily infested hive can therefore introduce an unhealthy dose of virus-riddled mites to your apiary. These will rapidly spread to your other hives. I therefore routinely treat swarms with suitable miticides soon after they arrive, well before any brood is sealed. I don’t look for DWV symptoms or bother searching for signs of phoretic mites, I just treat. Due to work commitments this swarm had to be treated on the third day after arrival, before I was even certain whether the queen was laying or not. Within the first 24 hours after treatment (with sublimated oxalic acid) there were about 40-50 mites on the board, with more falling over the next couple of days. It’s far easier and more effective to treat when there’s no brood present and so give the colony the very best chance of getting well established without a pathogenic virus load.

Finally, after a day of heavy rain, I took advantage of the bees being all ‘at home’, sealed the entrance and relocated them to another apiary to make space for a replacement bait hive on the same spot … on the off chance that swarming here isn’t over yet.

If it is, then there’s always the same time, next year.


Same time, next year was a 1978 romantic comedy starring Alan Alda and Ellen Burstyn about a couple, married to others, who meet by chance, develop an “instant rapport” or at least “really hit it off” (one of the quotes from the film) and then meet again, year after year, both gradually changing, ageing and dealing with life’s crises.

Bee shed inspections

A brief update on how things have been progressing in the bee shed. This is my first full season keeping colonies full-time within a shed or building though I’ve successfully overwintered mini-nucs in an unheated greenhouse in the past.

Under construction ...

Under construction …

When installed at the end of last season there was almost no need to open the hives, so it’s only this Spring that the pros and cons of the bee shed have begun to be properly understood.

The colonies are completely enclosed with simple tunnels leading to exits on the East/South East face of the shed. All the colonies are housed in standard National cedar boxes or poly nucs. Other than clear perspex insulated crownboards, there is no additional insulation and the shed is not heated. The shed is situated in open parkland with woodland and arable land nearby containing good forage and there is a permanent water supply nearby.

Colony development and Varroa loads

Colonies went through the winter in single National brood boxes, fed with fondant and treated with oxalic acid by vaporisation in September (before moving them to the shed) and in midwinter. The first inspection was conducted in late March. Colonies were building up well and were significantly stronger than colonies headed by sister queens in the same apiary or in my other apiary. Between late February and early May colonies dropped only 3-4 mites in total, with Varroa boards located within pull-out trays in the hive floor. I’m sure I missed a few mites, but doubt it was very many. We’ve recently uncapped a full frame of drone brood – each cell uncapped individually – and found no Varroa present. Mite levels are therefore reassuringly low – for reasons to be discussed in a future post – with no signs of DWV-related disease.

Varroa tray ...

Varroa tray …

Since mid-April colony development has been very good and they are now on double National brood boxes with 2-3 supers. A fourth super went onto one colony on the 25th of May and the stack now nearly reaches the shed roof. A four frame nuc has been split off one colony already to cool it down a little. Quite a bit of developing brood has also been harvested at weekly intervals for our research, usually by simply cutting a big slab out of the middle of a frame. This has probably also held the colonies back a bit and it’s only now I’m starting to plan for swarm prevention/control.

Inspections

Inspections have been easier than expected. These colonies are headed by queens with reasonable genetics (Heinz queens – local mongrels of 57 varieties, reared by me in 2015). The bees are steady on the comb and tend not to fly up at you when the crownboard is lifted. They’re nothing particularly special, but would be considered reasonably placid and non-aggressive.

The colony is gently smoked from outside the shed (through the entrance tunnel) and a small amount is wafted under the crownboard or between the QE and the bottom super. After allowing them to settle the supers and crownboard are removed and placed outside on an overturned roof. The queen excluder and adherent bees are also left standing outside (unless it’s cold when the bees are shaken off into the open hive).

Inspecting the colony is straightforward. Any frames removed to make space are rested on the hive stand. Double brooded colonies are split into two, with one box stood aside on an eke on the roof of an adjacent hive roof. Inevitably, the queenless half of the split tends to get tetchy within a few minutes, so it’s best to deal with them first. When frames need to be shaken free of bees this can be done either over the open hive or, better still, directly into a gap between the frames. If done outside many of the nurse bees on the frame fail to get back to the hive (they’ve probably not been on orientation flights yet).

The smoker is usually stood just outside the shed door … if you keep it in the shed during inspections you can end up being kippered 😎

Flying bees

Perhaps surprisingly, even going through all 22 frames in a double colony, the shed does not fill with a maelstrom of flying bees. Undoubtedly this is partly because they’re reasonably calm colonies. Those that do fly rapidly find the window or open door and make their exit. When I first started doing inspections in the bee shed I’d manually help the stragglers outside after reassembling the hive. It turns out that there’s really no need … almost all the bees quickly vacate the shed by making a beeline ( 😉 ) for the bright lights of the windows or doors.

The great escape ...

The great escape …

Just how quickly the bees leave the shed was emphasised last Sunday when selecting larvae for grafting. I opened and inspected a double brooded colony, found a suitable frame with 24 hour larvae on it and placed it in a two frame nuc for protection. Within 5 minutes I could work without a veil (I react very badly to stings to the face so take particular care over this) without interruption from flying bees.

Weather and temperature

I’m sure that the temperature influences the behaviour of the colonies in the shed. They certainly forage – or perhaps collect water to use fondant or crystallised stores – at lower temperatures than those situated outside. When inspections are conducted on a cold day (say 10-11°C) they are even more steady than usual. However, those that do fly take longer to leave the shed and they can end up clustering in small, rather pathetic, little groups which then need to be scooped up on a hive tool and dropped into the colony. On cool days I don’t leave the supers or QE outside the shed as the bees would rapidly get chilled. Work commitments mean that inspections must be conducted on certain days, so I don’t have the luxury of simply waiting until it’s a bit warmer. Although the shed is unheated the temperature differential between the inside and outside is significant – perhaps 4-8°C – or more if the sun is shining on the window side of the shed. On a warm, sunny day the temperature inside the shed can easily reach the mid-20’s which makes inspections a hot and sweaty activity.

Needless to say, inspections on damp or wet days are much better than on colonies located outside. I avoid days when it’s raining hard, partly for my own comfort to avoid getting wet accessing the apiary, but also because I’d prefer not to force the bees to fly on a really wet day. However, on damp or drizzly days, inspections proceed as normal.

And the bad news is …

Almost everything I’ve written above is positive and my overall initial impression is that the bee shed offers very significant advantages for the sort of beekeeping I need to do. However, there are some drawbacks and design issues that either currently cause problems, or might in the future.

The first is that it’s too small. The shed is 12 x 8 feet and I should have got one at least half as long again. This is largely because it’s also used for equipment storage and has a small table for working on. With four hives I need storage for 8-12 supers, additional brood boxes and spare frames. If I was starting again, knowing what I know now, I’d get an 18 x 10 shed with the intention of housing at least 6 colonies and some additional nucs (by contrast mine will accommodate 4 full colonies and 2 nucs down the sunny side of the shed, with the possibility of 2-3 additional nucs at a squeeze). It’s not only equipment storage that takes up the room … you need considerable room to work as well, with space for turning, stacking and temporary placement of hive parts. Working in the bee shed encourages an efficiency of movement – or causes a lot of collisions – I’d not expected.

Essential storage ...

Essential storage …

Secondly the lighting is – at best – variable. On a sunny morning there’s ample light to see eggs and tiny larvae. However, as the colonies have grown, the added supers restrict the amount of light getting through the windows. On an overcast day, or late in the afternoon, the lighting is pretty hopeless – good enough to see queen cups/cells, good enough to locate the queen, but (particularly on dark frames) too dim to see eggs, small larvae or to check frames for signs of disease. It’s not unusual to have to carry frames outside to inspect them fully. I’m currently investigating 12V LED systems run from a solar panel-charged caravan battery. My only concern is that this might disorientate the bees and slow their exit from the shed during inspections.

Multiple supers ...

Multiple supers …

Thirdly, I should have spent more time designing the hive stands. I made them an inch or so too low which caused some problems with locating the hive entrances centrally in the T&G planks, but was not insurmountable. More problematically, as a consequence of the leg locations it’s difficult to keep the floor clear of hive debris that falls through the OMF. With the Varroa boards in place this isn’t an issue, but when they’re out – which I prefer if there’s a chance of the shed getting very warm – the debris needs to be regularly swept up to keep the shed clean. Some sort of removable debris trays would have been a good addition, but are not easy to fit retrospectively. However, the overall hive stand design – with the legs going through the suspended floor to avoid vibrations – works very well.

Finally, swarm control has yet to be tackled. My preferred simple method is doing a vertical split (or using a Snelgrove board that I’m experimenting with this year) but this requires an upper entrance which, obviously, cannot easily be arranged. One possibility is using the Demaree method of swarm control. Alternatively, it would be straightforward to remove the queen into a nuc and let the colony requeen. Currently I’m trying to postpone the inevitable by removal of some brood, ensuring they have enough space within the brood boxes which I swap (top to bottom, bottom to top) periodically, ensuring they have sufficient space in the supers and keeping a close eye on them. The queens are clipped. If they do swarm they’re likely to end up in a lump outside the hive entrance – the ground is flagged and so they should hopefully be relatively easy to scoop up.


 

All together now

This is the last of a short series of related posts on rational Varroa control. It brings together the key points made on the choice of how and when to treat, coupled with a treatment strategy that minimises the influence of bees drifting between colonies. The latter is best summarised in three words … coordinated Varroa treatment.

Coordinated Varroa treatment makes sense

Abandoned hives

Abandoned hives …

Most beekeepers treat their own colonies together … it’s logical, easier and cost effective. But what about the other beekeepers in the shared association apiary? What about the colonies two gardens away? What about the large row of colonies in the bottom of the adjacent field? What about that abandoned hive in the hedgerow over the road? What about the feral colony in the church tower? All of these are a potential source of reinfestation. After a week or two of miticide treatment your own colonies are likely to be largely free of phoretic mites … but all those nearby untreated (or yet to be treated, or ineffectively treated … or just plain forgotten) colonies can act as a source of mites and viruses from drifting workers and drones. These will infest and infect your colonies. Robbing bees – not the maelstrom of foragers ripping a colony apart that most beekeepers would recognise, but the silent robbing that can occur largely unseen and unsuspected in many apiaries – will bring a smorgasbord of virus-loaded mites and workers to your recently-treated hives. Remember also, your colonies may well be robbing other untreated, mite-infested colonies nearby. If all colonies ‘within range’ (see below) were treated at the same time these bee behaviours (drifting, robbing) that cannot be altered would have far less impact in transferring mites and viruses.

Coordinated Varroa treatment – over a wide geographic area – hasn’t been widely investigated in the UK. In Europe there have been a number of coordinated treatment trials, for example in isolated mountain valleys, where the geography provides a barrier to bee movement. Due to the unregulated and often undocumented nature of beekeeping in the UK it may well be more difficult to organise effectively. However, this isn’t a reason coordinated Varroa treatment shouldn’t be attempted. There are precedents in the salmon farming industry where all cages within a single water catchment area must be coordinately treated – both in terms of time and (I believe) the compound(s) used for controlling sea lice. This isn’t voluntary because it’s been shown to be effective.

What’s ‘within range‘?

One mile radius ...

One mile radius …

Drifting of foragers and robbing etc. are distance-dependent activities. The more widely separated colonies are, the less likely they are to be an issue. This was amply demonstrated in the recent comments by Tom Seeley that feral colonies hived and co-located in apiaries succumbed to mite-transmitted virus infections, whereas those sited – individually – at least 30 metres apart had lower mite counts and survived better (Sharashkin, L [2016], ABJ 156:157). So perhaps all colonies within 30 metres should be treated together?

Clearly this is too low a limit. Firstly, we know bees can travel much further and the studies described by Seeley didn’t test whether colonies survived even better if spaced even further apart. Secondly, the feral colonies Seeley studies are naturally located approximately half a mile apart from each other. Whilst this is undoubtedly influenced by the availability of hollow trees it suggests that the range could usefully be extended to at least half a mile. I’ve certainly seen robbing occurring between colonies located at least 500 metres apart.

Since the effective limit over which re-infestation might occur isn’t known it perhaps make sense to throw the net a little more widely … a mile for example? This is a convenient distance … covering most beekeepers within a small village in a rural area, those sharing adjacent fields in farmland or perhaps a number of urban apiaries. It’s also a manageably small area, where personal contact and friendly agreement should be sufficient to coordinate treatment. Do you know the location of all of the colonies within a mile of your own? Google maps can help. So can local association membership, or simply accosting people you see wearing a beesuit. I knew of ~20 hives belonging to 4-5 beekeepers within a mile of my previous home apiary. Of course, with any sort of migratory beekeeping – bringing colonies back from the heather, taking them to orchards – or simply moving nucs from a split colony to a new apiary, there’s a possibility of colonies with low mite levels getting exposed to colonies with a high level of infestation. For proper coordinated treatment these movements would have to be taken account of.

In our bee virus research we’re investigating the benefits of large scale coordinated Varroa treatment by working with all the beekeepers on a large island, where the sea provides a natural barrier to mites entering the test area. Over the next three years we will see how mites, and more importantly the viruses they transmit, are controlled by coordinating Varroa treatment within this defined area.

Coordinated Varroa treatment helps mitigate the effects of drifting and robbing between colonies, activities that are usually underestimated and that are known to transmit mites and (inevitably) viruses and other pathogens. This isn’t rocket science. It’s a logical response to the biology of bees and the pathogens that they carry.

How to treat

Spot the difference ...

Spot the difference …

Use a miticide that is appropriate for the conditions, use it according the manufacturers instructions and keep records of the treatment. There are no hard and fast rules, but it’s worth taking account of the following:

  • Avoid using pyrethroid-based miticides if there’s any evidence of resistance. Just because you get a high mite drop with Apistan doesn’t mean there isn’t an even larger resistant population left infesting your colony¹ … there are ways of checking this, perhaps you should?
  • Avoid using Apiguard unless the temperature really is high enough for it to work effectively, which means an average of 15°C for a month. If used at a sub-optimal temperature you’ll be leaving mites behind …
  • Avoid trickling oxalic acid/Api-Bioxal if there’s brood (sealed or unsealed) in the colony. It’s toxic to unsealed brood and the mites in sealed brood will escape unscathed …
  • Avoid vaporising Api-Bioxal unless you enjoy cleaning the gunky mess™ from the vaporiser. If vaporising oxalic acid ensure that the colony is broodless, or be prepared to repeat treatment three times at five day intervals to catch both phoretic and emerging mites …
  • Be aware that some miticides stop the queen from laying. Perhaps try and avoid these when you’re dependent on the colony raising the all-important winter bees that are going to get it through to the following Spring. I don’t actually know how much of an issue this is for colony health and survival, but it always concerned me when the queen went on a go-slow at the very time I wanted her to keep laying strongly through late August/early September.
  • Don’t reduce treatment doses or times … partial treatments are partially effective. This is also a great way to select for miticide-resistant Varroa (though whether they arise depends upon the mechanism of action – resistance to oxalic acid, formic acid and thymol has not been observed).

When to treat

Bee working ivy ...

Bee working ivy …

Earlier than you perhaps think to protect the winter bees from viruses. When I lived in the Midlands I would treat immediately after taking the summer honey crop – perhaps mid/late August. There’s later forage available – himalayan balsam and ivy – both of which some beekeepers either like or have a market for, but collecting it risks exposing the developing winter bees to high levels of Varroa and pathogenic viruses. Now I live in Scotland I’m going to have to develop alternative treatment schedules for colonies going to the heather – brood breaks and/or creative use of a vaporiser in June/July.

Treatment is only part of the solution though …

These articles on Varroa control have focused almost exclusively on miticide treatment. There are also a range of beekeeping practices that can contribute significantly to effective Varroa control, reducing the necessity to treat with chemicals. These include enforced brood breaks, shook swarms, drone brood uncapping, queen trapping and others. A proper integrated pest management strategy involves both chemical and beekeeping interventions to prevent the build up of dangerously high mite levels in the colony. Some of these will be covered in more detail during the coming season.


¹I think there’d be a case to ban the sale and use of Apistan for three years out of every four … pyrethroid resistance in mites appears to be detrimental in the absence of selection i.e. resistance is lost if the miticide is not used for a few years. That way, when used it would be devastatingly effective. This compares to the current situation where Apistan resistance is very widespread, and constantly selected for by continuing use of pyrethroids. Of course, there’s no way to enforce this – despite the fact it would probably be a great benefit for bee health – but now we’re back to the unregulated and undocumented nature of UK beekeeping.

When to treat?

Preparing Apiguard

Preparing Apiguard …

When and how do you treat colonies to have the greatest effect in minimising Varroa levels? At the end of this longer than usual post I hope you’ll appreciate that this is a different – and much less important –  question than “When is the best time to treat?”.

You probably use one of the treatments licensed and approved by the Veterinary Medicines Directorate (VMD), which include Apistan, Apivar, Apiguard, MAQS and Api-Bioxal. I’ve discussed the cost-effectiveness of these treatments recently. If used correctly, all exhibit much the same efficacy, reducing phoretic mite levels by 90-95% under optimal conditions. That being the case the choice between them can be made on other criteria … the ease of administration, the cost/treatment, the likelihood of tainting the honey crop, the compatibility with brood rearing, whether they mess up your vaporiser etc. After using Apiguard for several years, with oxalic acid (OA) dribbled in midwinter, my current preference – used throughout the 2015 season – is OA sublimation or vaporisation. This change was based on four things – efficiency, cost, ease of administration and how well it is tolerated by a laying queen. The how? you treat is actually reasonably straightforward.

When, not how, is the question

DWV symptoms

DWV symptoms

OK, but what about when? Because, if the treatments are all much of a muchness if used correctly, the when is actually the more important consideration. When might be partly dictated by the treatment per se. For example, Apiguard needs an active colony to transfer the thymol throughout the hive so the recommendation is to use it when the ambient temperature is at least 15ºC (PDF guidance from Vita). It’s worth stressing that this is the ambient temperature, not the temperature in the colony, which in places will be mid-30’s even when it’s much colder outside. At low ambient temperatures the colony becomes less active, and in due course clusters, meaning that Apiguard is not spread well throughout the colony, and is therefore much less effective. If you’re going to use Apiguard you must not leave treatment too late.

For readers in Scotland it’s interesting to note that the SBA annual survey by Peterson and Gray shows significant numbers still use Apiguard in September and October, months in which the mean daily maximum temperature is ~14°C and 11°C respectively … so the average daily temperature will be well below the recommended temperature for effective Apiguard use.

However, the when should be primarily informed by the  why you’re treating in the first place. It’s not really Varroa that’s the problem for bees, it’s the viruses that the mite transfers between bees when it feeds on developing pupae that cause all the problems. Most important of these is probably Deformed Wing Virus (DWV), but there are a handful of other viruses pathogenic to bees that are also transmitted. DWV causes the symptoms shown in the image above … these bees are doomed and will be ejected from the hive promptly. However, although apparently healthy (asymptomatic) bees have low levels of DWV, it’s been shown by Swiss researchers that DWV reduces the lifespan of worker bees, and that high levels of DWV in a colony are directly associated with – and causative of – overwintering colony losses. Therefore, the purpose of late summer/early autumn treatment is to reduce the Varroa levels sufficiently so that high levels of the virulent strains of DWV are not transmitted to the overwintering bees. When? therefore has to be early enough that this population, critical for overwinter survival, will live through to the spring – however long the winter lasts and however severe it is. However, before discussing when winter bees are reared it’s worth considering what happens if treatment is used early or late.

What happens if you treat early?

Mid June

Mid June treatment …

For example, mid-season or after the first honey crop comes off. Nothing much … other than slaughtering many of the phoretic mites. This is what most beekeepers would call “a result” 😉  Aside from possible undesirable side effects of treatment – like tainting honey, or preventing the queen from laying or even, with some treatments, queen losses – early treatment simply reduces mite levels. It’s important to remember that the levels may well not be reduced sufficiently to negate the need for a treatment later in the season … as long as there is brood being raised the mites will be reproducing (for example, look at the mid-June treatment generated using BEEHAVE modelling – image above). Furthermore, avoiding those undesirable side effects might require some ‘creative’ beekeeping (for example, clearing the supers and moving them to another hive) and will certainly inform the choice of treatment but, fundamentally, if the mite levels are high then treating earlier than is usual will benefit the colony, at least temporarily. If the mite levels – estimated from the disappointingly inaccurate mite drop perhaps – are dangerously high you should treat the colony.

What happens if you treat late in the season?

Isolation starvation ...

Isolation starvation …

In midsummer workers only live for ~40 days. If mite levels are high, virus transmitted to these workers will shorten their lives, so reducing the colonies’ foraging ability and – possibly – ability to defend itself against wasps or robbing late in the season. However, if you delay treatment until very late the lifespan of bees raised at the end of the season – the overwintering bees – will be reduced with potentially more devastating consequences. The usual winter attrition rate of workers will be higher. The cluster size of the colony will shrink faster than a colony with low mite levels. At some point the colony will cross a threshold below which it becomes non-viable. The cluster is too small to move in cold periods to new stores, resulting in the beekeeper finding a pathetic little cluster of bees in a colony that’s succumbed to isolation starvation. A larger cluster, spread across a greater area and more frames, is much more likely to span an area of sealed stores and be able to exploit it.

When are winter bees reared?

The apiary in winter ...

The apiary in winter …

In the Swiss study referred to above they looked at the longevity of winter bees. The title of the paper is “Dead or alive: deformed wing virus and Varroa destructor reduce the life span of winter honeybees”. We can use their data to infer when winter bees start to be reared in the colony and when mite treatments should therefore have been completed to protect these bees. Their studies were conducted in Bern, Switzerland, in 2007/08 where the average temperature in November/December that year was 3ºC. They first observed measurable differences in winter bee longevity (between colonies that subsequently succumbed or survived) in mid-November. This was 50 days after bees emerged and were marked to allow their age to be determined. By the end of November these differences were more pronounced. Therefore, by mid-November Varroa and virus-exposed winter bees are already exhibiting a reduced lifespan. Subtracting 50 days from mid-November means these bees must have emerged in late September. Worker development takes ~21 days, so the eggs must have been laid in the first week of September, and the developing larvae capped in mid-September.

To protect this population of overwintering bees in these colonies, mite treatments would have had to be completed by the middle of September, so that mite levels were sufficiently low that the developing larvae weren’t capped in a cell with a Varroa mite carrying a potentially lethal payload of DWV. For Apiguard treatment (which takes 2 x 14 days) this means treatment should have been started in mid-August. For oxalic acid vaporisation (which empirical tests suggest is best conducted three times at five day intervals) treatment would need to start no later than early September and preferably earlier as it is effective for up to a month.

Of course, these figures and dates aren’t absolute – the weather during the study would have influenced when the larvae would be raised as winter bees, with the increased fat deposits and other characteristics that are needed to support the colony survival through the winter. Despite the study being based in Switzerland my calculations on dates are probably broadly relevant to the UK … for example, the temperature during their study period is only about 1ºC lower than the 100 year average for Nov/Dec in Eastern Scotland where I now live.

In conclusion

That was all a bit protracted but it hopefully explains why it’s important to be selective about when you administer Varroa treatments. Chucking in a couple of trays of Apiguard in mid-August or mid-October has very different outcomes:

  • in mid-August the phoretic mite population should be decimated, reducing the transmission of virulent DWV to the all-important winter bees that are going to get the colony through the winter. This is a good thing.
  • in mid-October the mite population will be reduced (not decimated, as it’s probably too cool to effectively transfer the thymol around the hive – see above) but many of the winter bees will already have emerged, probably with elevated levels of DWV to which they will succumb in December or January. This is a bad thing.

Perhaps perversely, treating early enough to prevent the expected Varroa-mediated damage to developing winter bees is not be the best way to minimise mite numbers in the colony going into the winter. Using BEEHAVE I modelled the consequences of treating in the middle of each month between August and November¹. I used the default BEEHAVE setup as described previously. Figures plotted are the average of 3 simulations, each ‘primed’ with 20 mites at the start of the year.

Time of treatment and mite numbers

Time of treatment and mite numbers

There’s a lot on this graph. To show colony development I plotted numbers of eggs, larvae and pupae (left axis) as dotted red, blue and black lines respectively. Mite numbers are shown in solid lines – treated with a generic miticide in mid-July (black), mid-August (blue), mid-September (brown), mid-October (cyan) and mid-November (green). In each case the miticide is considered to be 95% effective at killing phoretic mites. The gold arrowhead indicates the period during which winter bees are developing in the colony, based upon the data from Dainat.

Oxalic acid trickling

Oxalic acid trickling

Treating at or before mid-August controls the late-summer build up of mites in the colony – look how the blue line changes direction. Mites that are not killed go on to reproduce in late September and early October, resulting in levels of ~200 at the year end. Remember that mites present in midwinter can, in the absence of sealed brood, be effectively controlled by trickling or vaporising oxalic acid (Api-Bioxal), and that this Christmas miticide application is particularly important if the autumn treatment has not been fully effective. In contrast, treating as late as October and November (cyan and green lines) exposes the developing winter bees to the highest mite levels that occur in the colony doing the year, and only then decimates the phoretic mite numbers, with those that remain being unable to reproduce effectively as the brood rearing period is almost over. Starting treatment in mid-September isn’t much different, in terms of exposing the winter bees to high mite levels, than starting later in the year.

So, within reason, treating earlier rather than later both reduces the maximum mite levels and helps protect the winter bees from virus exposure. Of course, treating as early as mid/late August may not be compatible with your main honey crop (particularly if you take hives to the heather) … but that’s another issue and one to be addressed in a future post.


¹BEEHAVE makes a distinction between ‘infected’ and ‘uninfected’ Varroa, the proportions of which can be modified. This might (no pun intended) not accurately reflect the reality in the hive, where Varroa-mediated transmission of DWV results in the preferential amplification of virulent strains of the virus. I need to roll my sleeves up and delve into the code to see if the model can be altered to fully reflect our current understanding of the biology of the virus. This might take quite a while …

References

Overwintering honey bees: biology and management from the Grozinger lab.

Managing Varroa (PDF) by the National Bee Unit

 

Vaporising Api-Bioxal

Vaporising Api-Bioxal leaves a burnt caramelised residue in the vaporiser. This is difficult to clean. Does this damage the vaporiser or make it work less efficiently?

Forget it ...

Forget it …

I remortgaged the house, took my kids out of university and cancelled both trips to Mauritius later this year, all so I could afford some Api-Bioxal (a snip at £10.99 for 35g from Thorne’s). Api-Bioxal is the VMD-approved oxalic acid-containing miticide. Only ‘containing’ as – according to the manufacturers instructions – only 88.9% of the dodgy-looking white crystalline powder is actually oxalic acid (OA). The remaining ~11% is a mixture of glucose and powdered silica (VMD documentation [MS Word]) . As cutting agents go, these are relatively harmless. Nevertheless, some have expressed concern that the presence of glucose might leave a horrible gunky mess (a widely accepted technical term) in the bottom of the vaporiser. Let’s see …

Since I’d promised to help a friend with vaporising a few hives that were disappointingly Varroa-riddled when treated earlier in the winter, this seemed a good opportunity to do a side-by-side comparison of Api-Bioxal and OA vaporisation – in terms of residues, not efficacy¹. My vaporiser is an ‘active’ model (made by Sublimox) in which the vaporised oxalic acid is forced out through a small nozzle in about 20-30 seconds (see video). In use, the OA crystals are dropped into a preheated pan – by inverting the Sublimox – so the temperature change from ambient to 157ºC happens more or less instantaneously. Any comments below therefore might not apply to the passive vaporisers like the “Varrox”, or the plethora of home-grown ones² on the forums or variants listed on eBay. In the majority of these types the powder is added to a pan which is then heated to the sublimation temperature³.

At the start of the trial the pan of the Sublimox was clean, contained no residues and was only slightly tarnished (from historical use). This machine has been used dozens of times previously and in each case has been washed out with clean water after use as instructed by the manufacturers.

After a single colony was treated with 1.6g of Api-Bioxal the pan of the Sublimox contained an obvious charred residue.

Single use ...

Single use …

We treated one further hive with Api-Bioxal and took another photograph of the vaporiser ‘pan’ which now contained an even more obvious charred caramelised deposit, bubbled and lumpy in places. This wasn’t a loose flaky deposit, it was burnt onto the base and lower sidewalls of the vaporiser ‘pan’.

Two treatments ...

Two treatments …

In use the ‘collar’ around the plastic (delrin?) cups used to deliver the OA/Api-Bioxal usually have slight traces of the powder left around them. These were particularly obvious when using Api-Bioxal though I’m not sure any greater amount of powder was left here … it just looked a lot worse. It was also more difficult to clean off than ‘pure’ OA.

Plastic cup ...

Plastic cup …

The caramelised charred residues remaining in the vaporiser after two Api-Bioxal treatments needed a combination of scraping with a knife and repeated rinsing with boiling water to remove it. This took several minutes and would clearly be impractical (and irritating) to do between treatments, meaning that the residues would build up quickly over time. Compare the first and second image in the series above to see how much residue builds up at each use (and see the note below regarding the amount vaporised).

Cleaned vaporiser ...

Cleaned vaporiser …

I then added 1.6g of standard oxalic acid dihydrate (Thorne’s) and vaporised it before immediately photographing the unwashed pan and cup. The photo below should therefore be compared directly with the first in this series. You can see the traces of OA powder at the end of the nozzle of the vaporiser, but the pan is completely clean and contains no additional charred and caramelised residues. This vaporisation was done ‘in the open’ (i.e. not into a hive) and it was interesting to see how long it took the extensive cloud of crystals – perhaps 5 x 2 x 2m in extent – to dissipate as it gently drifted away downwind.

Single OA use ...

Single OA use …

But it gets worse …

I actually used much less Api-Bioxal per hive than the manufacturers recommended 2.3g per colony (this is partly because there is published evidence that ~1.4g is sufficient and double that amount provides no increase in mite killing). I didn’t weigh the Api-Bioxal but used one measuring scoop that – from previous tests – is known to contain ~1.6g of OA when full. Had I used the full recommended dose of Api-Bioxal I would have therefore expected the residue build up to be about 50% worse than shown above. On a vaguely brighter note, the powdered Api-Bioxal pours easily and smoothly, presumably because of the anti-caking agents it contains.

What are the implications of this?

I am very disappointed with the amount of residues left in the vaporiser after using even a single (less than recommended) dose of Api-Bioxal. I’m also disappointed with how difficult these are to clean out of the vaporiser. Might these residues damage the vaporiser, for example by blocking the nozzle, or reduce the effectiveness of vaporisation, for example by not allowing the pan to heat as evenly or quickly? I think both of these are a distinct possibility. An advantage of vaporisation is the ease and speed with which OA can be administered. If the vaporiser needs to be cleaned between every (or even every few) hives it would significantly reduce the attractiveness of this type of Varroa treatment. Remember, if you take your PPE seriously – which you should when vaporising oxalic acid – you’ll be wearing gloves, a respirator/mask and goggles throughout this entire procedure, including cleaning out the residues from the hot vaporiser.

No thanks.


 

Update … 22/2/16

Chris Strudwick kindly sent me before and after photographs of a Bioenoxal vaporiser that had been used once with Api-Bioxal. The ‘before’ image (left) shows the machine after vaporising 1.6g of Api-Bioxal. The ‘after’ shows the “result of 5 minutes with a nylon pan scourer and water after an initial scraping with a hive tool” … so the gunk can be cleaned off, but it takes time.

Many thanks Chris


¹This would have entailed treating hives with a known Varroa-load with either Api-Bioxal or OA. This was not done.

²Some of the DIY vaporisers are either spectacularly dangerous or have been designed without an appreciation of the temperature control required to vaporise oxalic acid.

³If you have a “Varrox”-type vaporiser I’d be interested to hear your experience with using Api-Bioxal.

Time to BEEHAVE

BEEHAVE ...

BEEHAVE …

I’ve been dabbling with BEEHAVE, a computer simulation of a honeybee colony. It’s not beekeeping, but it’s about as close as you can get in the middle of winter. BEEHAVE was developed by Matthias Becher in the University of Exeter and the paper that describes the model is published and Open Access [PDF]. The model includes a wealth of user-modifiable variables such as forage availability, climate, beekeeping activities and pathogens, and outputs information on colony size, speed of development, age structure, honey stores etc. The BEEHAVE simulation is implemented in the open source language NetLogo and is freely available. The parameters that influence colony development – egg laying rate, drone/worker ratios, forage (nectar and pollen) availability, mite replication rate etc. are all based on measured and published data (or logically extrapolated from this if they don’t exist) so that the in silico performance is a fair reflection of what might be expected in the field.

If you can, do … if you can’t, simulate it 🙂

I’m interested in the rational and effective use of miticides to control Varroa-mediated transmission of DWV (and other viruses) in the hive. Using BEEHAVE and a standardised set of conditions allows predictions to be made of how effective a particular Varroa control might be. For example, here’s a simple question we can try and answer:

How important is a midwinter mite treatment if you’ve treated earlier in the year?

Using BEEHAVE set to all the default conditions and ‘priming’ the colony with just 20 mites on the 1st of January it’s possible to see what happens if no treatments are applied over one or more years. It’s then possible to repeat the predictions with the inclusion of a Varroa treatment. For the purpose of this brief introduction to BEEHAVE I’ve used a miticide which is applied and active for a total of 28 days and which kills 95% of phoretic mites. This might broadly reflect Apiguard treatment (2 x 14 days) or vaporised oxalic acid (OA; 3 treatments at 5 day intervals, but documented to kill mites for up to one month). I’ve additionally looked at the application of a single treatment with oxalic acid in midwinter, again killing 95% of phoretic mites, the sort of effect that OA trickling might achieve if there’s no brood present.

No treatment … they’re doomed

No treatment

No treatment

BEEHAVE modelling is based on a series of underlying probabilities (e.g. likelihood of a developing pupa to become mite associated, likelihood of that being a drone or worker pupa) so doesn’t produce the same results every time it is run¹. For example, the graph above shows adult bee numbers (left axis, blue lines) in an untreated colony for three simulations of up to five years each (horizontal axis), together with the associated mite number (right axis, red lines). Mite number build up strongly as new brood is reared each spring, with mite numbers peaking at ~24,000 in the fourth summer. In the third and fourth winters mite number per bee range from 2-4. The default conditions of 20 mites, coupled with a minimum viable colony size of 4000 bees, results in one colony succumbing in the fourth winter and the two remaining dying in the fifth winter (bee numbers drop to zero). Real studies – with untreated hives in the field – have shown similar outcomes (Martin, 1998 [PDF]) though colonies tend to die between winters 2 and 3, presumably because the input mite populations are higher². In all subsequent graphs the data plotted is the average of three simulations.

One treatment … better than nothing

It’s worth remembering at this point that the advice from the National Bee Unit is that mite numbers in the colony should be maintained below 1000 (Managing Varroa [PDF]). To try and achieve this we need to investigate the influence of applying miticides in the simulation – in mid-June (left graph), mid-September (middle) or late December (right). I appreciate mid-June is very early in the season, but it emphasises an important point.

That’s a bit better 🙂 These plots show the averages of adult bee and mite numbers (using the format shown above, blue for bees, red for mites). None of the in silico colonies expired during the simulation though the mite numbers are dangerously high irrespective of the treatment during the mid/late summer months. Note that range of the scale on the right hand (mite numbers) axis differs in each graph. Treatment in mid-June (left) delays the summer exponential rise in mite numbers and, in terms of overall impact on mite numbers (and consequent adult bee losses) is measurably better than only treating in midwinter (right). Of the conditions tested, mid-September (centre) is clearly the best … Varroa levels are reduced at the same time as the colony starts to contract, leaving the remaining mites less opportunity to reproduce. Maximum colony size remains about the same year on year and Varroa numbers never reach more than one third of those seen in either mid-summer or midwinter treatments. However, not everything is rosy … Varroa levels are dangerously high from the third summer on, and levels are increasing each winter. Remember that these simulations were started with just 20 mites in the colony².

Do your colonies have only ~20 mites in them this winter?

Two treatments … a double whammy

Two optimal treatments

Two optimal treatments

It’s only when you combine early autumn and midwinter treatments that mite numbers are really well controlled. Under the highly optimised conditions – both treatments were set to be 95% effective against phoretic mites – Varroa numbers remain below the NBU recommended maximum of 1000 for the duration of the simulation. Clearly the combination of the mid-September slaughter of phoretic mites, coupled with a midwinter mopping up – when there’s little or no brood present – provides really tight control of Varroa levels. However, the importance of this is perhaps even more apparent when you consider the consequences of a sub-optimal mid-September treatment.

The graph on the left shows the consequences of using a miticide that achieves only 85% efficacy … perhaps reflecting Apiguard usage when the ambient temperature is too low for the thymol to be spread throughout the colony. Under these conditions mite numbers rapidly get out of control. Compare that with the graph on the right which includes an additional midwinter treatment where mite numbers are far better controlled … though only to about the same level as is seen with a 95% knockdown of mites in mid-September (centre graph in the ‘one treatment only’ section, above).

And the answer is …

Occupied bait hive

Occupied bait hive …

Although the majority of miticides are broadly similar in their maximum published efficacy, I suspect that they are often used in a way or under conditions that do not routinely achieve these maxima. For example, the 30 year average September temperature in England is just below 13°C, much lower than the temperatures in which Apiguard efficacy reached the reported maximum of 99%, and lower than the Vita-recommended minimum temperature (15°C). Therefore, the answer to the original question (which was How important is a midwinter treatment if you’ve treated earlier in the year?) is … if there’s any chance the late summer/early autumn treatment was sub-optimal then a midwinter treatment is very important to prevent Varroa levels building up in the colony, resulting in the spread of virulent strains of DWV and other viruses. The other broad conclusion is that miticides are much more effective – in terms of impact against the total mite population – when brood levels are low or absent. That’s why brood breaks coupled with miticide treatments e.g. applying vaporised oxalic acid to a recently hived swarm or one that has moved in to a bait hive, are a very powerful combination to reduce the impact of mites, and the viruses they transmit, on the colony.

There are additional considerations which influence the choice and timing of miticide treatments. In a future post I’ll address the timing of the autumn treatment and the critical development of the overwintering bees that get the queen and the colony through to the following Spring.


¹BEEHAVE provides the ability to model colony development based upon measured and measurable parameters within a honeybee colony. Of course, in the real world a host of factors influence our bees – climate, forage availability, bad beekeeping, good beekeeping, integrated pest management, swarming, queen longevity etc. These are all variable within BEEHAVE but have been left unaltered from the defaults for the purpose of this post in which only the timing and efficacy of miticide treatment was altered. All the data for this post were generated using the rather verbosely numbered BEEHAVE_BeeMapp2015 version.

²Mite levels were deliberately started at a very low level to emphasise how quickly they build up if not controlled. Running the simulations with a higher mite input simply shifts all the graphs to the right e.g. increasing input mites to 200 (not an unreasonable number for many midwinter colonies) with no treatment, results in the virtual colony dying in early December of the third year, with mite levels having reached ~5300 in the first summer and ~19000 in the second.

This is the second in a series of related posts about Varroa control. The first was on drifting in honeybees. I’ve created a separate page that lists these and other posts on the how, why and when of Varroa treatment.