Category Archives: Diseases

Botulism

Do not feed to infants

Do not feed to infants

I was recently asked, Why can’t you give young babies honey?

You can.

But just because you can doesn’t mean you should.

And on this point the NHS guidelines are very clear. You should not give honey to babies under 12 months of age because there is a risk that they might get botulism.

Bacteria, toxins and Botox

Botulism is a serious, sometimes fatal, disease caused by infection with a bacterium called Clostridium botulinum. As it grows, C. botulinum produces neurotoxins which cause a flaccid (floppy) paralysis and can result in respiratory failure. About 5-10% of cases are fatal, but infections thankfully very are rare.

Symptoms include fatigue, weakness, blurred vision and difficulty speaking and swallowing. The paralysis is ‘descending’, generally starting in the head and neck, then moving to the shoulders, arms, chest and lower limbs.

Botulinum toxin

Botulinum toxin

Unusually for a bacterial infection there is no fever. This reflects the fact that there’s probably only limited bacterial growth (which typically induces fever) and the potent neurotoxicity of the botulinum toxin. This toxin stops the release of the neurotransmitter acetylcholine from the nerve endings, thereby causing paralysis.

Botulinum toxin is one of the most acutely lethal toxins known. The lethal dose depends upon the route of administration, but is between 1.3 and 13 ng/kg 1.

Remember, botulinum toxin is the active ingredient in Botox.

No thanks. I’ll stick with the wrinkles 😉

Botulism cases in the UK/Europe

Botulism is a notifiable disease. Consequently, we have good data on the incidence of botulism in the UK and Europe. In 2014 there were 91 confirmed cases in the EU, with 14 cases reported in the UK between 2010 and 2014. Other than injecting drug users, a significant proportion of the cases are in infants – see below.

C. botulinum is widespread in the environment and infection usually occurs by ingestion of improperly prepared food e.g. undercooked or improperly canned foods, in which the bacteria survives.

Clostridium botulinum

Clostridium botulinum

The bacteria grows in the absence of oxygen and produces the toxin during growth. Although the toxin is heat-inactivated if properly cooked (over 85°C), the bacterium also produces heat-resistant spores during growth. These spores can withstand temperatures over 100°C for long periods and usually require both high temperatures and pressures to inactivate them.

As a consequence of this the spores are also very widespread in the environment … cue the Jaws soundtrack … just waiting to encounter the correct conditions to germinate and initiate a new round of bacterial growth (and toxin production).

Botulism cases in children

About a third of all cases of botulism are in the 0-4 age group. I’ve been unable to find a more detailed breakdown by age, but there have been 19 cases of infant (children less than 12 months old) botulism in the UK since 1978.

In many cases of infant botulism the source of the spores is unknown. However, other than well-documented cases of contaminated milk powder, honey is the only food regarded as a significant risk factor. About 60% of cases of infant botulism are in babies with a history of honey consumption 2 and, in several cases, epidemiological follow-up has confirmed that honey was the source of the infection.

Treatment is not with antibiotics as it’s the toxin that causes the symptoms, not the bacteria. Instead patients are treated with immunoglobulin (antibodies) specific for the toxin. These inactivate toxicity fast and recovery is usually complete, but can be protracted.

C. botulinum spores in honey

Oxygen inhibits the growth of C. botulinum. So do acidic conditions. Honey is acidic, with a pH of about 3.9, which is too low for the bacterium to grow. However, the spores remain viable at low  pH. It is this contamination of honey with C. botulinum spores that poses a risk for infants.

It is possible to microbiologically examine honey for contamination with C. botulinum spores. When this has been done, 6-10% of honey samples tested were contaminated, with contamination levels estimated at 5 to 80 spores per gram of honey. The infectious dose for a human is estimated at 10-100 spores 3.

So … much less than one teaspoon of contaminated honey.

Despite this, there is no requirement for honey to carry a label warning that it should not be fed to infants. Instead, the Food Standards Agency recommend honey carries a warning that it is unsuitable for children under one year of age.

Why is infant botulism so rare?

If up to 10% of honey is contaminated with C. botulinum spores, why are there not many more cases of botulism in infants? After all, European paediatricians have even been known  to recommend honey – a long-standing traditional solution – as a means of soothing crying babies4.

The intestine of the developing baby is full of bacteria – the so-called commensal microbiota – all competing to get established and to lead a long, happy and healthy association with their human host. The spores of C. botulinum have to germinate and establish an infection in the face of this competition and, usually, they fail. A likely possibility is that infant botulism only occurs in babies in which the commensal microbiota have not properly developed … either because they are so young, because broad-spectrum antibiotic use has prevented the development of the microbiota or for a pre-existing genetic condition.


 

Survival of the fattest

Winter bees have high levels of vitellogenin, a glycolipoprotein 1, deposited in their fat bodies which act as a food reservoir for the long winter.

These fat winter bees are essential for the successful overwintering of the colony.

Last week I discussed the major points that need attention for overwintering i.e. strong, healthy colonies with ample food in a weathertight hive.

This week I want to explore the relationship between colony strength, health – specifically with regard to Varroa and deformed wing virus (DWV) – and isolation starvation.

Isolation starvation describes the phenomenon where a small colony of tightly clustered honey bees gets isolated from the honey stores laid down in autumn, resulting – typically during protracted cold periods – in the colony starving to death.

Isolation starvation ...

Isolation starvation …

It’s both a pathetic and distressing sight. Bees, with their heads crammed into the bottom of cells searching for food, dying from starvation when literally inches away from capped stores.

Deaths and births

In temperate climates the winter is characterised by low temperatures and little or no forage for the bees. The queen usually stops laying sometime in autumn and starts again around the turn of the year. During the intervening period she may lay intermittently, but generally in limited amounts.

The fat bodied winter bees that are reared in late summer and early autumn are long-lived (about 6 months) and are responsible for getting the colony through the winter. They protect the queen, thermoregulate the hive and they help rear the brood raised in the autumn and through the winter.

In their absence – or if there are just too few of them – the colony will perish.

Winter bees do not all live for 6 months. The usual figure quoted is ~175 days 2. Some live shorter lives, some longer … up to 9 months under certain conditions.

Importantly, in studies I’ve discussed at length previously, high levels of DWV reduces the lifespan of winter bees. We know this because, in Varroa-infested colonies, researchers 3 have shown that the winter bees die off faster 4.

Live fast, die young

Winter bees with high levels of DWV don’t really live fast … but they do die young. In the studies above the average lifespan of winter bees was reduced by 20% in the colonies that died overwinter.

There are a couple of important things to note here. Dainat and colleagues were not looking at bees in the presence or absence of Varroa, or in the presence or absence of high or low levels of DWV. They simply looked at hives that succumbed in the winter or that survived, then measured DWV and Varroa levels. It’s a subtle but important difference. Their surviving colonies still had Varroa and DWV.

From analysis of hives that died or survived, and having marked known numbers of bees in late summer, they could determine the life expectancy of workers – in their surviving colonies it was ~88 days, in those that died it was ~71 days.

Healthy colonies

The gradual death of bees through the winter coupled with the reduced lifespan of winter bees with high levels of DWV explains why colonies need to be strong and healthy.

The following graphs are based upon modelled data 5, but show the influence of colony size and winter bee lifespan.

The first graph – the least important – simply shows the lifespan of bees. The graph plots the number of bees (on the vertical axis) in a population that die at a particular time (on the horizontal axis) after the start of the experiment. The blue bees have a longer average lifespan than the red bees 6.

Lifespan of winter bees

Lifespan of winter bees

In the following graphs remember that the blue bees are healthy, with low levels of Varroa and – consequently – low levels of DWV. The red bees are unhealthy and have high levels of Varroa and DWV.

Using this lifespan data we can look at the influence on the total number of winter bees in a colony (on the vertical axis) over time (horizontal). Imagine that the horizontal axis is the long, dark, wet and cold months of winter. Starting in early September and running through until late March.

Brrrr 🙁

Winter bee numbers in healthy (blue) and unhealthy (red) colonies

Winter bee numbers in healthy (blue) and unhealthy (red) colonies

It is clear, and of course entirely predictable, that the numbers of bees in the healthy (blue) colony are higher than those in the unhealthy colony at each time point. If the average lifespan is reduced (by disease) more bees will have died by a particular time point when compared with a healthy colony at the same timepoint.

Finally, consider that the shaded section of the graph represents the lower limit of bee numbers for viability. If the number of bees in the colony drops into this region the colony will perish.

Simplistically – and in reality – starting with similar numbers of bees a healthy colony will survive longer than an unhealthy colony.

Strong colonies

Using a similar approach we can also look at the influence of the average lifespan of winter bees on the survival of strong or weak colonies.

The following graph shows the numbers of bees in the colony over time for a strong colony (solid line) and a weak colony (dashed line) where worker bee lifespan is identical 7.

Winter bee numbers in strong and weak colonies.

Winter bee numbers in large (strong) and small (weak) colonies with the same average lifespan.

The shaded section of the graph again represents colony oblivion.

Large (strong) colonies take longer to drop below the threshold for viability and so – all other things being equal – will survive longer 8.

Mix’n’match

A strong colony with high levels of Varroa and DWV might actually survive less well than a weak but healthy colony.

Strong unhealthy colonies might survive less well than weak healthy colonies.

Large unhealthy colonies might survive less well than small healthy colonies.

In this graph the weak but healthy colony drops below the ‘viability threshold’ after the strong but unhealthy colony 9.

Winter bees and brood rearing

This is modelled data, but it makes the point clearly. Large and/or healthy colonies retain more of the all-important winter bees and so survive longer.

Simples.

The differences might not appear marked. However, for convenience 10 I’ve omitted the influence of winter bee numbers on the ability of the colony to rear brood.

If there are more winter bees, the colony is able to thermoregulate the hive better. It’s therefore able to keep any brood present warm. It’s therefore able to rear more brood.

As a consequence, the differences in bee numbers between the large or small, or the healthy and unhealthy, colonies will be much more striking.

Critically 11 the strength of the colony coming out of the winter is often the rate-limiting determinant for spring build-up to exploit early season nectar flows. Weak colonies develop less well.

Isolation starvation

Finally, returning to that pathetic little cluster of starving bees in the image at the top of the page. What is the relationship between colony health, strength and isolation starvation?

It’s now time to dust off my weak-to-non-existent Powerpoint skills …

Isolation starvation schematic

Isolation starvation schematic

Again, it’s straightforward. A large (strong) overwintering colony (A above) only has to move a short distance to access stores in midwinter. In contrast, a small (weak) overwintering colony has to move much further.

Consequently, small colonies become isolated from their stores during long, cold periods when the colony is clustered.

Prediction

Many beekeepers will be familiar with isolation starvation of overwintering colonies.

Most would explain this in terms of “very cold weather and the cluster was unable to reach its stores”.

Some would explain this in terms of “the colony was far too small to reach the stores when clustered”.

Very few would explain this in terms of “the Varroa and DWV levels were too high because of poor disease management last autumn. Inevitably most of my winter bees died off early in the winter, leaving a very small cluster of bees that were unable to reach the stores..

I suspect the real cause of isolation starvation is probably disease … specifically poor management of Varroa levels and consequently high levels of DWV in the colony.


Colophon

Herbert Spencer

Herbert Spencer

Another post, another poor pun in the title. Survival of the fittest encapsulates the Darwinian evolutionary principle that the form of an organism that survives is the one able to leave the most copies of itself in future generations. Darwin didn’t actually use the term until the 5th edition (1869) of his book On the origin of the species. Instead, the phrase was first used by Herbert Spencer in 1864 after reading Darwin’s book. Whilst ‘survival of the fittest’ suggests natural selection, Spencer was also a proponent of the inheritance of acquired characteristics, Lamarckism.

They think it’s all over!

We’re gently but inexorably segueing into early autumn after an excellent beekeeping season. The rosebay willow herb is almost over, the farmers are busy taking in the harvest and colonies are – or should be – crowded with under-occupied workers.

Rosebay willow herb

Rosebay willow herb

Drones are being ejected, wasps are persistently looking for access and there’s a long winter – or at least non-beekeeping period – ahead.

There’s a poignancy now in being in the apiary conducting the last few inspections of the season. Only a few short weeks ago, during late May and early June, the apiary was a scene of frenetic productivity … or complete turmoil, depending upon your level of organisation or competence.

Now there’s little activity as there’s not much forage available.

Colonies are busy doing nothing.

The most important time of the season

But that doesn’t mean that there’s nothing to do.

Rather, I’d argue that late August and early September is probably the most important period during the beekeeping year.

However well or badly the season progressed, this is the time that colonies have to be prepared for the coming winter. With good preparation, colonies will come through the winter well. They’ll build up strongly in spring and be ready to exploit the early season nectar flows.

In Fife, this is about 8 months away 🙁

This explains the poignancy.

There are some colonies inspected last weekend that probably won’t get properly opened again until mid/late April 2019. Queens I saw for the first time in August won’t get marked or clipped until next spring 1.

Au revoir!

Spot the queen ...

Spot the queen …

To survive the winter and build up well in the spring the colony has few requirements. But they are important. A lack of attention now can result in the loss of the colony later.

To appreciate their needs it’s important to understand what the colony does during the winter.

Suspended animation

Honey bees don’t hibernate in winter. In cold weather (under ~7°C) they cluster tightly to conserve energy and protect the queen and any brood in the colony.

At higher temperatures the cluster breaks but they largely remain within the hive. After all, there’s little or no forage available, so they use their honey and pollen stores.

The fat-bodied overwintering bees that are reared in autumn have a very different physiology to the ephemeral summer workers. The latter have a life-expectancy of 5-6 weeks whereas overwintering bees can live for many months 2.

But they’re not immortal.

Throughout the winter there’s a slow and steady attrition of these workers. As they die off the clustered colony gradually reduces in volume, shrinking from the size of a medicine ball, to a football, to a grapefruit … you get the picture.

Some brood rearing does occur. The queen often stops laying after the summer nectar flows stop 3 and laying might be sporadic through the autumn, dependent upon weather and forage availability.

Late summer brood frame from a nuc ...

Late summer brood frame from a nuc …

However, by the turn of the year she starts laying again. At a much reduced level to her maximum rate, but laying nevertheless and, with sufficient workers in the colony and as forage become available, this rate will increase.

The amount of brood reared during the winter period (late autumn to early spring) isn’t enough to make up for the losses that occur through attrition. This explains why colonies are much smaller in the spring than the early autumn.

Strong, healthy, well-provisioned and weathertight

Knowing what’s happening in the colony during the winter makes the requirements that must be met understandable.

  • Strong colonies start the winter with ample bees. Assuming the same attrition rate, a larger colony will get through the winter stronger than a smaller one. There will be more workers available to ‘reach’ stores (I’ll deal with this in the next week or two) and keep the queen and brood warm. Hence there will be more foragers to exploit the early crocus, snowdrop and willow.
  • Healthy colonies will have a lower attrition rate. The overwintering workers will live longer. High levels of deformed wing virus (DWV) are known to shorten the life of winter bees. To minimise the levels of DWV you must reduce the levels of Varroa in the colony. Critically, you must protect the overwintering bees from Varroa exposure. Treat too late in the season and they will already be heavily infected …
  • Well-provisioned colonies have more than enough stores to survive the winter. The clustered colony will have to move relatively short distances to access the stores. As a beekeeper, you won’t have to constantly meddle with the colony, lifting the lid and crownboard to add additional stores in midwinter.
  • Weathertight colonies will be protected from draughts and damp 4.The hive must be weathertight and, preferably, not situated in a frost pocket or damp location 5.

Winter preparation

Once the honey supers are off all activities in the apiary are focused on ensuring that these four requirements for successful overwintering are achieved in a timely manner.

Clearing bees from wet supers ...

Clearing bees from wet supers …

Weak colonies are united with strong colonies. At this stage in the season – other than disease – the main reason a colony is likely to be weak is because the queen isn’t up to the job. If she’s not now, what chance has the colony got over the winter or early spring? 6

Varroa treatment is started as early as reasonably possible with the intention of protecting the overwintering bees from the ravages of DWV. This means now, not early October. Use an appropriate treatment and use it correctly. Apiguard, oxalic acid (Api-Bioxal), Apivar etc. … all have been discussed extensively here previously. All are equivalently effective if used correctly.

All colonies get at least one block (12.5kg) of bakers fondant, opened like a book and slapped (gently!) on the tops of the frames. An eke or an empty super provides the ‘headspace’ for the fondant block. All of the Varroa treatments listed above are compatible with this type of feeding simultaneously 7.

Hopefully, hives are already weathertight and secure. Other than strapping them to the hive stands to survive winter gales there’s little to do.

They think it’s all over!?

It is … almost 🙂


Colophon

They think it’s all over! is a quote by Kenneth Wolstenholme made in the closing stages of the 1966 World Cup final. Some fans had spilled onto the pitch just before Geoff Hurst scored the the last goal of the match (England beat West Germany 4-2 after extra time), which Wolstenholme announced with “It is now, it’s four!”. This was the only World Cup final England have reached, whereas Germany have won four.

As Gary Lineker says “Football is a simple game; 22 men chase a ball for 90 minutes and at the end, the Germans win.”

Sphere of influence

How far do honey bees fly? An easy enough question, but one that is not straightforward to answer.

The flight range of the honeybee ...

The flight range of the honeybee …

Does the question mean any honey bee i.e. workers, drones or the queen? As individuals, or as a swarm?

Is the question how far can they fly? Or how far do they usually fly?

Why does any of this matter anyway?

Ladies first …

Workers

The first definitive experiments were done by John Eckert in the 1930’s. He located apiaries in the Wyoming badlands at increasing distances from natural or artificial forage 1. Essentially the bees were forced to fly over a moonscape of rocks, sand, sagebrush and cacti to reach an irrigated area with good forage. He then recorded weight gain or loss of the hives located at various distances from the forage.

Wyoming badlands

Wyoming badlands …

The original paper can be found online here (PDF). The experiments are thorough, explained well and make entertaining reading. They involved multiple colonies and were conducted in three successive years.

Surprisingly, Eckert showed that bees would forage up to 8.5 miles from the colony. This means they’d be making a round trip of at least 17 miles – and probably significantly more – to collect pollen and nectar.

However, although colonies situated within 2 miles of the nectar source gained weight, those situated more than 5 miles away lost weight during the experiments.

Gain or loss in hive weight ...

Gain or loss in hive weight …

Therefore, bees can forage over surprisingly long distances, but in doing so they use more resources than they gain.

John Eckert was the co-author (with Harry Laidlaw) of one of the classic books on queen rearing 2. His studies were probably the first thorough analysis of the abilities of worker bees to forage over long distances. Much more recently, Beekman and Ratnieks interpreted the waggle dance (PDF) of bees to calculate foraging distances to heather. In these studies, only 10% of the bees foraged ~6 miles from the hive, although over 50% travelled over 3.5 miles.

Queens

Queens don’t get to do a lot of flying. They go on one or two matings flights, perhaps preceded by shorter orientation flights, and they might swarm.

Heading for a DCA near you ...

Heading for a DCA near you …

I’ll deal with swarms separately. I’ll also assume that the orientation flights are no greater than those of workers (I don’t think there’s any data on queen orientation flight distance or duration) at no more than ~300 metres 3.

On mating flights the queen flies to a drone congregation area (DCA), mates with multiple drones and returns to the colony. DCA’s justify a complete post of their own, but are geographically-defined features, often used year after year.

There are a number of studies on queen mating range using genetically-distinguishable virgin queens and drones in isolated or semi-isolated locations. They ‘do what they say on the tin’, drone congregate there and wait for a virgin queen

In the 1930’s Klatt conducted studies using colonies on an isolated peninsula and observed successful mating at distances up to 6.3 miles

Studies in the 1950’s by Peer demonstrated that matings could occur between queens and drones originally separated by 10.1 miles 4. These studies showed an inverse relationship between distance and successful mating.

More recently, Jensen et al., produced data that was in agreement with this, with drone and queen colonies separated by 9.3 miles still successfully mating 5.

However, this more recent study also demonstrated that more than 50% of matings occurred within 1.5 miles and 90% occurring within 4.6 miles.

Just because they can, doesn’t mean they do 🙂

Drones … it takes 17 to tango …

Seventeen of course, because that’s one queen and an average of 16 drones 😉

There’s a problem with the queen mating flight distances listed above. Did the queen fly 9 miles and the drone fly just a short distance to the DCA?

Or vice versa?

10 miles ... you must be joking!

10 miles … you must be joking!

Or do they meet in the middle?

Do queens choose 6 to fly shorter distances because it minimises the risk of predation and because they are less muscle-bound and presumably less strong flyers than drones?

Alternatively, perhaps drones have evolved to visit local DCAs to maximise the time they have aloft without exhausting themselves flying miles first?

Or getting eaten.

It turns out that – at least in these long-distance liaisons – it’s the queen that probably flies further. Drones do prefer local DCAs 7 and most DCAs are located less than 3 miles from the ‘drone’ apiary 8.

Swarms

I’ve discussed the relocation of swarms recently. Perhaps surprisingly (at least in terms of forage competition), swarms prefer to relocate relatively near the originating hive. Metres rather than miles.

The sphere of influence

Effective foraging – in terms of honey production (or, for that matter, brood rearing) – occurs within 2-3 miles of the hive. This distance is also the furthest that drones usually fly to occupy DCAs for mating.

Queens can fly further, but it’s the law of diminishing returns. Literally. The vast majority of matings occur within 5 miles of the hive.

In fact, other than under exceptional circumstances, a radius of 5 miles from a colony probably represents its ‘sphere of influence’ … either things that can influence the colony, or that the colony can influence.

Why does this matter?

Worker flight distances are relevant if you want to know the nectar sources your bees are able to exploit, or the pollination services they can provide. In both cases, closer is better. It used to also be relevant in trying to track down the source of pesticide kills, though fortunately these are very much rarer these days.

Closer is better ...

Closer is better …

Workers not only fly to forage on plants and trees. They also fly to rob other colonies. I don’t think there are any studies on the distances over which robbing can occur, but I’ve followed bees the best part of a mile across fields from my apiary to find the source of the robbing 9.

All of these movements can also transport diseases about, either in the form of phoretic Varroa mites piggybacking and carrying a toxic viral payload, or as spores from the foulbroods.

Drone and queen flight distances are important if you’re interested in establishing isolated mating sites to maintain particular strains of bees. My friends in the Scottish Native Honey Bee Society have recently described their efforts to establish an isolated queen mating site in the Ochil Hills.

And I’m interested as I now have access to a site over 6 miles from the nearest honey bees in an area largely free of Varroa.

It’s not the Wyoming badlands, but it’s very remote 🙂


 

CSI: Forensics in Fife

This is a long post. If you can’t be bothered to read it in its entirety the conclusion is …

It’s the viruses wot done it … probably.

But the take home message is that you can learn from colonies lost in midwinter.

Introduction

I always have mixed feelings about midwinter hive losses, or deadouts as they’re called in the US. Of course, I regret losing the colony and wonder whether I could have managed them differently, or prepared them better, to increase their chances of survival. At the same time I’d much prefer a weak colony perishes in midwinter rather than having to mollycoddle them through the spring.

Winter colony losses in the UK vary from year to year but have averaged about 20% over the last decade (BBKA figures and SBA data). Whether these accurately reflect real losses is unclear to me – there are no statistics and they are usually by self-selected beekeeper reporting, so possibly unreliable.

Mollycoddling weak colonies in the Spring … a wasted effort?

The reality is that weak colonies in the spring require a lot of support and still may not survive. Even if they do, there’s little chance they will be strong enough to exploit anything other than the late season nectar flows.

If the colony is weak because of queen problems then they are likely to need re-queening. This requires a spare queen early in the season – not impossible, but it needs planning or is likely to be expensive. In my view you’d be better off using the ‘spare’ queen to make up a nuc from a prolific colony, rather than trying to rescue what might be a basket case.

If the colony is weak because of disease then having them limp on through the spring is a potential disaster for your other colonies. If it doesn’t recover quickly it’s likely that neighbouring colonies will rob it out, both destroying the weak colony and transferring whatever it was ailing from around the apiary.

Finally, the colony might be weak due to poor management e.g. insufficient stores in the early spring when the queen was gearing up to lay again. In this case you might be able to rescue the situation by boosting it with syrup, brood and bees. However, care is needed not to weaken your other colonies. Two half-strength colonies are more work and much less use – they will collect  less honey – than one full strong colony.

Learning from your losses

So, rather than pampering weak colonies in Spring – particularly if they’re struggling because of disease or queen problems – I’d prefer they expired in the winter. That sounds cruel but isn’t meant to. The reality is that a proportion of all colonies are likely to be lost in the winter … on average ~20%, but significantly more in long hard winters (or perhaps with more accurate surveys!). As beekeepers, all we can do is manage them in ways to minimise these losses – keep strong, healthy colonies and provide them with sufficient stores – and learn from those that are lost.

How do you learn from the losses? By examining the ‘deadout’ and trying to work out what went wrong. You then use this information in subsequent seasons to try and avoid a repeat performance.

For example …

A life in the year of a June swarm

Sometime in early/mid January a colony of mine died.

The colony was alive in early December when treated for mites. It was suspiciously quite at the end of that month when other colonies were flying. It looked moribund by mid-January on a quick visit to the apiary.

I took the hive apart on the 30th of January to see what might have gone wrong.

But before the autopsy, here’s the history of the deceased …

  • 7th June – a small to medium sized swarm arrives in a bait hive. The bait hive had foundationless frames so it was difficult to estimate the amount of bees in terms of ‘frames covered’. My notes state ” … 4-5 frames of bees (ish) …”.
  • 8th June – very brief inspection, two frames nearly drawn. Queen not seen.
  • 9th June – vaporised with an oxalic acid-containing treatment. 21 mites dropped in the first 24 hours. Not monitored after that as I replaced the solid floor of the bait hive with an open mesh floor.
  • 19th June – unmarked queen laying well. My notes state “… suspect this is a 2015 Q on account of the lousy weather we’ve had …”. There was already sealed brood present.
  • 27th June – clipped and marked the queen blue. At this point there were over 7 frames of brood in all stages – eggs, larvae and sealed brood – present.
  • Between then and mid-August the colony continued to build up very well … so well I ‘harvested’ three frames of brood and bees to make up nucs for circle splits.
  • In mid/late August I treated three times at five day intervals with a vaporised oxalic acid-containing miticide. My notes in August state ” … KEEP AN EYE ON THIS ONE … very high mite levels …” and (after a second round of treatment) in late September, ” … mite levels still high …”. Actual mite counts weren’t recorded.
  • In late September/early October the colony was fed with fondant. An additional block of fondant was left on into the late autumn.
  • Further miticide treatment was added in early December. Mite drop was high but not outrageous – about 44/day averaged over the first 5 days, dropping to 3/day over the subsequent five day period.
  • By late December the mite drop was less than one per day … however, by this time I was beginning to be concerned as there was very little activity from the colony on the warm days around Christmas.

The autopsy

I took the roof off the hive and removed the remaining fondant. Some of the fondant had dripped down between the frames but, in taking these apart, it was clear there were sufficient stores present.

The frames were clean. There were no sign of Nosema, which usually appears as distinctive faecal smearing and marking on the top bars, the face of the frames and – in a heavily infected colony – the front of the hive.

The frames were almost devoid of bees. There were a hundred of so clinging to the middle pair of frames. I brushed these away to reveal a dozen or so corpses stuck headfirst down in the cells. This is characteristic of starvation. These particular bees likely died of starvation, but the majority did not (or they would have also been wedged headfirst into the comb).

Having removed the frames the few hundred dead bees lying on the open mesh floor could be seen. Amongst these was the blue marked and clipped queen. The bees on the floor weren’t showing any obvious signs of disease – no characteristic shrivelled wings seen with overt deformed wing virus infection for example.

I’d walked to the apiary so couldn’t take the hive away with me. I therefore sealed up the entrance to prevent any robbing just in case there was disease present that could be transmitted to another colony. In due course I’ll burn old frames, I’ll treat some of the good drawn comb with acetic acid fumes and I’ll clean and sterilise the hive.

Elementary, my dear Watson

There’s no really obvious smoking gun, but there are some reasonably strong clues. I’m pretty sure why and how this colony succumbed.

But first, what didn’t kill off the colony? Well, Nosema didn’t and nor did starvation. Sure, a handful of bees in the middle frame died of starvation, but the majority were on the floor and there were ample stores in the hive and some fondant remaining. It had also been warm enough to move about within the colony to get to these stores.

It’s not possible to deduce much about the queen. The colony was strong in late August but not fully inspected after that. My notes state that brood levels were low in late September, but they were in all the colonies I peaked in at that time. She could have failed catastrophically soon afterwards. If she had I might have expected to find signs of attempted replacement e.g. a queen cell, as there were a few warm periods in the autumn. The fact that she was still present is far from conclusive, but suggests to me that she didn’t fail outright. The colony wasn’t full of drones late in the season suggesting she was poorly mated, though there are plenty of other ways the queen can fail. At least until late August she was laying very well.

I think the two main clues are the Varroa levels in autumn and December, coupled with the small number of bees present on the floor of the colony. Taking those in reverse order. There were far fewer bees than I would have expected from a colony that entered the autumn with about 8 frames of brood. This suggests to me that the bees were dying off at a faster rate than expected. Many bees that died off earlier in the winter would have been carried out and discarded on the warmer days.

The Varroa levels were very high in autumn and quite high (at least for my colonies) in December. In the autumn I didn’t record the numbers. A couple of hundred dropped after treatment in December isn’t huge … but I suspect that the colony was much reduced in size by now meaning the percentage infestation was likely significant.

The damage was already done …

Miss Scarlett in the ballroom with the lead piping

I think this colony died due to the viruses transmitted by Varroa, in particular deformed wing virus (DWV). DWV is known to shorten the life of overwintering honey bees – see this study by Dainat et al., for full details.

DWV symptoms

DWV symptoms

Why are there no symptomatic bees visible in the carpet of bees littering the floor of the hive? That’s easy … you only get these symptoms in very young bees that have emerged from Varroa-infested brood. In the winter there’s little or no brood (and there certainly wasn’t for some time in this colony). Any bees in this colony that had emerged with DWV symptoms would have been discarded from the colony months earlier. The colony was inspected every 7-10 days from mid-June to late July but there were no obvious signs of DWV symptomatic bees. They might have been missed, but I’m reasonably experienced at spotting them.

My interpretation is that the colony arrived with high Varroa levels and that – despite treatment shortly after arrival – these persisted through to mid-August. The mites transmitted the usual cocktail of pathogenic DWV strains within the colony. High Varroa levels are known to result in the massive amplification of virulent DWV strains in exposed bees. This late summer/early autumn period is a critical time for a colony. It’s when the overwintering bees are being reared. I discuss this at length in When to treat?. These overwintering bees, now infected with virulent strains of DWV, subsequently died off at a higher rate than normal. Despite the colony appearing reasonably strong in late August, many of the bees were carrying a lethal viral payload.

Bees that died in late autumn would have been carried out of the hive and discarded in the usual manner. Overall bee numbers continued to dwindle, leaving just a few hundred and the queen by the year end. We’ve had some hard frosts in the last month. The expired colony is in the same apiary as the bee shed which has a max/min thermometer inside. The lowest temperature seen was -6°C during this period. This probably finished them off.

So no crime scene … just another reminder that the viruses will get you if the Varroa levels are allowed to get too high.

Learning from my mistakes

I think I made one big mistake with this colony … way back on the 10th of June, just three days after it moved into the bait hive. I treated for mites, monitored mite drop after 24 hours and then left the colony with an open mesh floor. I should have monitored for longer (mite drop after vaporisation is often greater after the first 24 hours). I would have then realised how badly infested they were and could have taken greater care to reduce mite levels. By mid/late August mite levels were probably catastrophically high. They were hammered down with miticides but the damage was already done. I suspect that this is also a good example of why the timing of treatment is critical.

It’s sobering that an apparently minor oversight in midsummer probably resulted in the loss of the colony in midwinter.

If you got this far, well done. It wasn’t my intention to write so much. Swarms are a significant source of potential disease and carry a disproportionately high mite load … something I’ll discuss in the future.


 They are not unreliable because they are reported by beekeepers 😉  Not entirely anyway. They are unreliable because they are generally a self-selecting group that report them. The BBKA or SBA ask for beekeepers to complete a survey. Some do, many do not. The BBKA do not report – at least in their press releases – the number of respondents. Self-selecting bias in surveys means they may not be entirely (or at all) accurate. The SBA surveys by Magnus Peterson and Alison Gray are more thorough. For their 2014 report for example the sample size was 350, with a total of 213 respondents (for comparison, there are about 4000 beekeepers in Scotland). With this information, coupled with some additional data – for example, knowing that only 87% of respondents were actually keeping bees during the survey period! – you can determine how representative the survey is.

 Sherlock Holmes never uses this phrase in any of the books by Conan Doyle though he does use a number of similar expressions. The phrase “Elementary, my dear Watson” was first used by PG Wodehouse in Psmith, Journalist which was published contemperaneously (1909). The actor Clive Brook used the phrase in the 1929 film The return of Sherlock Holmes.

Drifting in honeybees

During previous research on deformed wing virus (DWV) biology and its transmission by Varroa I’ve moved known Varroa-free colonies (sourced from a region of the UK which the mite has yet to reach and maintained totally mite-free) into apiaries in the countryside. Within 2-3 weeks Varroa was detectable in sealed brood, showing that mite infestation occurs very readily. I know other researchers who have made very similar observations. Where do these mites come from?

They’re not all ‘your’ bees

The obvious source would be the phoretic mites transported on workers ‘drifting’ from nearby infested colonies, or on drones which are known to travel quite long distances and may be accepted by almost any colony. If you want to see how frequent this is try marking a few dozen drones with a dab of paint. To avoid confusion use the colour used to mark queens next year. There are unlikely to be 4+ year old queens in the apiary and the drones will all perish before the end of the current season. Over the next few days and weeks the drones will appear in adjacent colonies, and some will likely leave the apiary and be accepted in your neighbours colonies.

How to encourage drifting ...

How to encourage drifting …

Beekeepers are usually aware that colonies at the ends of rows often ‘accumulate’ bees that have drifted when returning to the hive. In shared association apiaries some crafty beekeepers will site their colonies at the ends of rows to take advantage of the ‘generosity’ of other colonies. However, many beekeepers probably do not appreciate the extent to which drifting occurs. Pfeiffer and Crailsheim (1998) report that 13-42% of the population of a colony are ‘alien’ i.e. have drifted from adjacent hives, depending upon the time of season. Remember that drifting occurs in both directions simultaneously, so the overall numbers of bees in a colony may not be adversely affected (or boosted). In other studies Sekulja and colleagues (2014) showed that ~1% of marked bees drifted between colonies over a three day observation window. Interestingly, American foulbrood (AFB) infected bees drifted slightly more than uninfected bees. Spread of foulbroods during drifting is one reason the bee inspectors check nearby apiaries when there is an outbreak. These studies were all on workers where drifting primarily occurs during orientation flights before the bees become foragers. Drones drift two to three times more than workers (Free, 1958).

The likelihood of drifting must be closely related to the separation of hives and apiaries. Although workers will forage up to 2-3 miles from the hive I suspect the proportion of bees that drift this distance is extremely small. However, unless you’re very isolated I expect there are other apiaries within a mile or so of your own. Drones are known to fly up to about five miles to reach drone congregation areas for queen mating and are accepted by all colonies. I’ve regularly found drones appearing in (relatively) isolated mini-nucs. I’m not aware of studies that have formally tested drifting between apiaries (though it is reported in passing in the Sekulja et al., 2014 paper cited above).

Consequences of drifting

So, your hives probably contain workers and drones from other nearby colonies, and you can only really be sure that they’re all “your” bees if you live – as the sole beekeeper – on an isolated island. Not only does your neighbour generously exchange bees with you, he or she also kindly shares the phoretic mites those bees are carrying, the viral payload the bees and mites are infected with and – if you’re really unlucky – the Paenibacillus larvae spores responsible for causing AFB infection (and vice versa of course).

There are lessons here that should inform the way we conduct our integrated pest management to maintain healthy colonies. 

This post provides background information for an article (“Viruses and Varroa: Using our current controls more effectively” by David Evans, Fiona Highet and Alan Bowman) in the December 2015 issue of Scottish Beekeeper, the monthly magazine for members of the Scottish Beekeepers Association.

More later …

 

Deformed Wing Virus

Deformed wing virus (DWV) is probably the most important viral pathogen of honeybees. In the presence of Varroa the virus is amplified to very high levels in the colony, resulting in newly emerged workers – those that survive long enough to emerge – exhibiting the classic symptoms familiar to most beekeepers. These include deformed or atrophied wings, a stunted abdomen, additional deformities or paralysis of appendages and (not visible) learning impairment. There is a clear association between high Varroa levels, high levels of DWV symptomatic bees and overwintering colony losses.

Classic DWV symptoms

Classic DWV symptoms

These images are of workers from a colony treated for a month with Apiguard to reduce mite numbers. Many bees remained with symptoms. I suspect the high levels of mites pre-treatment resulted in the amplification of virulent strains of DWV which continued to cause disease even after the mite numbers were reduced. This emphasises the need to monitor mite numbers and treat appropriately with Apiguard, oxalic acid or – during the season – other appropriate integrated pest management practices such as drone brood culling.

Worker with immature mite

Worker with immature mite …

DWV symptoms and mite

DWV symptoms and mite