The Fight Continues

Winter is nearly over

It won’t be long until spring has sprung and beekeepers across the land start to open hives and assess how their bees did over the winter. I could be more cynical than the average walrus, but I strongly suspect that most beekeepers are like gamblers with winter loss figures. They are happy to talk about the good times but stay very quiet when losses are mounting. It’s human nature. The same probably applies to honey yields. That’s why I like to mix up my sources of information – some anecdotal and some from research projects. We are not always great at working out the cause of death; anything is better than “I didn’t manage varroa properly, so they snuffed it.”

Knowledge Seeking

As somebody floundering around seeking knowledge about bees, I understand that anecdotal information can be a powerful thing, especially from commercial beekeepers who manage hundreds of colonies. Few research projects directly target what I want to know, and many suffer from small sample sizes. Large research projects involving hundreds of colonies over several years are not easy to come by. 

German bee monitoring project

A good one that springs to mind is ‘The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies.’ This four-year study involved 1,200 honey bee colonies from 120 apiaries across Germany starting in 2004. It specifically investigated the causes of winter mortality. Without going into all of the details, the factors involved were (i) high varroa mite infestation levels (ii) deformed wing virus infections in autumn (iii) acute bee paralysis virus infections in autumn (iv) old queens and (v) relative colony weakness before the winter.

The most significant factor of all was good ole Varroa destructor. The measure of ‘mites per 100 bees’ in October was critical. There is a nearly exponential increase in winter losses from 0% – 20% infestation rates in October. A threshold of 6% (6 mites per 100 bees) keeps colony losses below 10%. In other words, if you do an alcohol wash in October and find more than 6% mite infestation, you are in trouble. 

Regarding the viruses, there was some evidence that high levels can cause winter losses even with low varroa counts, but often the two go together (high varroa levels + virus = death). I found it interesting that the research showed that colonies headed by young queens have a significantly higher chance of winter survival than those with older queens. The reason is elusive but may be to do with producing more brood and stronger colonies going into winter; the survivors went into winter with more bees than those that died.


The research paper concludes:

Based on the results presented, it is safe to state that Varroa destructor is the dominant killer of honey bee colonies during winter. In addition to high varroa infestation levels, DWV and ABPV infections in autumn significantly lower the winter survival of honey bee colonies as do old queens heading overwintering colonies. That a weak colony has not the best chance to survive the winter is rather trivial, but the fact that we observed such winter losses due to colony weakness shows that beekeepers still winter weak colonies. 

Brood Interruption

So, the fight continues against the mighty mite. I recently declared an interest in trying out a brood interruption method described by Ralph Buchler at the National Honey Show. He was very much involved in researching this on 370 colonies across 11 locations in Europe over two seasons. The paper, ‘Summer brood interruption as integrated management strategy for effective Varroa control in Europe’ shows promise. I was contacted by another researcher, Raffaele Dall’Olio, who pointed me to other work done in Europe on queen caging. It all boils down to timing and working the method to what’s right for one’s local conditions. 

My aim is to cage queens for a limited time, probably in June, and to then blast those hives with oxalic acid by sublimation once there is no sealed brood. I can do that with some colonies and treat the others normally, then compare the results. The queen is caged for 25 days in the study, and oxalic acid is used straight after she’s released. Encouragingly, at the nearest experimental location to me, Ireland, the efficacy was over 92% (2016 and 2017). Across all sites, it was 88.25%, with a range of 72% to 99%.

What about honey?

However, it’s all very well killing mites, I hear you say, but what about honey production? It’s crucial for those hoping to sell honey that trapping the queen does not lead to a lower honey harvest. I’m mainly interested in testing the impact on honey; I don’t need convincing that using oxalic acid on broodless colonies kills most of the mites. The essential things to check will be how much extra time it takes and whether taking her majesty off laying duties for a few weeks buggers up the harvest. Theoretically, if you get the timing right, the bees have no larvae to feed, so they crack on with foraging.

My main concerns are that I live in a place famous for unpredictable weather. If I release the queen after 25 days and we get a long spell of cold and wet weather, will she lay enough brood to build up nice and strong for the winter? I suppose I’d have to feed syrup in such circumstances, and maybe even pollen. To counterbalance this fear, I have the satisfaction of possibly being able to manage varroa with just oxalic acid. Beecraft has expressed an interest in publishing what I find, as long as I put it into some sort of cohesive order and take some pictures. It’s nice to have a project!

9 thoughts on “The Fight Continues

  1. Hi Steve

    I suspect half of UK beekeepers will be trying queen caging this season 😉 There’s been a discussion on it recently on Bee-L, with comments on very variable efficiency and significant influence of the climate on how well it works. There’s also a worthwhile set of comments on how long you actually need to cage the queen for. Why 25 days? That must be to allow drone eggs laid just before caging to complete their brood cycle. Get the timing wrong and you have too few bees for the summer flow … tricky. I’ve bought some cages and am going to try a slightly different approach, hoping to only miss about two weeks of laying.

    I’ve often noticed that strong queenless colonies get a bumper crop, so I’m not unduly worried about caging her in the middle of the strong flow.

    The high virus/low Varroa is what happens when you treat colonies too late in the season. The bees with high levels of virus die a month or two too early and the colony fails to make it through the winter. RIP.

    Finally, I think the annual reviews of winter hive losses are like Amazon reviews … the beeks who lose the lot want to share their tales of woe, those that lose none want to brag about it. You often don’t get the full picture (and the sample size is too low). The BeeInformed partnership in the US have reported figures of up to 35% recently, and I suspect we’re closer to that than the 8% of whatever was reported by the BBKA last winter.


    • Thanks David, very interesting as always. I saw Randy Oliver’s comment about 2 week trapping then OA a week later – is that your plan? Just re-read all of the comments on Bee-L and it made me question my sanity. The 2 week caging sounds less risky…

  2. Hi Steve, some interesting thoughts, enjoyed it again. I’m wondering though why the brood cycle seems to be very in vogue currently and the extra work it invariably entails ?. Coupled with the totally alien concept (to the bees I’m assuming) of using such an enforced break to their natural rhythm? As a newbie I’m wondering why not simply stick with good observance (checking of mite levels), something like apivar (to tackle mites in brood as well) applied at the correct time and duration and if required a suitable “mid winter” treatment too ? When i have carefully considered my winter losses (a single nuc each season for the past couple of years), its been because I’ve grown too attached to a wee small colony that really was never going to make it or to a hive that that was so damp and waterlogged that i should have known better !

    • Hi Keith, I have doubts about the whole thing, for sure. I suppose many unmanaged colonies swarm annually giving a brood break that way, whereas I’m trying to avoid swarming (May/June here) and artificially doing the brood break without the loss of foragers. It does sound like madness and I’m doing it on some hives not all. I think beekeepers are too prone to fiddling about and hate to think that I’m falling into that trap…but on the other hand, what happens when Apivar resistance kicks in? Curiosity killed the walruses bees! I think I’ll do some at 25 days caging as per the research and another set with 14 days in the cage and OA after a further 8 days. The only problem with the latter is a few capped drone cells, but I can scrape them open before treating.

  3. greengage1

    I listened to an interesting podcast by Dr. Samuel Ramsey. He described forced brood breaks as not being such a good idea. He said that forcing varroa out of their natural reproductive cycles and moving them all, virtually at once, to host on the bees within a colony could cause a rapid spread of viruses within your colony and spike virulence. well worth a listen to, I think it was an American bee podcast I heard it on.

  4. Peter Walker

    Hi Steve, thanks for another informative piece. I’ll be fascinated to see how your enforced brood break trial goes. As so often with these things, the theory looks sound, but the practicalities and possible unintended consequences need to be explored. Kudos for giving it a go!

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