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.”
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.
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!