Honey bee colony numbers fell by 16.7% in the winter of 2018-19, according to an international study led by the University of Strathclyde.
The survey of 28,629 beekeepers in 35 countries found that, out of 738,233 colonies being managed at the start of winter, 122,953 were lost, through a combination of circumstances including natural disaster, colonies that died, and unsolvable problems with a colony’s queen.
Larger beekeeping operations, with more than 150 colonies, including operations run by professional beekeepers, saw lower loss rates than smaller ones, while beekeepers who did not migrate their colonies at least once in 2018 had lower losses than those who did.
A change of queen bee, which occurred in 55% of colonies going into winter, was also a significant factor. Higher percentages of young queens in a beekeeper’s colonies corresponded to lower overall losses, excluding those caused by natural disaster.
Slovenia saw the highest loss rate, at 32%, while the lowest was 5.8% in Bulgaria. There were losses of between 20% and 25% in Portugal, Greece, Iran, Croatia, Spain and Serbia; the loss rate in Serbia the previous winter had been 7.4%. There were rates below 10% for 2018-19 in Finland, Israel, Norway, England, Denmark and Wales. The loss rate for Scotland was 18.9%, higher than for the rest of the UK.
The study, based on voluntarily submitted information, covered 31 countries in Europe – including the four nations of the UK – along with Iran, Algeria, Israel and Mexico.
It has been published in the Journal of Apicultural Research and was carried out by researchers in the colony loss monitoring group of the international honey bee research association COLOSS, which is based in the Institute of Bee Health at the University of Bern.
Dr Alison Gray, a Lecturer in Strathclyde’s Department of Mathematics & Statistics, led the study. She said: “It’s widely believed that a 10% rate of honey bee colony loss is an acceptable level. Our study has found a rate above this but there is considerable variation between countries.
“We had not previously investigated the effect of the age of the queen bee on winter losses. If bees in a colony sense their queen is ailing, they may start to raise a new queen by building queen cells. A beekeeper may also make the decision to introduce a new queen, although this has to be done carefully.
"The proportions of wintered colonies with a new queen are not strictly comparable between all beekeepers, as beekeeping operations vary considerably in size. However, overall, across many beekeeping operations of varying size, we found that operations with higher percentages of young queens saw significantly lower loss rates.
“Due to our methodology, it is not possible to say whether the lost colonies are the ones that had the older queens but our findings suggest that replacing the queen in more than 50% of a beekeeper’s colonies is best. Young queens may be better in colony build-up, due to greater fertility or better health status, are often less likely to contract diseases and, in general, produce more healthy bees.”
The previous study, of the winter of 2017-18, showed a very similar overall level of loss, although the pattern of loss rates across the participating countries was different and the number of colonies studied in the new research was larger. Iran was a new participant in the study, with 1653 beekeepers submitting data on 230,093 colonies.