Improved treatment timing reduces honey bee losses to Varroa mites

Honey bee mortality can be significantly reduced by ensuring that treatments for the parasitic Varroa mite occur within specific timeframes, a new study reveals.

The mites—belonging to the species Varroa destructor—feed on the larvae of bees and can destroy colonies if not treated at key time points to reduce or remove infestations.

But researchers have found that more than a third of beekeepers surveyed in England and Wales deviate from recommended treatment guidelines, often missing these application windows.

They further observed that beekeepers who mistimed Varroa mite treatments experienced exacerbated colony losses, with this effect occurring across a wide range of medications.

“The main finding here was that a major cause of honeybee mortality could, in theory, be quite easy to reduce,” said Dr Thomas O’Shea-Wheller, lead author of the study, from the University of Exeter.

“We talk a lot about honey bee colony losses in terms of novel diseases and emerging threats, but this is actually something that’s been under our nose the whole time.”

The researchers used the National Bee Unit’s BeeBase—a register of beekeepers in England and Wales—to analyse data from 4,339 beekeepers across 37 counties, encompassing a total of 18,700 colonies from 2016 to 2020.

They looked at the range of Varroa treatments that beekeepers were using, how they were applying these treatments, and what influence this had on colony survival and honey yield.

They found that colony losses were predicted not just by the type of Varroa treatment that a beekeeper used, but also by their level of adherence to recommended treatment regimens.

“Although beekeepers are frequently treating for Varroa, a considerable proportion of these are missing key time points and thus experiencing higher losses than expected,” said Dr O’Shea-Wheller.

Outlining why timing is so important in administering Varroa treatments, Dr O’Shea-Wheller explained: “Mite numbers build up across the season in a kind of exponential curve. You have to target this at key junctures in order for treatments to be effective.

“If you miss these, and apply a treatment too early or too late, then it will fail to properly supress the mite population and growth will continue unchecked. Missing the recommended treatment window is, I think, the largest cause of poor prognostic outcomes and associated colony mortality.”

Dr O’Shea-Wheller warned that deviating from recommended Varroa treatment guidelines could also lead to the mites developing increased resistance.

“Varroa has been the primary cause of honey bee mortality for some years, and it seems that we’re in a sort of arms race as the mites become increasingly resistant to acaricides. Like with antibiotics, it’s important to use Varroa treatments precisely as directed, because mistiming, overdosing, or underdosing can all increase the chances of the mites developing resistance.”

The paper, published in the journal Entomologia Generalis, is entitled: “A large-scale study of Varroa destructor treatment adherence in apiculture”.