Selective breeding sustainably protects honey

image: Beekeepers transport colonies to support large-scale agriculture. Image taken in South Dakota.
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Credit: Thomas O’Shea-Wheller

A new breed of bee is a major step forward in the global fight against the parasite Varroa mite, new research shows.

The invasive mite, which has spread to every continent except Australia and Antarctica, has been the biggest threat to bees since its initial spread 50 years ago.

In the study – conducted by the universities of Louisiana and Exeter and the Agricultural Research Service of the United States Department of Agriculture (USDA) – “Pol-line” bees, selected for their resistance to the mite in the part of a rigorous 20-year breeding program, were tested alongside a standard variety in a large-scale pollination operation.

Mite-resistant bees were more than twice as likely to survive the winter (60% survival versus 26% in standard honey bees). While standard honey bees suffered high losses unless extensive chemical miticide treatments were used.

“The Varroa the mite is the greatest threat to managed honey bee colonies globally,” said Dr. Thomas O’Shea-Whellerof Institute of Environment and Sustainability at the Penryn Campus of Exeter in Cornwall.

“So far, new methods of controlling mites – and the diseases they carry – have had limited success, and mites are becoming increasingly resistant to chemical treatments. It’s a ticking time bomb.

“By selectively breeding bees that identify and eliminate mites from their colonies, our study found a significant reduction in mite numbers and, importantly, a two-fold increase in colony survival.

“Although this is the first large-scale trial, continued breeding and use of these bees has always yielded promising results.

“This type of resistance offers a natural and lasting solution to the threat posed by Varroa mites, and does not rely on chemicals or human intervention.”

The study took place in three US states (Mississippi, California and North Dakota), where commercial beekeepers move tens of thousands of colonies each year to provide pollination for large-scale agriculture.

Varroa mites are native to Asia, so European honey bees (the most common species kept for pollination) did not evolve alongside them, and therefore lack effective resistance.

Like humans, managed bees are largely “decoupled” from natural selection, Dr. O’Shea-Wheller said, so they cannot develop resistance as they might in nature.

However, managed bees sometimes react to the mites (which reproduce in the cells of bee larvae) by expelling the infested larvae – killing both the larvae and the mites, in a behavior known as Varroa– sensitive hygiene (VSH).

By breeding selectively for this trait, colonies can be produced that automatically protect against infestation, while maintaining large colony sizes and abundant honey production.

“The great thing about this particular trait is that we learned that honey bees of all types express it at some level, so we know that with the right tools it can be promoted and selected for all bees. “, said the molecular biologist researcher. Dr. Michael Simone-Finstrom, USDA Agricultural Research Service.

Survival of colonies through winter is particularly important for beekeepers, as honey bees are in high demand in early spring, a key time for pollination of high-value crops such as almonds.

The study also looked at virus levels associated with Varroa mites in bee colonies.

Colonies bred for Varroa resistance showed lower levels of three major viruses (DWV-A, DWV-B and CBPV).

Interestingly however, when examined separately from mite infestation levels, these viruses were not good predictors of colony losses.

“A lot of research focuses on viruses, with perhaps not enough focus on the mites themselves,” Dr. O’Shea-Wheller said.

“Viruses are clearly important, but we have to step back and be rigorous to get the best practical results, because if you control the mites, you automatically control the viruses they transmit.”

Dr O’Shea-Wheller said breeding and testing bees was expensive and time-consuming, but breeding mite-resistant bees was profitable in the long run and would probably be the only sustainable solution for face the Varroa pandemic.

The study was funded by the USDA.

The article, published in the journal Scientific reportsis titled: “A Derived Honey Bee Strain Confers Resistance to Varroa destructor and associated viral transmission.”

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Jeanetta J. Stewart