A new breed of honey bees provides a major advance in the global fight against the parasitic Varroa mite, new research shows. The invasive mite, which has spread to all continents except Australia and Antarctica, has been the prime threat to honey bees since its initial expansion 50 years ago.
In Egypt, honey bees are considered to be the holiest and most important insects, however, today the Varroa mite is the most significant threat to them. The Varroa mite is a species of most devastating pest of Western honey bees that was first documented in the US in the early 1960s. The Varroa mite is a self-replenishing imitator of the honey bee and has been reported to be responsible for most of the diebacks on honey bees’ nests. It is also believed that when conditions are right, it can survive in areas where other organisms cannot.
In the study — by the universities of Louisiana and Exeter, and the Agricultural Research Service of the US Department of Agriculture (USDA) — “Pol-line” bees, bred for resistance to the mite in a rigorous 20-year breeding program, were trialed alongside a standard variety in a large-scale pollination operation.
The mite-resistant bees were more than twice as likely to survive the winter (60% survival compared to 26% in standard honey bees). While the standard honey bees experienced high losses unless extensive chemical miticide treatments were used.
“The Varroa mite is the greatest threat to managed honey bee colonies globally,” said Dr Thomas O’Shea-Wheller, of the Environment and Sustainability Institute at Exeter’s Penryn Campus in Cornwall.
“So far, new methods to control the mites — and the diseases that they carry — have had limited success, and the mites are becoming increasingly resistant to chemical treatments. It’s a ticking time-bomb.
“By selectively breeding bees that identify and remove mites from their colonies, our study found a significant reduction in mite numbers, and crucially, a two-fold increase in colony survival.
“While this is the first large-scale trial, continued breeding and use of these bees have shown consistently promising results. “This kind of resistance provides a natural and sustainable solution to the threat posed by Varroa mites, and does not rely on chemicals or human intervention.”
The study took place across three US states (Mississippi, California, and North Dakota), where commercial beekeepers move tens of thousands of colonies annually to provide pollination for large-scale agriculture.
Varroa mites originated in Asia, so European honey bees (the most common species kept for pollination) have not evolved 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 like they might in the wild.
However, managed bees sometimes respond to mites (which reproduce in the cells of bee larvae) by expelling infested larvae — killing both the larvae and the mites, in a behavior known as Varroa-sensitive hygiene (VSH).
By selectively breeding for this trait, colonies can be produced that automatically protect themselves from infestation, while maintaining large colony sizes and ample honey production. In fact, the increased number of colonies results in the individual cells becoming more equipped to fight off infection. It has been shown that when larger colonies are perpetuated, it is easier for the species as a whole to maintain its population size and keep the cell count high. This ability to maintain a large cell count allows for greater efficiency in fighting off infection and can be advantageous in terms of survival rates.
“The great thing about this particular trait is that we’ve learned 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 in everyone’s bees,” said research molecular biologist Dr. Michael Simone-Finstrom, of the USDA Agricultural Research Service.
Honey bee colonies are usually small, so providing enough hay, eggs, and water will help colony size in the spring. Colony survival over the winter is particularly important for beekeepers because honey bees are in high demand in the early spring — a key time for pollinating high-value crops such as almonds. Therefore, due to lack of survival, colonies can be wiped out causing genetic damage and a decline in populations, which can then have a negative economic impact.
The study also examined levels of viruses associated with Varroa mites in bee colonies. The colonies bred for Varroa resistance showed lower levels of three major viruses (DWV-A, DWV-B, and CBPV).
Interestingly, however, when examined separately from levels of mite infestation, these viruses were not strong predictors of colony losses.
“A lot of research is focussed on the viruses, with perhaps not enough focus on the mites themselves,” Dr. O’Shea-Wheller said.
“The viruses are clearly important, but we need to take a step back and be rigorous in delivering the best practical outcomes because if you control the mites, you automatically control for the viruses that they transmit.”
Dr. O’Shea-Wheller said bee breeding and testing is expensive and takes time, but that breeding mite-resistant bees is cost-effective in the long term and is likely to be the only sustainable solution to deal with the Varroa pandemic.
In terms of fighting the Varroa mite, at least for now, future work should focus on designing methods to prevent its spread in both North America and Europe. The work of developing methods to prevent the spread of Varroa mite should be directed towards two goals: preventing the spread of bloodsucking creatures and managing the populations of beneficial insects. Furthermore, as someone who has been following the developments in this field, it will be important to see how effective these methods are in combination with public health measures to stop the spread.
The study was funded by USDA.
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- Thomas A. O’Shea-Wheller, Frank D. Rinkevich, Robert G. Danka, Michael Simone-Finstrom, Philip G. Tokarz, Kristen B. Healy. A derived honey bee stock confers resistance to Varroa destructor and associated viral transmission. Scientific Reports, 2022; 12 (1) DOI: 10.1038/s41598-022-08643-w
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