Honey bees are pollinators of flowering plants all over the world and are therefore essential for the production of food crops. Over the past few decades, honey bees have been subjected to numerous environmental and management stresses that have impacted their survival rates. Of these, the most devastating was infection with the parasitic mite destructive varroa.
Varroa is an external parasite of Asian and European bees. Not only does it attach to the bodies of adult and larval bees and suck up nutrients, but it also carries at least five viruses that are devastating to bees. The original host of the parasite was Apis ceranathe Asian honey bee, but it was introduced into the European honey bee, Audder melliferain the mid-1800s. Since then, it has spread to every continent and region of the world except Australia and Antarctica.
Interestingly, Asian honey bees exhibit certain behavioral traits that help control varroa infestations. These behaviors include grooming mites on their own bodies and those of other bees, as well as removing larvae that have been infected with mites. Such hygienic strategies have genetic components and are hereditary. It is likely that since Asian honey bees have been exposed to varroa infestations during evolution, natural selection has gradually favored the survival of colonies where the bees practice these behaviors. As a result, Asian honey bees are largely resistant to the parasite.
In the United States, scientists have spent 20 years breeding a line of European honey bees that also show hygienic behavior towards varroa infestations. Beekeepers in this country, like many others, have used pesticides and other chemical agents to control varroa mites in bee colonies. This means that the natural selection of more resistant bees could not take place and that beekeepers are faced with large losses in the number of colonies each year.
In a new study, published today in the journal Scientific reports, experts from the universities of Louisiana and Exeter, and the Agricultural Research Service of the US Department of Agriculture (USDA), field-tested the survival rates of these varroa-resistant “Pol-line” bees. The study focused on three US states (Mississippi, California, and North Dakota), where commercial beekeepers move tens of thousands of colonies each year to provide pollination services for large-scale agriculture. The survival of varroa-resistant bee colonies was compared to the survival of standard bees that had not been selectively bred for hygienic behavior.
“The Varroa The mite is the greatest threat to managed honey bee colonies globally,” Dr Thomas O’Shea-Wheller from the Institute for Environment and Sustainability at the Penryn Campus in Exeter told 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. »
Study results showed that mite-resistant bee colonies were more than twice as likely to survive the winter (60% survival versus 26% in standard honey bee colonies). Standard honey bees suffered high losses unless intensive chemical treatments were used. Survival of colonies through the winter is particularly important to beekeepers because honey bees are needed in early spring to pollinate many valuable crops.
“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,” explained Dr O’Shea. -Wheller. “Although this is the first large-scale trial, continued breeding and use of these bees has consistently shown promising results.”
Although European honey bees are not naturally resistant to Varroa infestation, they exhibit some of the specific hygienic behaviors that are common in Asian honey bees. The incidence of cleaning behaviors is very low, but this indicates that the genes for these behaviors are present. By breeding selectively for these behavioral traits, colonies can be produced that automatically protect themselves 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.
According to Dr. O’Shea-Wheller: “This type of resistance provides a natural and lasting solution to the threat posed by Varroa mites, and does not rely on chemicals or human intervention.
The current study also monitored the levels of three debilitating viruses present in varroa resistant colonies. These viruses (DWV-A, DWV-B and CVPV) are carried by mites and transmitted to bees. The results showed that colonies bred for varroa resistant behavior also had significantly lower levels of these viruses. This is because the cleaning behavior of bees removes mites from colonies and therefore also removes viruses.
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 is expensive and time-consuming, but breeding mite-resistant bees pays off in the long run and is probably the only sustainable solution to deal with the Varroa pandemic.
By Alison Bosman, Terre.com Personal editor