Honey bees are pollinators of flowering plants all over the world and are therefore critical for the production of food crops. In recent decades, honey bees have been subjected to numerous environmental and management stresses that have impacted their survival rates. Among these, the most devastating has been infection by the parasitic mite Varroa destructor.
Varroa is an external parasite of Asian and European honey bees. Not only does it attach to the bodies of adult and larval bees and suck out nutrients, but it also carries at least five viruses that are devastating to the bees. The parasite’s original host was Apis cerana, the Asian hive bee, but it was introduced to the European honey bee, Apis mellifera, in the mid-1800s. Since then it has spread to all continents and regions of the world, except Australia and Antarctica.
Interestingly, Asian honey bees show certain behavioral traits that help keep infestations of varroa mites under control. These behaviors include grooming mites off their own and other bees’ bodies, as well as getting rid of larvae that have been infected with mites. Such hygienic strategies have genetic components and are heritable. It is likely that, since the Asian honey bees have been exposed to varroa mite infestations over evolutionary time, natural selection has gradually favored the survival of colonies where bees practice these behaviors. As a result, Asian honey bees are largely resistant to the parasite.
In the U.S., scientists have spent 20 years breeding a line of European honey bees that also shows hygienic behavior towards infestations of varroa mites. Beekeepers in this country, as in many others, have resorted to using pesticides and other chemical agents to control varroa mites in bee colonies. This has meant that natural selection for more resistant bees has not been able to take place and apiculturists are faced with large losses in colony numbers each year.
In a new study, published today in the journal Scientific Reports, experts from the universities of Louisiana and Exeter, and from the Agricultural Research Service of the US Department of Agriculture (USDA), tested the survival rates of these varroa resistant “Pol-line” bees in the field. The study took across three US states (Mississippi, California, and North Dakota), where commercial beekeepers move tens of thousands of colonies annually to provide pollination services for large-scale agriculture. The survival of colonies of varroa resistant bees was compared with the survival of standard bees that had not been selectively bred for hygienic behavior.
“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.”
The results of the study showed that the mite-resistant bee colonies were more than twice as likely to survive the winter (60% survival compared to 26% in standard honey bee colonies). The standard honey bees experienced high losses unless extensive chemical treatments were used. Colony survival over the winter is particularly important for beekeepers, because honey bees are needed in early spring to pollinate many valuable crops.
“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,” explained Dr. O’Shea-Wheller. “While this is the first large-scale trial, continued breeding and use of these bees has shown consistently promising results.”
Although European honey bees are not naturally resistant to varroa mite infestations, they do show some of the specific hygienic behaviors that are common in Asian honey bees. The incidence of the cleaning behaviors is very low but it does indicate that the genes for these behaviors are present. By selectively breeding for these behavioral traits, colonies can been produced that automatically protect themselves from infestation, while maintaining large colony sizes and ample honey production.
“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.
According to Dr. O’Shea-Wheller: “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 current study also monitored the levels of three debilitating viruses present in the varroa resistant colonies. These viruses (DWV-A, DWV-B and CVPV) are carried by the mites and transmitted to the bees. The results showed that the colonies bred for varroa resistant behavior also had significantly lower levels of these viruses. This is explained by the fact that cleaning behavior by the bees removes mites from the colonies and therefore also removes the viruses.
Interestingly however, when examined separately from levels of mite infestation, these viruses were not strong predictors of colony losses.
“A lot of research is focused on the viruses, with perhaps not enough focus on the mites themselves,” said Dr. O’Shea-Wheller.
“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 that 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.