Strong against bloodsuckers by breeding

The varroa mite weakens bee colonies by draining bodily fluids from adults and larvae. © ARS-USDA

Hope for the plagued beneficial insects: the resistance of bee colonies to the varroa mite can be significantly increased through selection, as the success of a breeding line shows. The scientists report that the so-called “pole-line honey bees” are more than twice as likely to survive the winter as standard honey bees due to their resistance. The success is based on the increased hygiene behavior of the insects, in which they specifically remove offspring infested with the parasites.

Our valued honey producers and pollinators are faring badly: many bee colonies are weakened and loss rates are high. According to studies, a mixture of different causes is responsible for this: lack of food, pesticides and various pathogens give the insects a hard time – but their biggest problem is a stubborn parasite. In the last 50 years, the varroa mite, originally from Southeast Asia, has spread mainly in Europe and America. The parasites weaken the honey bees by draining bodily fluids from both the adults and the larvae. They also transmit viral pathogens that cause additional problems for the bees. Above all, the parasite infestation limits the colonies’ ability to survive the winter.

In order to avoid total failures, beekeepers often only have to reach for the chemical club. However, combating mites with pesticides is problematic: residues of the active ingredients can end up in the honey and the mites are also increasingly developing resistance to the substances. Similar to crop protection, bee protection is therefore also looking for the most gentle and sustainable alternatives possible. One approach is the development of bee breeding lines that have increased resistance to parasite infestation. International beekeepers and various organizations have been dedicated to this goal for years and have already had considerable success.

“Pol-line honey bees” in the test

Now a team of researchers reports new test results of a varroa mite-resistant breeding line developed over the past 20 years by the US Department of Agriculture’s Agricultural Research Service. The resilience of the so-called pole-line honey bees has now been extensively tested for the first time in commercial beehives that are intended to provide pollination services and produce honey. Above all, the ability of the insects to survive the winter despite the burden of the mites was examined. The experiments took place with pole-line colonies and conventional control bees in the US states of Mississippi, California and North Dakota.

As the scientists report, the superiority of the Pol-line honey bees was clearly evident: According to the results of the investigation, the colonies of the breeding line were significantly less affected by the parasites than the controls and this was also reflected in the winter survival rate: in the Mite-resistant bees were, on average, more than twice as likely to survive the winter as standard honey bees, the researchers report. Even if chemical control measures were carried out against the mites, there was still a clear advantage, according to the evaluations. “This ability to achieve high colony survival rates with reduced or no varroa treatment can save beekeepers a great deal of money and time,” says co-author molecular biologist Michael Simone-Finstrom of the Genetics and Physiology Laboratory in Baton Rouge.

Increased hygiene behavior

As further investigations showed, the varroa mite resistance of the pol-line bees is also reflected in a lower burden of the viral pathogens transmitted by the parasites. It was confirmed: “If you control the mites, you automatically control the viruses that they transmit,” says lead author Thomas O’Shea-Wheller, who now works at the University of Exeter. According to the researchers, the successful results make it clear that the many years of breeding efforts have paid off and should now be continued consistently.

As they explain, the resistance effect of the pol-line bees is based on the behavior of the so-called Varroa-sensitive hygiene (VSH): If nurse bees perceive mites when caring for the brood, they sometimes eliminate infested larvae and thus ensure less pollution with the parasites in the hive. Naturally, however, the behavior is not pronounced enough. However, it is based on genetic predispositions, so that the tendency towards VSH can be increased through selection. In the course of the last 20 years, the breeding line of the Pol-line bees was created through targeted crossings. Care was also taken to ensure that, in addition to their increased resistance to mites, they retained their honey yield performance.

“This type of resistance offers a natural and sustainable solution to the Varroa mite threat. It doesn’t rely on chemicals or human intervention,” says O’Shea-Wheller. “Beekeeping and testing the lines are expensive and time-consuming. But breeding mite-resistant bees is cost-effective in the long term and probably the only sustainable solution to combating the Varroa pandemic,” the scientist is convinced.

Source: US Department of Agriculture – Agricultural Research Service, University of Exeter, Article: Scientific Reports, doi: 10.1038/s41598-022-08643-w

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