Strong against bloodsuckers by breeding

Hope for plague beneficial insects: the resistance of bee colonies to Varroa mites can be greatly enhanced through selection, as shown by the success of the breeding line. Scientists report that so-called “polar line honey bees” are more than twice as likely to survive the winter as standard honey bees because of their immunity. The success is based on the increased hygienic behavior of insects, in which they remove the offspring attacked by parasites.

Our valuable honey producers and pollinators are doing poorly: many bee colonies are weakened and loss rates are high. According to studies, a mix of different causes is responsible: lack of food, pesticides and various pathogens make it difficult for insects to live – but their biggest problem is the persistent parasite. Over the past 50 years, the Varroa mites from Southeast Asia have mainly spread to Europe and America. Parasites weaken honey bees by sucking body fluids from both adults and larvae. They also carry viral pathogens that cause additional problems for the bees. First of all, parasite infestation reduces the colony’s ability to survive the winter.

To avoid total failure, beekeepers often only have to reach for a chemical club. However, controlling mites with pesticides is problematic: residues of the active ingredients can be found in the honey, and the mites are also becoming more resistant to these substances. As with plant protection, bee protection also looks for the most gentle and sustainable alternatives possible. One approach is to develop bee breeding lines that have increased resistance to parasite infestation. International beekeepers and organizations have dedicated themselves to this goal for years and have already had considerable success.

“Honeybees from the Pol line” in the test

Now, a team of scientists is reporting on new test results for a Varroa mite-resistant breeding line developed over the past 20 years by the Agricultural Research Department of the US Department of Agriculture. The immunity of the so-called polar line honey bees was first extensively tested in commercial hives to provide pollination services and produce honey. First of all, the ability of insects to survive winter despite being loaded with mites was investigated. The experiments were conducted on polar line colonies and conventional control bees in the US states of Mississippi, California and North Dakota.

According to the scientists, the superiority of the honey bees from the Pol line was clearly clear: according to the results of the research, the colonies of the breeding line were much less affected by parasites than the control animals, which was also reflected in the winter period. Survival rate: Mite-resistant bees were on average more than twice as likely to survive winter as standard honey bees, the researchers said. Even when chemical mite control measures were carried out, there was still a clear advantage according to estimates. “This ability to achieve high colony survival rates with little or no treatment for varroa can save beekeepers a lot of money and time,” says co-author molecular biologist Michael Simone-Finstrom of the Baton Rouge Genetics and Physiology Laboratory.

Increased hygienic behavior

As further studies have shown, resistance to pol-line bee varroa mites also translates into a lower burden of viral pathogens transmitted by parasites. Confirmed: “If you control mites, you automatically control the viruses they carry,” says lead author Thomas O’Shea-Wheller, who now works at the University of Exeter. According to the researchers, the successful results clearly show that many years of breeding efforts have paid off and should now be consistently continued.

They explain that the immune action of the Pol bees is based on the so-called Varroa-sensitive hygiene (VSH): if the nurse bees spot mites while caring for brood, they sometimes eliminate infected larvae and thus ensure a lower parasite load in the hive. Of course, this behavior is not clear enough. However, it is based on a genetic predisposition, so that the tendency to VSH can be increased by selection. In the last 20 years, the Pol-line bee breeding line has been created through targeted crosses. It was also ensured that in addition to increased mite resistance, they retained their honey yield.

“This type of immunity offers a natural and permanent solution to the threat of Varroa mites. It doesn’t rely on chemicals or human intervention, ”says O’Shea-Wheller. “Beekeeping and line testing are expensive and time consuming. However, keeping bees resistant to mites is profitable in the long run and is probably the only sustainable solution to fight the Varroa pandemic, ‘the scientist believes.

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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