After the great death of bees in the early 2000s, the number of honey bees in Germany slowly but surely increased again. In 2007 there were only 670,000 bee colonies in Germany, today there are almost a million again.
But now scientists are warning of a new threat: a variant of the deadly deformed wing virus (DWV). According to the results of their study, recently published in the journal International Journal for Parasitology: Parasites and Wildlife appeared, the mutated pathogen is spreading rapidly worldwide. “The new mutant DWV-B is more deadly and possibly also easier to transmit than the original pathogen,” says Robert Paxton, a zoologist at the Martin Luther University in Halle-Wittenberg. “Infected animals usually die after 10 to 14 days.”
How the virus kills the bees is not entirely clear. In any case, the deformed wings typical of the disease are not the main problem. In any case, they only occur in animals that have already been infected as pupae and then hatch with defective wings due to the infection. But even adult bees, which you can’t tell from the outside, can become infected with the pathogen and die from it. “Sick animals move much less and can no longer perform their tasks in the hive as well,” says Paxton.
It takes two to three years for the virus to wipe out the entire population. “Most of the infected colonies then die in the winter,” says Paxton. There is still no drug against the virus itself. But against its carrier, the varroa mite. The parasites, which are about one millimeter long and one and a half millimeters wide, suck the blood, more precisely the hemolymph, of the bees and transmit the deformed wing virus in the process. “It’s a bit like mosquitoes and the pathogen that causes malaria,” says Paxton. The bloodsuckers themselves are unpleasant, but not deadly. What is really dangerous are the pathogens that they transmit.
The new virus variant is now present on almost all large land masses
According to the authors of the study, there is evidence that the new mutant DWV-B, unlike the original virus, is not only transmitted by the varroa mite, but that it also multiplies in the parasite. “This could represent an additional mechanism that increases the infection rate in DWV-B compared to DWV-A,” the scientists suspect in their study.
The B variant of deformed wing virus was first detected in 2001 in the Netherlands. 16 percent of their genome is changed compared to the A variant. These changes seem to give the virus tremendous benefits. This is the only way to explain why the aggressive DWV-B variant spreads so quickly and suppresses the original virus.
To find out how widespread the new variant is, Paxton and his team evaluated genetic data and publications on DWV viruses that were found in honey bees worldwide between 2008 and 2021. In addition, the researchers in Germany, Great Britain and Italy took samples themselves. Their result: “The new virus variant is now present on all major land masses on earth except Australia.” In the 2000s, DWV-B spread mainly in Europe and Africa. It has also been rampant in North and South America since 2010 and has been detectable in Asia since 2015. “In Germany there is now only the B variant,” says Paxton. You have completely replaced the A variant.
Wild bees become infected from flowers that an infected honey bee has previously visited
What does this mean for beekeepers in Germany? “They have to pay more attention to hygiene in the hive than before to prevent an infestation with the varroa mite,” says Paxton. Because Varroa mites infected with the deformed wing virus transmit the pathogen to the bees with almost a hundred percent probability. If a colony is already infested with mites, it should be treated with special insecticides that will kill the mites but not the bees – twice a year if necessary.
In this way, beekeepers not only protect their own bee colonies, but also wild bees, of which there are around 580 different species in Germany. Wild bees are at least as important pollinators as the honey bee, and unlike them, many species are in fact threatened with extinction. Some species, such as the common bumblebee, are known to be able to get deformed wing virus. Laboratory studies have shown that infected bumblebees also die earlier than healthy ones.
“The wild bees become infected when they visit flowers where an infected honey bee has previously collected and left excrement,” says Paxton. So if it is possible to keep the incidence of the new virus variant in the honey bee colonies low, the risk of wild bees becoming infected also decreases. Fortunately, transmission of honey bees to their wild relatives by varroa mites is rare or even impossible, since these parasites primarily affect honey bees.
But wild bees are more threatened by many other dangers than the honey bee, which humans take care of in an emergency. One of the main reasons for the sharp decline in many wild bee species is intensive agriculture. On the one hand because of the many insecticides and herbicides, the effects of which are never limited to those organisms that farmers want to control. Just recently, an investigation in the science journal Science showed that the pesticide glyphosate also harms bumblebees in a roundabout way. Perhaps the biggest problem for wild bees is habitat loss. The intensively cultivated fields are like a green desert for the insects, where they simply cannot find anything to eat.