Fungal infection instead of poison club: Researchers have bred a fungal pathogen that can effectively kill varroa mites and is therefore suitable for use as a gentle bee protection agent. Using special breeding methods, the scientists have succeeded in making the previously sensitive fungus resistant to the high temperatures in the beehive. In the future, the pathogen could replace the problematic chemicals in the fight against the bee parasites, the first test successes give reason to hope.
The mite Varroa destructor is an important contributor to the so-called bee mortality: In addition to lack of food, pesticides and pathogens, these parasites have caused great damage to bee colonies all over the world in the last few decades. The mites weaken the honeybees by draining body fluids from both the adult animals and their larvae. They are also considered to be carriers of viral pathogens. So far, beekeepers have usually tried to get rid of the stubborn parasites with the “poison club”. But these chemical control measures are problematic: They weaken the bees further and residues of the active substances can accumulate in the honey. In addition, the mites have increasingly developed resistance to so-called miticides. Similar to crop protection, we are therefore looking for alternatives that are as gentle and sustainable as possible in bee protection.
Heat-sensitive mite enemies
The targeted use of natural enemies of harmful mites comes into question. Among other things, the representatives of the metarhician fungi came into focus. These are so-called entomopathogenic fungi, which with their mold-like network can penetrate and kill insects or mites. These pathogens have already proven to be effective in the fight against some harmful insects. In principle it was already known that the fungus Metarhizium brunneum also infects the Varroa mites and can kill them. Bees, on the other hand, are completely immune to this pathogen. Thus, this fungus initially seemed to be an ideal biological remedy against the parasites. So far, however, there was a catch that prevented effective use: “We knew that metarhicium can kill mites, but unfortunately these fungi cannot tolerate the heat of 35 degrees Celsius that typically prevails in the beehive,” says Steve Sheppard of Washington State University at Pullman.
Apparently, M. brunneum had not yet adapted to a targeted infestation of the bee parasites. In order to close this gap, Sheppard and his colleagues have now helped evolution a little. To do this, they specifically propagated those lines of M. brunneum in the laboratory that corresponded to the desired properties – aggressiveness against d
en mites and heat tolerance. In addition, the process of so-called directed evolution was used, which can produce a particularly large number of mutants with changed properties by means of random mutagenesis and subsequent selection. For this purpose, the fungi were specifically exposed to substances in the laboratory that lead to more genetic diversity without genetic engineering and thus in a more natural way.
Promising breeding line
The result of the breeding process was ultimately a strain that proved to be extremely promising in tests: Compared to the original lines, the cultivated form of the fungus thrives considerably better under beehive conditions, the scientists report. This was at the expense of the mites and thus served the well-being of the bees, as tests with colonies in the field confirmed. According to this, the new breeding strain of Metarhizium brunneum can eliminate the Varroa mites just as well as conventional treatment measures. In contrast to these, the biological control strategy is not associated with the problematic side effects of the chemical substances.
The next step now is to apply for permits from environmental protection agencies to use metarhicon on beehives, the scientists say. The team will also work on simplified administration methods so that beekeepers can treat their beehives as practically as possible. “We hope that in ten years’ time Metarhizium will have largely replaced chemical mite control agents in order to control the Varroa mites,” says Sheppard, “and that the problem will finally lose its horror with it”.
Source: Washington State University, Article: Scientific Reports, doi: 10.1038 / s41598-021-89811-2