Biological control: An effective fungus against Varroa, the bee killer

A new strain of Metarhizium brunneum, an entomopathogenic fungus found in soils around the world, effectively combats Varroa destructor—the devastating ectoparasite of honeybees—without the use of chemical acaricides. Unlike other strains of Metarhizium, this one can survive in a hot environment, such as in beehives where temperatures typically hover around 35 °C.   For decades now, honeybee populations have been in decline. As a central pillar of many ecosystems, the disappearance of pollinators is a cause for concern. One of the main culprits is believed to be the ectoparasite Varroa destructor. Far ahead of other factors such as pesticides, stress, and pathogens that decimate bee populations and insect populations in general, this mite is the cause most frequently identified by beekeepers facing the loss of their colonies. Varroa destructor feeds on adult and immature honeybees by piercing their exoskeletons with its sharp mouthparts and consuming their tissues through extraoral digestion. The bees, severely weakened, are less effective at foraging, more susceptible to infections, and die prematurely. Furthermore, Varroa-infected bees drift to neighboring hives, introducing the mite and associated viruses into uninfected colonies. This is particularly problematic for commercial beekeeping, where thousands of hives belonging to different beekeepers from different areas are seasonally concentrated in orchards and cultivated fields. Without treatment, colonies infected with Varroa have an average lifespan of one to three years. These parasites are therefore considered the greatest threat to beekeeping worldwide. Consequently, they endanger global crops that depend on pollination, which, economically speaking, is worth some $238 billion. Currently, beekeepers rely heavily on chemical acaricides to combat Varroa despite the danger these chemicals pose to bees, other insects, and ecosystems as a whole—not to mention the resistance developed by the parasite. Furthermore, interactions between different chemical acaricides can increase their toxicity to bees. In search of an alternative solution, researchers explored the development of a strain of the fungus Metarhizium specifically designed to control Varroa. Indeed, the mitospores of this entomopathogenic fungus have proven effective at eliminating the mite within apiaries where it proliferates. After the mitospores adhere to and germinate on the mite, the hyphae penetrate its exoskeleton and proliferate within its internal tissues, killing it from the inside. Since bees have high immunity to the spores, using these fungi is a safe and natural way to combat this devastating pest. However, this treatment is often expensive, and the fungi do not survive long in beehives, which are too hot for them. In fact, average temperatures there generally hover around 35 °C. However, through directed evolutionary selection, the research team has succeeded in creating a new strain of Metarhizium that is more heat-resistant. Metarhizium brunneum can not only survive higher temperatures but has also been selected for its virulence against Varroa. The researchers hope that the fungus will be approved by the Environmental Protection Agency as a biological control agent so that, within the next ten years, this natural acaricide will largely replace chemical products and the Varroa parasite will be nothing more than a bad memory.