Directed Evolution of Metarhizium Fungus Improves its Biocontrol Efficacy Against Varroa Mites in Honey Bee Colonies

Thursday 1 July 2021

Development and testing of novel varroa mite controls is an ongoing process in the beekeeping industry. In the past, varroa mites have developed resistance to certain chemical treatment products. This emphasizes the need for better management of this honey bee pest, including the integration of various control measures beyond synthetic miticides. Reducing reliance on these synthetic chemicals reduces the risk of mites developing resistance to them, thereby prolonging the effective lifespan of these products in beekeeping. One novel varroa mite control product that is upcoming in development and testing is a fungus called Metarhizium. Keep reading below to learn more about how this fungus may be an effective biocontrol in the battle against varroa mites.

Directed Evolution of Metarhizium Fungus Improves its Biocontrol Efficacy Against Varroa Mites in Honey Bee Colonies *

A specific type of fungus (Metarhizium brunneum) that causes disease in varroa mites has been the focus of a study working on developing a biological control that is able to function effectively as a control for varroa mites under typical honey bee hive conditions. This fungus is likely to control varroa mites by adhering spores to the exoskeleton of the mite which then germinate and grow a long branching filament called a ‘hypha’ (collectively referred to as mycelium). The hypha penetrates the mite exoskeleton and proliferates in the mite’s internal tissues, effectively killing the mite. Initially, this fungus showed promise as a biocontrol for varroa mites but was unable to function effectively due to the high temperatures within honey bee hives. In response to this result, researchers created new strains of the Metarhizium brunneum fungus using techniques of traditional selection and directed evolution. This process involved collecting mites that were killed by the fungus and then growing and reproducing the fungus under conditions designed to increase its heat tolerance and overall effectiveness as a biocontrol of varroa mites in functioning hives. The resulting strains were able to survive, germinate, and grow in honey bee hive temperature conditions (35C) with efficacy of mite control greater than the original fungus strain and similar to currently used treatments.

There is an ongoing shift away from the use of synthetic chemical treatments in honey bee hives for reasons beyond the risk of pest and disease resistance development. Concerns over the use of these synthetic chemicals have been raised in the beekeeping industry in terms of their long-term impacts on honey bees and their potential to contaminate hive products intended for human use. Biological controls are an attractive option for pest control due to their typically high specificity in affecting only the intended target and low toxicity towards non-targets. This fungal biocontrol also exhibits minimal residual effects both in agricultural settings and in the natural environment. There are currently no biological controls available for use in Canadian beekeeping.

As a final note on the topic of upcoming honey bee pest control products, only treatment products that are registered for use in beekeeping in Canada are to be used in Canadian beekeeping practices.


* Han J. O. et al. (2021) Directed evolution of Metarhizium fungus improves its biocontrol efficacy against Varroa mites in honey bee colonies. Sci. Rep. 11, 10582. Full text available ONLINE.

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