The symbiotic bacteria Frischella perrara in honey bees mitigate varroa mite infection

Honey bees are vulnerable to various pathogens and pests, and ectoparasitic Varroa destructor has become the main cause of population decline of western honey bees (Apis mellifera) worldwide. Currently, chemical acaricides are the primary methods for controlling Varroa mites (V. destructor), and developing non-chemical control technique is urgently needed for beekeeping industry. We aimed to evaluate if the symbiont gut bacteria from bee colonies were attributed to Varroa tolerance and could be developed as a control agent. We sequenced 16S rDNA of gut microbiota between surviving colonies at a non-acaricide treated apiary (i.e., anti-mite colonies, AMCs) and conventional mite-susceptibility colonies (CMSCs) at an apiary with routine acaricide treatment. We found that AMCs harbor higher abundance of Frischella. By supplementing CMSCs with F. perrara in the field, we found decreased Varroa infestation intensity with more fallen phoretic Varroa and more injured pupae to be removed (an indicator of hygienic behavior). By supplementing honey bees with F. perrara in laboratory, we found increased proboscis extension reflex frequency and expression of odor-binding proteins in their antennae (OBP 11, 12, 16, and 21), indicating a potential molecular mechanism of enhanced hygienic behavior. This study suggests a novel function of the gut symbiont F. perrara in mitigating Varroa parasitism, implying its potential to serve as a new method to control Varroa mites through enhancing bees' olfactory sensitivity.

The parasitic mite of honey bees, Varroa destructor, is the major challenge of beekeeping industry across the world. We propose a practical control method, applying a gut symbiont bacterium (F. perrara) to bee colonies to address this global challenge without the side effects of using chemical treatment methods. We found F. perrara treatment can lead to a higher number of fallen mites from adult bees' body. The potential mechanism is to improve honey bees' hygienic behavior to clean mites through promoting their olfactory sensitivity, increasing the chances to detect mites. Varroa control by administrating F. perrara does not harm honey bee health and avoid product contamination and pesticide resistance to mites. This method may provide a sustainable Varroa control tool in realistic beekeeping industry.