Varroa destructor weakens honey bee external immunity by impairing melittin production

Social insects employ venom as an external immune defence against pathogens and parasites. Like other Hymenoptera, the venom gland of honey bee serves as a reservoir of antimicrobial substances, primarily melittin. This study investigates the role of venom associated with grooming behaviour as an external immune defence in Apis mellifera workers infested by Varroa destructor . Using a multi-step approach, we first confirmed the presence of venom on bees' bodies using melittin as a marker. We then examined how grooming facilitates the distribution of venom on the bee's body. Further assays compared melittin levels on the bodies of Varroa -free and Varroa -infested workers and assessed the effects of bee-venom on mite activity. Our findings confirmed the occurrence of "venom bathing" in A. mellifera , whereby bees coat their bodies with antimicrobial substances through selfgrooming. excluding social components or environmental contamination. Infested bees spread larger amounts of venom on their bodies compared to uninfested bees and bee-venom significantly also reduced mite activity, suggesting venom functions as an external defence. However, Varroa negatively impacts melittin production. Our study reveals a previously unknown negative effect of V. destructor : impairment of honey bees' external immune defence through reduced melittin production.