Varroa destructor weakens the external immunity of western honey bees by impairing melittin production.

Social insects employ venom as an external immune defence against pathogens and parasites. Like other Hymenopterans, the venom gland of honey bee (Apis mellifera) serves as a reservoir of antimicrobial substances, primarily melittin. This study investigated the presence and origin of venom on honey bee body that could act as an external immune defence in honey bee workers infested by the ectoparasite mite, 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 could facilitate the distribution of venom on the bee's body through behavioural observations. Further assays were used to compare melittin levels on the bodies of Varroa-free and Varroa-infested workers and assess the effects of bee-venom on mite activity. Our findings confirmed the occurrence of "venom bathing" in A. mellifera, excluding social components or environmental contamination, with bees likely coating their bodies with antimicrobial substances through self-grooming. Our results further suggest that infested bees spread higher amount of venom on their bodies compared to uninfested bees, and bee-venom significantly reduced mite activity, suggesting that the venom functions as an external defence. However, Varroa negatively impacted melittin production. Our study reveals a previously unknown negative effect of V. destructor: impairment of honey bees' external immune defence through reduced melittin production.