Lactic acid impairs Varroa destructor grip skill: fitness costs and effects on behaviour under artificial conditions

Varroa destructor is one of the major parasites threatening the Western honeybee Apis mellifera if not managed. Yet, sustainable treatments are rare due to the difficulty to impact the mite without disturbing the host. One current soft acaricide used by beekeepers is lactic acid, although its way of action on the parasite remains poorly understood. We undertook the first investigation about lactic acid on V. destructor at the individual level under artificial conditions. We found that lactic acid does not kill V. destructor but rather interferes with its attachment skills. In particular we focused on its arolia, mite’s soft pads, and highlighted shape and fluid modifications induced by lactic acid. We also analysed fitness costs and behavioural effects of impaired attachment due to lactic acid on mites. We observed a reduced locomotion as well as a potential lower reproduction but no impact on nutrition. Even if conducted under artificial conditions, these results significantly improve our understanding of lactic acid effect on the parasite’s biological abilities. They also provide new perspectives to apply differently this organic acid in the field against V. destructor.