The parasitic mite Varroa destructor remains the greatest threat to honey bee health. Once established in a country, beekeepers must typically treat against Varroa to keep the colony alive. New, highly effective and easy to apply varroacides with limited unwanted side effects are thus highly desired. Lithium chloride (LiCl) has shown excellent potential as a new treatment agent, as feeding this compound to bees in syrup or candy results in high Varroa mortality yet is easy and safe to use with no pesticide residues in honey. We investigated the onset of the toxic effect in cage trials using individual bees parasitized by single mites. Within 24 hrs of starting the 50 mM LiCl feeding, 78% of the mites died, whereas within 48 hrs 95% mortality was achieved. A colony-level Summer treatment in brood-free colonies eliminated 77-98% of the mites, depending on the LiCl feeding duration. Unfortunately, lithium disturbs the development of the honey bee brood, leading to concentration dependent brood removal of up to 61%. Further investigations are needed to find an application method to minimize this brood loss. The combination of a brood break and treatment with LiCl remains an effective and practical control method.
