From consumption to excretion: Lithium concentrations in honey bees (Apis mellifera) after lithium chloride application and time-dependent effects on Varroa destructor.

Background: Owing to its systemic mode-of-action and ease of application, lithium chloride (LiCl) is an ideal varroacide for the control of Varroa destructor infestations in honey bee colonies. To better understand how LiCl functions within a colony, we screened different parts of honey bee anatomy for lithium accumulation. We wanted to elucidate the time-dependent effects of LiCl on V. destructor and its metabolism within honey bees when they were fed continuous LiCl treatments, as well as evaluate potential adverse effects such as accumulation in the hypopharyngeal glands of nurse bees, which could negatively impact queens and larvae.

Results: Cage experiments reveal rapid acaricidal onset, with >95% mite mortality within 48 h of treatment. Bee hemolymph analysis supports these observations, showing a rapid increase in lithium concentration within 12 h of treatment, followed by stabilization at a constant level. Lithium accumulates in the rectum of caged bees (≤475.5 mg kg after 7 days of feeding 50 mm LiCl), reflecting the bees' metabolic and excretion process. Despite concerns about potential accumulation in hypopharyngeal glands, low lithium levels of only 0.52 mg kg suggest minimal risk to the queen and 1- and 2-instar larvae. Cessation of LiCl treatment results in a rapid decline in mite mortality in the first 5 days, which increases again thereafter, resulting in mite mortality of 77-90% after 10 days.

Conclusion: These findings help optimize LiCl application in colonies to achieve high Varroa mortality without unwanted adverse effects and provide important baseline data for future registration. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.