Lithium chloride leads to concentration dependent brood damages in honey bee hives (Apis mellifera) during control of the mite Varroa destructor

Abstract

Lithium chloride (LiCl) has a high efficacy against Varroa destructor and a good tolerability for adult bees but the effect of LiCl on the honey bee brood has not been taken into consideration yet. We quantified the mortality of larvae fed with different concentrations of LiCl. For artificially reared larvae already, a concentration of 1 mM had significant toxic effects while under colony conditions, 10 mM was well tolerated. However, a chronic application of the effective concentration of 25 mM elicited brood mortalities between 60 and 90%. Shorter feeding periods of 2 or 4 days reduced the brood damages significantly. Measurements of the lithium concentrations in larvae and pupae during a chronic exposure with 10, 17.5 and 25 mM LiCl revealed respective lithium levels in 5th instar larvae of 7, 13 and 15 mg/kg. No lithium was detectable in 2-day old larvae indicating that pure worker jelly from the hypopharyngeal gland is not contaminated with LiCl. Based on these results, applications of LiCl in colonies with brood should be avoided.