Larval Apis mellifera L. (Hymenoptera: Apidae) mortality after topical application of antibiotics and dusts

Beekeepers apply various dusts to honey bee, Apis mellifera L., colonies to dislodge parasitic mites and control bacterial brood diseases. Anecdotal reports by beekeepers indicate that the antibiotic oxytetracycline (OTC) can be toxic when applied in powdered sugar to cells containing immature bee brood, but it was not known whether the purported toxicity is caused by the antibiotic or the sugar carrier. Additionally, the toxicity of various dusts, proposed for parasitic mite control, is poorly known. In the current studies, we tested OTC and two other antibiotics (tylosin and lincomycin, candidate compounds for use in honey bee colonies) in a powdered sugar carrier for larval toxicity. We also tested for larval toxicity, several dusts that have been proposed for mite control. OTC caused significant brood mortality of approximately 80% at the concentrations used in the hive (200 mg in 20 g sugar). In contrast, tylosin and lincomycin at the 200 mg dose were both similar to untreated controls, and only five times that concentration (1000 mg) caused significant brood mortality of approximately 65%. The addition of dusts, wheat flour, talc, and a commercially available protein supplement, BeePro, resulted in mortality levels between 65 and 80%, similar to that seen with OTC. The common antibiotic carrier, powered confectioners sugar, was nontoxic. The use of 100 unsealed brood cells was demonstrated to be a reliable means of assessing potential adverse affects of dry compounds on larval honey bees. Two new candidate antibiotics for use in honey bee colonies were less toxic to larval bees than the currently labeled antibiotic, OTC.