Effect of the Propolis Components, Cinnamic Acid and Pinocembrin, on Apis Mellifera and Ascosphaera Apis

The value of propolis as a disinfectant agent in the bee hive is considerable. We investigated the fungistatic effect of cinnamic acid and pinocembrin on the pathogen Ascosphaera apis causing chalkbrood disease in bee colonies in the laboratory with an agar diffusion test on malt extract agar. The minimal fungicide inhibitory concentration was 0.5% for cinnamic acid and 1.5% for pinocembrin (MIC -values). Both substances were tested by individual application for adult bee tolerance in a range of dosages and for brood tolerance (L1 - L5) referring to the MIC values. The dermal toxicity of adult bees for cinnamic acid and pinocembrin was low, a maximum of 3.3% even in the highest dosage of 1000 μg (concentration 20%) and 250 μg (concentration 5%) per bee, respectively. The mortality rates showed no significant differences to the controls (z-test, p≤0.05). First results on brood tolerability after application of test substances resulted in larval mortality but primarily in the group of young larvae. The mortality reduced simultaneous with the increasing larval weight. The total larval mortality 72 hours after the application over all stages was 28.7% for cinnamic acid and 13.1% for pinocembrin (positive control group 24.8%; negative control group 15.4%). The test groups showed no significant increased mortality compared to the control groups (z-test, p≥0.05). Both substances have in vitro fungicide potential and can be considered as potential active agents against A. apis in honey bee drugs. The results provide high tolerance of adult bees and an indication for larval tolerability most notably in elder larvae.