Efficacy of natural propolis extract in the control of American Foulbrood.

Paenibacillus larvae is the causative agent of American Foulbrood (AFB), a severe disease that affects larvae of the honeybees. Due to the serious effects associated with AFB and the problems related to the use of antibiotics, it is necessary to develop alternative strategies for the control of the disease. The aim of the present work was to evaluate the effect of a propolis ethanolic extract (PEE) against P. larvae and its potential for the control of AFB. In vitro activity of PEE against P. larvae isolates was evaluated by the disk diffusion method and the minimum inhibitory concentration (MIC) was determined. Toxicity for honeybees was evaluated by oral administration of PEE and its lethal concentration was assessed. Lastly, colonies from an apiary with episodes of AFB on previous years were divided into different groups and treated with sugar syrup supplemented with PEE by aspersion (group one), sugar syrup by aspersion (group two), fed with sugar syrup supplemented with PEE (group three) and fed with sugar syrup only (group four). All isolates were sensitive to PEE and the MIC median was 0.52% (range 0.32-0.64). PEE was not toxic for bees at least at 50%. Field assays showed that 21 and 42 days after the application of the treatments, the number of P. larvae spores/g of honey was significantly lower in colonies treated with PEE compared to the colonies that were not treated with PEE. To our knowledge, this is the first report about the use of propolis for the treatment of beehives affected with P. larvae spores.