Advances in the apiary control of the honeybee American Foulbrood with Cinnamon (Cinnamomum zeylanicum) essential oil

The activity of Cinnamomum zeylanicum Nees essential oil (cinnamon oil) against Paenibacillus larvae (Withe) was evaluated in the laboratory and in a field experiment in order to improve the biological control of the Apis mellifera L. disease American foulbrood (AFB). The MICs (Minimal Inhibitory Concentration) against P. larvae were determined by the tube dilution method. Bee lethality was estimated using cages with approximately 374 bees, fed with the essential oil at different concentrations diluted in ethanolic syrup. The security index was calculated as LD50 of A. mellifera/MIC of P. larvae strain at 24, 48 and 72 h in both tests. The apiary trial, using three groups of five hives each, was carried out in April-May 2006 in an experimental apiary of J. J. Nágera coastal station (Mar del Plata, Argentine). The first group was treated with cinnamon oil (two doses of 1000 μg/ml per hive), the second with oxytracycline-HCl (three doses of 0.4 g per hive) and the third was left untreated as a control. All of the treatments were performed at 7-day intervals. The evaluation of treatment efficacy was made by counting the number of infected brood cells in both sides of a central comb, using a brood surface of 360 cm (18 x 20 cm). For the Mar del Plata AFB strain the C. zeylanicum essential oil and antibiotic MICs were 50 μg/ml and 3.125 μg/ml, respectively. Regarding the systemic administration method, bees LD50 at 24 h was 456.07 μg of essential oil of cinnamon/bee. LD50 estimated at 48 h showed a slight decrease with respect to that recorded at 24 h. Security index was 9.1214 (ml/bee) for 24 h. In relation with field trials, after 24 and 31 days from the beginning of treatments the cinnamon oil-treated hives showed a lesser incidence of infected larvae (7.89% and 52.42%) than the control group highlighting a clear efficient control. No significant differences between the two treated groups were recorded. These results represent a further proof of the potential of cinnamon oil to control American Foulbrood with only minor toxicological risks to bees. Furthermore, the use of cinnamon oil avoids problems with antibiotic residues in honey. This is important in marketing honey in the EU where antibiotics in honey are generally forbidden by law.