Antimicrobial activity of plant extracts against the honeybee pathogens, Paenibacillus larvae and Ascosphaera apis and their topical toxicity to Apis mellifera adults.

AIMS: To explore alternative nonchemical control measures against two honeybee pathogens, Paenibacillus larvae and Ascosphaera apis, 37 plant species were screened for antimicrobial activity. METHODS AND RESULTS: The activity of selected plant extracts was screened using an in vitro disc diffusion assay and the minimal inhibitory concentration (MIC) was determined by the broth microdilution method. The results showed that 36 plant extracts had some antibacterial activity on P. larvae by disc diffusion assay. Chromolaena odorata showed the greatest antibacterial activity against P. larvae (MIC 16-64 µg ml-1 ). Of the 37 tested plants, only seven species, Amomum krervanh, Allium sativum, Cinnamomum sp., Piper betle, Piper ribesioides, Piper sarmentosum and Syzygium aromaticum had inhibitory effects on A. apis (MICs of 32-64 µg ml-1 ). The results demonstrated that promising plant extracts were not toxic to adult bees at the concentrations used in this study. CONCLUSIONS: The results demonstrate the potential antimicrobial activity of natural products against honeybee diseases caused by P. larvae and A. apis. Chromolaena odorata in particular showed high bioactivity against P. larvae. Further study is recommended to develop these nonchemical treatments against American foulbrood and chalkbrood in honeybees. SIGNIFICANCE AND IMPACT OF THE STUDY: This work proposes new natural products for the control of American foulbrood and chalkbrood in honeybees.