Natural extracts as potential control agents for Nosema ceranae infection in honeybees, Apis mellifera

• Nine of twelve plant extracts had high antiparasitic activity against N. ceranae. • Crude extracts inhibited the development of Nosema spores under field conditions. • Extracts did not show negative affects on bees under whole colony conditions. Nosema disease is one factor that can cause colony decline in honeybees ( Apis mellifera L.) worldwide. Nosema ceranae has outcompeted Nosema apis in the Western honeybee ( A. mellifera ) which is its original host. Fumagilin is an effective antibiotic treatment to control Nosema infection but currently it is forbidden in many countries. In this study, 12 plant extracts were evaluated for their toxicity to adult bees and antimicrosporidian activity under laboratory and field conditions. N. ceranae -infected adult bees were fed ad libitum with 50% sucrose solution containing 1% and 5% (w/v) of each plant extract. Bee mortality in N. ceranae -infected groups fed with plant extracts was higher than that in the control group treated with fumagilin. The results demonstrated that 9 of 12 extracts had high antimicrosporidian activity against N. ceranae and their efficacies were comparable to fumagilin. Spore reduction in infected bees was 4–6 fold less after extract treatment. Following laboratory screening, Annona squamosa, Ocimum basilicum, Psidium guajava and Syzygium jambos were tested in honeybee colonies. Plant extracts of 2% concentration (w/v) inhibited the development of Nosema spores after 30 days of treatment. At the end of experiment (90 days), spores in the plant extract treated groups were lower than in group treated with fumagilin but there was no significant difference. Although, extracts tested in this study showed high toxicity to bee in laboratory cages, they did not show negative affects on bees under whole colony conditions. Therefore, the effectiveness of plant extracts tested in this study was notable and warrants further study as potential Nosema control agents in honey bees. Plant extracts would offer a non-antibiotic alternative for Nosema control and help reduce the overuse of antibiotics in livestock.