List of volatile organic compounds found for each variety of hops, MIC and MBC average values for all the extracts with each strain of P.larvae tested and Survival curves at 24, 48 and 72 hours of adult bees fed with three increasing concentrations of Bullion (B), Nugget (N), Victoria (V) and Cascade hop extracts

Abstract

Paenibacillus larvae is the causative agent of American Foulbrood (AFB), a devastating disease that affects honey bee larvae. Plant-based antimicrobials that do not harm bees or hive products represent a natural alternative, particularly for diseases related to food provision such as AFB. This study aimed to evaluate the antibacterial properties of Humulus lupulus extracts against P. larvae and assess their toxicity on both honey bee larvae and adults. We investigated the antibacterial activity of hop extracts (Humulus lupulus L.) against P. larvae and analyzed their total polyphenol content, flavonoid levels, antioxidant activity, and volatile organic compounds across 24 Soxhlet-extracted samples. The extracts obtained using dichloromethane as a solvent exhibited the highest antimicrobial activity and showed no toxicity to adult bees or A. mellifera larvae. In these extracts, we identified a diverse array of volatile compounds with antibacterial properties, whose synergistic effects likely contributed to the enhanced antimicrobial efficacy observed.