Two strains of Pseudomonas fluorescens bacteria differentially affect survivorship of waxworm ( Galleria mellonella ) larvae exposed to an arthropod fungal pathogen, Beauveria bassiana

Two strains of Pseudomonas fluorescens were found contaminating a biopesticide used in a previous study against Varroa destructor infestations in honey bee hives. 15 In that study, the biopesticide, a formulation of a fungal pathogen of arthropods, Beauveria bassiana, failed to have any negative impact on the mite infestation despite successful results in previous studies using uncontaminated batches of the same biopesticide. The objective of the present research was to determine whether the bacteria may have interfered with the infectivity and/or virulence of B. 20 bassiana in a simplified system; positive results in that system would then provide a rationale for further work under more complex conditions. Galleria mellonella late instar larvae treated topically with both a bacterial suspension of 6.8 to 7.0 107 cfu/ml and a fungal suspension of 2.5107 or 2.5108 B. bassiana conidia/ ml showed, in the case of one of the bacterial strains, significantly increased 25 survivorship compared to larvae treated with just the B. bassiana suspension. When larvae were immersed in a bacterial suspension prior to application of B. bassiana suspension using a spray tower, a significant positive effect of the same P. fluorescens strain on larval survivorship was observed at 2.5108 conidia/ml. Neither the bacterial suspensions alone nor blank control solutions had any effect 30 on larval survivorship. These results show that an interaction between the bacteria and the pathogen may explain some of the results from the prior field trial.