Bio-pesticides: Harmful or harmless to ammonia oxidizing microorganisms? The case of a Paecilomyces lilacinus-based nematicide

Bio-pesticides are considered as low-risk compounds, a belief mainly based on their natural origin rather than on experimental evidence. Thus there is a need to explore the ecotoxicity of bio-pesticides and mostly their impact on soil microbes which is largely unknown. The effect of Quillajia saponaria plant extract (QLAgri®) and Paecilomyces lilacinus strain 251 (PL251, BIOACT®) on the microbial community was investigated comparatively to the synthetic nematicides oxamyl and fluensulfone. Particular attention was given to potential effects on ammonia-oxidizing bacteria (AOB) and archaea (AOA). No effect of QLAgri®, oxamyl and fluensulfone on the soil microbial community and AOB/AOA was observed. In contrast BIOACT® stimulated the growth of copiotrophic Gram negative bacteria and fungi as determined by phospholipid fatty acids (PLFA) analysis. Terminal restriction fragments length polymorphism (TRFLP) and qPCR analysis of the amoA gene showed a significant time-dependent inhibitory effect of BIOACT® on the abundance of AOB/AOA up to 20 days post application. Further qPCR analysis indicated that PL251 did not proliferate in soil. These results suggested the establishment of complex interactions between BIOACT® and AOB/AOA which were further explored. In a follow-up study BIOACT® and its co-formulants, glucose (78%) and skimmed milk powder (6%), both induced a transient inhibitory effect on potential nitrification and abundance of AOB/AOA, whereas no effect was seen when PL251 spores were used. Overall, our data suggest that the transient effect of BIOACT® on nitrifiers was the result of a competitive exclusion by copiotrophic microorganisms feeding on co-formulants rather than a direct toxicity effect. •Soil microbial ecotoxicity of bio-pesticides is largely unknown.•Soil microbial ecotoxicity of selected bio- vs synthetic pesticides was studied.•BIOACT®, a Paecilomyces lilacinus-based pesticide reduced abundance of nitrifying microbes.•Co-formulants of BIOACT® (glucose) stimulated copiotrophic exclusion of nitrifiers.•More thorough investigation of soil microbial toxicity of bio-pesticides is needed.