Non-linear effects of Fontelis 20 SC (fungicide) on gut microbiota relate to changes in reproduction of Folsomia candida (Collembola)

Soils face significant pressure from agriculture, often involving pesticides. Especially, fungicides can substantially harm fungal-feeding soil organisms. One of them, Fontelis 20 SC (penthiopyrad) is globally used, yet little is known about its toxicity, especially to soil animals. Using Folsomia candida (Collembola) as a model organism, we investigated (1) the effects of Fontelis 20 SC on F. candida 's life-history (juvenile number, relative growth, compensatory growth, egg number, egg volume, hatching success, reproduction investment, growth-reproduction-trade-off, and compensatory growth-reproduction trade-off), (2) their recovery in a Fontelis-free environment, (3) the impact on gut bacteria during and after treatment, and (4) whether changes in microbiota can be related to changes in reproduction (juvenile number, egg number, egg volume, hatching success, reproduction investment). The effects of the fungicide treatment on the life-history and microbiota composition were mostly non-linear; with fungicide treatment groups often exceeding the control values at intermediate concentrations, but returning to the control level at higher concentrations. However, microbiota alpha-diversity in the fungicide-free (control) environment was higher than in all other concentrations. The reproduction (egg number, egg volume, hatching success, reproduction investment) of the animals were able to recover or even overshoot the control group, except for the highest concentration. While microbe abundance, Vagococcacea, and Caulobacteraceae were negatively related to reproduction parameters, the opposite was true for Saccharimonadales and Enterococcacea. We conclude that microbiota may drive the non-linear effects on reproduction in Collembola, but further experiments with microbiota manipulation are needed to confirm causality.