# Synergistic effect of the next generation insecticide flupyradifurone with a fungal pathogen in the ant Lasius niger

## TL;DR

A new insecticide, flupyradifurone, increases ant mortality and makes them more vulnerable to a fungus, raising concerns about its environmental impact.

## Contribution

This study reveals a synergistic interaction between a novel insecticide and a fungal pathogen in ants, showing sublethal pesticide effects can increase pathogen susceptibility.

## Key findings

- Chronic exposure to flupyradifurone (≥100 ppm) significantly increased ant worker mortality.
- Sublethal flupyradifurone exposure (5-50 ppm) made ants more susceptible to fungal infection, with mortality hazard ratios more than doubling.
- The study demonstrates an indirect sublethal effect of the insecticide via synergy with a pathogen.

## Abstract

Pesticides are important drivers of global insect declines, and their detrimental effects can be amplified by synergistic interactions with other stressors. While there is growing evidence of adverse impacts of the next-generation insecticide flupyradifurone (FPF) on pollinators, its effects on other insects and interactions with other stressors remain poorly understood. We investigated the effects of chronic exposure to a range of FPF concentrations, alone and in combination with the entomopathogenic fungus Metarhizium brunneum, in the non-target ant species Lasius niger. Twenty-day exposure to FPF concentrations ≥ 100 ppm significantly increased worker mortality, whereas lower concentrations (≤ 50 ppm) had no direct lethal effect. Furthermore, worker survival following acute exposure to M. brunneum was altered by prior pesticide exposure: compared to unexposed controls (0 ppm), workers exposed to sublethal FPF concentrations (5 and 50 ppm) for 10 days before fungal challenge were more susceptible to infection, with hazard ratios for fungal-induced mortality rising from 1.69 in controls to more than 3.5 in pesticide-exposed individuals. This demonstrates an indirect sublethal effect of FPF via synergy with the pathogen, raising further concerns about the environmental impact of novel pesticides and their implications for insect conservation.

The online version contains supplementary material available at 10.1038/s41598-025-20393-z.