Ultra-low concentrations of a botanical insecticide blend alter microbiota composition and gene expression in the ladybeetle Propylea japonica

Botanical pesticides are increasingly promoted as environmentally benign alternatives to synthetic chemicals in integrated pest management. However, the ecological safety of their persistent ultra-low residues remains poorly understood, particularly regarding non-target natural predators. In this study, we investigated the chronic toxicological effects of an ultra-low concentration (ULC; 1/100 of LC₃₀ of 15.106 mg/mL) of a botanical pesticide mixture—composed of commercially available tea saponin and matrine mixed at a ratio of 10:1 (v:v)—on the generalist Propylea japonica. Long-term exposure significantly prolonged larval development, reduced pupal mass, and compromised female fecundity, indicating potential disruptions to population stability and biocontrol efficacy. Gene expression analyses revealed stage-specific alterations in detoxification and hormone-related pathways: fourth-instar larvae exhibited elevated expression of GST and DuoX, downregulation of JHAMT1 and PjIRS1, and upregulation of EcR, whereas adult females showed marked suppression of JHDK, FOXO, PjIRS1, and Vg. These changes point to endocrine disruption and impaired reproductive capacity. Moreover, 16S rRNA sequencing revealed that ULC exposure significantly decreased microbial diversity and altered symbiont composition, particularly in adult beetles. Collectively, our findings demonstrate that even ultra-low residues of botanical pesticides can impose sublethal physiological stress and reshape symbiotic microbial communities in beneficial insects. This work underscores the need for refined ecological risk assessments of green pesticides and advocates for pest management strategies that balance effective control with the conservation of natural enemies.