Integrated analysis of lethal and sublethal effects of rice pesticides on the natural predator Microvelia Pulchella (Insecta: Hemiptera: Veliidae): insights into direct and indirect effects.

Abstract

This study examines the lethal and sublethal effects of commonly used rice crop pesticides-cypermethrin, butachlor, and carbendazim-on the broad-shouldered water strider, Microvelia pulchella. Conducted under controlled conditions, the research aimed to elucidate the direct and indirect impacts of these pesticides on key life-history traits of this aquatic predatory insect. Initial dose-response bioassays revealed that cypermethrin was the most toxic, with a median lethal concentration (LC50) of 0.79 ppm, followed by butachlor (LC50 = 2.53 ppm) and carbendazim (LC50 = 15.63 ppm). Subsequently, using Structural Equation Modelling (SEM), we investigated causal relationships between sublethal pesticide exposure and life-history traits in M. pulchella. The results demonstrated that sublethal concentrations induced significant biological disruptions, with pronounced sex-specific effects on wing development, body size, longevity, fecundity, and offspring hatching success. Cypermethrin exposure markedly reduced female body size and longevity in both sexes, indirectly diminishing egg production and offspring hatching success. Butachlor similarly reduced body size in both sexes and male longevity, with cascading effects on reproductive output. Specifically, decreased egg production was linked to smaller female body size, reduced male longevity, and an indirect effect of butachlor on female longevity mediated by body size. In contrast, carbendazim exposure led to reductions in body size and significant impairments in wing development. Its effects on fertility, however, were dose-dependent, showing a slight decline at lower concentrations but an unexpected increase at higher doses, suggesting a complex, non-linear relationship between exposure levels and reproductive outcomes. These findings highlight the detrimental ecological consequences of cypermethrin, butachlor, and carbendazim on M. pulchella, disrupting its role as a natural predator and impairing its ability to regulate pest populations. The study concludes with evidence-based recommendations for Integrated Pest Management (IPM) practices and regulatory policies aimed at preserving beneficial predatory species like M. pulchella and fostering sustainable agricultural systems.