Dual-Geotaxis bioassay as a behavioral indicator of sublethal pesticide exposure in mosquito larvae (Culicidae)

Understanding sublethal pesticide effects on aquatic organisms is important because behavioral impairment may occur before overt mortality and may influence ecological fitness. This study evaluated the behavioral and toxicological responses of fourth-instar Culiseta longiareolata larvae following nominal imidacloprid exposure using a dual-geotaxis bioassay. Larvae were identified morphologically and individually exposed to nominal concentrations of 0, 1, and 2 mg L⁻¹ imidacloprid under controlled laboratory conditions. Behavioral activity was recorded for 10 min following acclimation, and geotactic responses were quantified as negative geotaxis (time spent swimming upward or remaining near the surface) and positive geotaxis (time spent moving downward or remaining near the bottom). Acute toxicity was assessed separately after 24-hour exposure to nominal concentrations of 0, 0.5, 1, and 2 mg L⁻¹. Under control conditions, larvae exhibited predominantly surface-oriented behavior, spending approximately 75% of the observation period in negative geotaxis (p < 0.01). Imidacloprid exposure produced concentration-related behavioral changes characterized by reduced negative geotaxis and increased positive geotaxis at both tested concentrations (p < 0.05). Probit analysis estimated a 24-hour LC₅₀ of 1.349 mg L⁻¹, while exploratory polynomial modelling identified a geotactic equilibrium concentration of 0.523 mg L⁻¹, at which predicted time allocation between positive and negative geotaxis was equal. This value closely corresponded to the estimated LC₃₁ (0.519 mg L⁻¹). The findings demonstrate that dual-geotaxis measurements can detect measurable behavioral alterations under sublethal imidacloprid exposure and may provide a complementary endpoint to conventional lethality-based assessments in aquatic ecotoxicology. Although mechanistic pathways and analytical verification of exposure concentrations were not evaluated, the assay offers a practical framework for rapid behavioral screening of neuroactive contaminants.