Altered Negative Geotaxis in Culex sp. Larvae Exposed to Pymetrozine and Manganese: Implications for Behavioural Aquatic Ecotoxicology

Abstract

Geotaxis is the behaviour of an organism in response to gravitational forces. Mosquito larvae display innate negative geotaxis in water. This study explored how exposure to pymetrozine (PYM) and manganese (Mn) affects the geotaxis of Culex sp. larvae in water, and whether this behaviour could be a useful endpoint in aquatic ecotoxicology. Culex sp. larvae were exposed to varying concentrations of both chemicals, and their geotaxis was monitored for ten min. Additionally, 24-hr mortality bioassays were carried out to obtain novel data on the lethal effects of both chemicals on Culex sp. larvae. Mortality experiments revealed that PYM caused significant larval mortality with an LC50 of 60.75 (54.29–67.98) mg/L. Mn showed a moderate lethal effect with an undetermined LC50 value due to insufficient mortality across the tested concentration range (i.e., 0–345.50 mg Mn/L). Geotaxis testing revealed that in the control, the mosquito larvae spent, on average, 78.23% of their time in negative geotaxis. Exposure to Mn significantly decreased this innate rate to 62.71% in the 172.75 mg Mn/L treatment (p < 0.05), whereas exposure to PYM lowered it further to 54.54% in the 8.25 mg PYM/L treatment (p < 0.05). These significant reductions in negative geotaxis in contaminated water samples suggest that geotaxis could serve as a useful endpoint for the quick assessment of water pollution. However, because the geotaxis dose–response curves were biphasic, validation using other aquatic life forms may be required, especially at lower concentrations, to ensure that systemic shutdown in the larvae at higher concentrations is not misinterpreted as genuine geotaxis responses.