Time matters: Multiple observations over time uncover differences in the development of carbaryl toxicity in 7 aquatic invertebrates.

The widespread use of pesticides in agriculture also results in the exposure of non-target organisms. These non-target organisms can differ widely in their sensitivity to pesticides. The sensitivity of organisms to a compound is typically assessed using standardized ecotoxicological tests that rely on fixed-time metrics such as four-day LC50 values. These metrics ignore the time-dependency of toxicity and provide little mechanistic understanding of the observed differences in species sensitivity to toxic substances. To quantify potential differences in the development of toxicity over time, seven aquatic arthropod species were exposed to graded concentrations of the acetylcholinesterase-inhibiting insecticide carbaryl. Immobility was recorded at multiple time points, and the data were analyzed using the General Unified Threshold model of Survival (GUTS) with the software OpenGUTS. The GUTS model highlighted two different groups within our test species: five species showed fast kinetics in the development of effects, compared with two species that showed significantly slower kinetics. The species showing slow kinetics did not reach their incipient EC50 within the test duration, indicating that standard test durations for acute tests may underestimate the toxicity for this species-compound combination. Interpretation with the GUTS model ensured the use of all the observations over several time points, as well as the degradation dynamics of the compound in the water. This helps guide further hypothesis-driven mechanistic work, such as measurements of uptake and elimination rates, target-site interactions, or OMICs, to understand whether toxicity is more toxicokinetic- or toxicodynamic-driven.