Hazard evaluation of ten organophosphorus insecticides against the midge, Chironomus riparius via QSAR

Toxicities of ten organophosphorus (OP) insecticides were measured against midge larvae (Chironomus riparius) under varying temperature (11, 18, and 2 5 degrees C) and pH (6,7, and 8) conditions and with and without sediment. Toxicity usually increased with increasing temperature and was greater in t he absence of sediment. No trend was found with varying pH. A series of uni dimensional parameters and multidimensional models were used to describe th e changes in toxicity. Log K-ow was able to explain about 40-60% of the var iability in response data for aqueous exposures while molecular volume and aqueous solubility were less predictive. Likewise, the linear solvation ene rgy relationship (LSER) model only explained 40-70% of the response variabi lity, suggesting that factors other than solubility were most important for producing the observed response. Molecular connectivity was the most usefu l for describing the variability in the response. In the absence of sedimen t, (1)chi(v) and (3)kappa were best able to describe the variation in respo nse among all compounds at each pH (70-90%). In the presence of sediment, e ven molecular connectivity could not describe the variability until the par titioning potential to sediment was accounted for by assuming equilibrium p artitioning. After correcting for partitioning, the same molecular connecti vity terms as in the aqueous exposures described most of the variability, 6 1-87%, except for the 11 degrees C data where correlations were not signifi cant. Molecular connectivity was a better tool than LSER or the unidimensio nal variables to explain the steric fitness of OP insecticides which was cr ucial to the toxicity.