title: How closely do acute lethal concentration estimates predict effects of toxicants on populations?
abstract: Acute lethal dose/concentration estimates are the most widely used measure of toxicity and these data often are used in ecological risk assessment. However, the value of the lethal concentration (LC50) as a toxicological endpoint for use in ecological risk assessment recently has been criticized. A question that has been asked frequently is how accurate is the LC50 for prediction of longer-term effects of toxicants on populations of organisms? To answer this question, Daphnia pulex populations were exposed to nominal concentrations equal to the 48-h acute LC50 of 6 insecticides, Actara, Aphistar diazinon, pymetrozine, Neemix, and Spinosad; and 8 agricultural adjuvants, Bond, Kinetic, Plyac, R-11, Silwet, Sylgard 309, Water Maxx, and X-77; for 10 d. None of the D. pulex populations exposed to the acute LC50 of these insecticides were 50% lower than the control populations at the end of the study; exposure to diazinon resulted in populations that were higher than expected (91% of the control). Exposure to Actara and Aphistar resulted in populations that were < 1 and 29% of the control, respectively. Exposure to Fulfill, Neemix, and Spinosad resulted in extinction. Extinction occurred after exposure to all of the adjuvants, except Silwet L-77 where the population was 31% of the control. These results corroborate other studies that indicate that the LC50 is not a good predictor of effects on population growth. Although lethal concentration estimates have their place in toxicology, namely to compare intrinsic toxicity of chemicals among species or susceptibility of a species to different chemicals over short time periods, population growth and growth-rate studies are necessary to predict toxicant effects on populations.
authors.affiliation: Washington State University, Puyallup Research and Extension Center, Puyallup, Washington 98371, USA. starkj@wsu.edu
authors.forename: John D
authors.initials: JD
authors.lastname: Stark
mesh_terms: D000818:Animals; D003621:Daphnia; D023381:Endpoint Determination; D007306:Insecticides / Q000633:toxicity*; D007928:Lethal Dose 50; D011157:Population Dynamics; D015203:Reproducibility of Results; D018570:Risk Assessment; D014873:Water Pollutants / Q000633:toxicity*
chemical_list: D007306:Insecticides; D014873:Water Pollutants