Comparative Toxicity of Chlordane, Chlorpyrifos, and Aldicarb to Four Aquatic Testing Organisms M. T. Moore, 1 D. B. Huggett, 1 W. B. Gillespie, Jr., 1 J. H. Rodgers, Jr., 1 C. M. Cooper 2 1 Department of Biology, University of Mississippi, University, Mississippi 38677, USA 2 USDA-ARS, National Sedimentation Laboratory, Oxford, Mississippi 38655, USA Received: 15 May 1997/Accepted: 9 September 1997 Abstract. Laboratory toxicity data contrasting responses of aquatic organisms to insecticides are important for focusing on sensitive species (steepest exposure-response slope) exposed to aqueous concentrations of these insecticides in field studies. These data also allow prediction of expected responses of aquatic species to a range of insecticide concentrations in situ. Aqueous 48-h toxicity tests were performed to contrast re- sponses of Daphnia magna Straus, Hyalella azteca Saussure, Chironomus tentans Fabricius, and Pimephales promelas Rafinesque to acetylcholinesterase-inhibiting insecticides: chlor- pyrifos, aldicarb, and chlordane. As expected, invertebrates tested (H. azteca, C. tentans, and D. magna) were  200 times more sensitive than the vertebrate P. promelas to chlorpyrifos exposures. H. azteca was approximately 3.5 times more sensi- tive to chlorpyrifos (453% mortality/μg/L) than D. magna (128% mortality/μg/L). For both aldicarb and chlordane, C. tentans was the most sensitive species tested (2.44 and 2.54% mortality/μg/L, respectively). Differences in chlordane potency for test species varied only by a factor of approximately 2–3 (0.88% mortality/μg/L for H. azteca to 2.54% mortality/μg/L for C. tentans). Although point estimates of population re- sponses such as LC50s, NOECs, and LOECs are of some utility for predicting effects of pesticides in aquatic systems, exposure- response slopes are also useful for extrapolation of laboratory data to diverse field situations, especially where sediment sorption may regulate insecticide exposure or bioavailability. Comparative toxicity data provide important information on variations in responses of aquatic species to insecticides and are useful for determining margins of safety for aquatic biota, either prospectively (before manufacture and use) or retrospectively (after manufacture and use) (Adams 1995; Graney et al. 1994). Laboratory toxicity data also provide insight into expected effects of accidental spills, cropland runoff, pesticide aerial drift, or other events potentially adversely affecting nontarget organisms. In this series of laboratory experiments, effects of chlordane (1,2,4,5,6,7,8,8-octachloro-2,3,3a,4,7,7a-hexahydro- 4,7-methano-1H-indene), chlorpyrifos [phosphorothioc acid O, O-diethyl O-(3,5,6-trichloro-2-pyridinyl) ester], and aldicarb [2-methyl-2-(methylthio) propanal O-[(methylamino)carbonyl) oxime] were determined for four commonly tested aquatic organisms in 48-h exposures (Table 1). Between the 1940s and 1970s, pesticide use increased almost 40-fold, with new products such as organochlorines (e.g. chlordane) becoming prominent (Nimmo 1985). In 1995, aldicarb and chlorpyrifos applications were approximately 600,000 kg and 3,000,000 kg active ingredient, respectively (Economic Research Service 1996). Organochlorines were designed to be persistent and manufactured inexpensively. Because of growing concerns for environmental and human health, organochlorine pesticides have been largely replaced with less persistent but relatively active organophosphorus and carbamate pesticides. Chlorpyrifos, an organophosphorus insec- ticide, is sold under the trade names Dursban TM and Lorsban TM . The carbamate aldicarb is manufactured with the trade name Temik TM as an insecticide, as well as an acaricide and nemati- cide. Because of its reported human oral and dermal toxicity, aldicarb is sold to certified applicators only in granular form, rather than as emulsified concentrates or liquids (EXTOXNET 1993). Before discontinuance of its manufacturing, chlordane was sold with trade names such as Octaklor TM as an insecticide. Insecticides associated with agriculture and domestic sites are used primarily in terrestrial systems; however, due to frequent proximity of croplands and homes to aquatic systems, concerns have arisen regarding the margin of safety for these materials in aquatic systems (Kersting and van Wijngaarden 1992). Because these insecticides are designed to evoke rapid responses in target populations and degrade rapidly, measure- ments of short-term effects (e.g., 48-h exposures) offer impor- tant information for evaluation of potential risks to aquatic systems. Although insecticides vary in their persistence in aquatic systems, a primary effect that is almost immediately apparent is lethality to nontarget species. Differential responses of organisms representing diverse physiological capabilities and niches in aquatic systems can help focus field studies where nontarget effects due to off-site movement of insecticides are suspected. The objectives of this study were to compare, contrast, and model responses (i.e., survival) of populations of Daphnia magna, Hyalella azteca, Chironomus tentans, and Pimephales promelas to short-term (48 h) aqueous laboratory Correspondence to: J. H. Rodgers, Jr. Arch. Environ. Contam. Toxicol. 34, 152–157 (1998) ARCHIVESOF E nvironment al Cont aminat ion and T oxicology  1998 Springer-Verlag NewYork Inc.