Recovery Following Pulsed Exposure to Organophosphorus and Carbamate Insecticides in the Midge, Chironomus riparius D. B. Kallander 1 , S. W. Fisher 2 , M. J. Lydy 3 1 Indiana Department of Environmental Management, 100 N. SenateAvenue, P.O. Box 6015, Indianapolis, Indiana 46206, USA 2 Department of Entomology, The Ohio State University, 1735 NeilAvenue, Columbus, Ohio 43210, USA 3 Department of Biological Sciences, Wichita State University, Wichita, Kansas 67260, USA Received: 10 August 1996/Revised: 14 October 1996 Abstract.The importance of recovery following pulsed and continuous exposure was determined by measuring the acute toxicity of two organophosphorus (parathion and malathion) and four carbamate (aldicarb, carbaryl, carbofuran and pro- poxur) insecticides. Two 1-h pulses caused significantly fewer symptoms of intoxication than 2 h of continuous exposure if at least 2 to 6 h in clean water were provided between doses for the four carbamates. Two 1-h pulses were equally toxic as a single 2-h continuous exposure for the two organophosphorus insecti- cides. Acetylcholinesterase activity in midges given two 1-h pulses of carbaryl separated by 24 h in clean water showed reactivation to control levels between the two exposures. These results contribute to the belief that episodic exposure to insecticides is less toxic if recovery in clean water is provided. Toxicity bioassays have focused primarily on assessing the effects of continuous exposure of xenobiotics on aquatic organisms (Pasco and Shazili 1986; Jarvinen et al. 1988). However, exposure of aquatic communities to pollutants is often pulsed rather than continuous (Siem et al. 1984; Poirier and Surgeoner 1988; Baughman et al. 1989; Parsons and Surgeoner 1991a). Studies have shown that the impact of pulsed exposure of aquatic organisms to pollutants can be quite different from continuous exposure. For example, Daphnia pulex suffered a higher mortality rate when exposed to pulsed doses of copper than when exposed to copper continuously (Ingersoll and Winner 1982), while the mortality of steelhead ( Oncorhynchus mykiss) and cutthroat trout ( Salmo clarki ) was also higher when the fish was given multiple pulses of ammonia than when given continuous exposure, although sublethal exposure allowed the fish to tolerate higher doses given subsequently (Thurston et al. 1981). In spite of these examples, some researchers have argued that pulsed exposure to pollution should be less toxic to aquatic organisms if sufficient time is allowed between exposures for recovery to take place (Mancini 1983; Wang and Hanson 1985). The purpose of this study was to observe the recovery of the midge, Chironomus riparius, after receiving pulsed exposure to organophosphorus (OP) and carbamate compounds. The acute toxicity of two OP and four carbamate insecticides was measured following continuous and pulsed exposure, where pulses were separated by up to 24 h in clean (undosed) water. To examine the effects of pulsed exposure enzymatically, acetylcho- linesterase activity was measured in midges exposed to two 1-h pulses of carbaryl separated by 24 h in clean water. Materials and Methods Organisms The midge, Chironomus riparius, was used throughout this study and was selected because it is easy to culture, and is important in aquatic food chains. Midge cultures were maintained at room temperature (23 6 2°C) according to the methods of Estenik and Collins (1978). Fourth instar larvae were used in all tests. Head capsule size was used to determine when midges were in fourth instar. Fourth instar head capsule width was between 0.63 and 0.71 mm wide and body length was 1.0 mm or longer. Compounds For measurement of acute toxicity in both continuous and pulsed exposure, two organophosphorus (malathion, diethyl mercaptosucci- nate s-ester with 0,0-dimethyl phosphorodithioate; and parathion, 0,0-dimethyl 0 ( p-nitro-phenyl) phosphorothioate), and four carbamate (aldicarb, 2-methyl-2-(methylthio propionaldehyde 0-(methylcar- bamoyl) oxime; carbaryl, 1-naphthyl N-methyl-carbamate; carbofuran, 2,3-dihydro-2,2-dimethyl-7-benzofuranyl methyl carbamate; and pro- poxur, 0-isopropoxyphenyl methyl-carbamate) insecticides of varying toxicity were examined. AChE reactivation studies were conducted with carbaryl. Pulsed exposure studies were conducted using the same compounds as for the toxicity measurements. All compounds were purchased from Chem Services Co., West Chester, PA, and were $99% purity. Correspondence to: M. J. Lydy Arch. Environ. Contam. Toxicol. 33, 29–33 (1997) ARCHIVES OF E nvironment al Cont aminat ion and T oxicology r 1997 Springer-Verlag NewYork Inc.