INSECTICIDE TOXICITY TO MACROBRACHIUM LAMARREI (H. MILNE EDWARDS) (DECAPODA, PALAEMONIDAE) BY G. S. SHUKLA and OMKAR Pollution Relevant Research Laboratory, Department of Zoology, University of Gorakhpur, Gorakhpur-273001, India INTRODUCTION Pesticides are at present used extensively for crop-protection, public health and other purposes. As a consequence of their usage, these compounds fre- quently reach the aquatic environment, either directly through spraying opera- tions or through run-off water from agricultural lands and thus contaminate the freshwater eco-system. These pesticides have been found to be extremely toxic not only to fish (Saunders, 1969; Mawdesley-Thomas, 1971; Anees, 1975) but also to crustaceans (Muncy & Oliver, 1963; Nebeker & Gaufin, 1964; Eisler, 1969; Sanders, 1969; Chaiyarach et al., 1975; Bluzat & Seuge, 1979; McLeese & Metcalfe, 1980; Shukla & Omkar, 1981). In the present investigation, three commonly used insecticides were tested for their short-term toxicity to a freshwater prawn, Macrobrachium lamarrei (H. Milne Edwards) which is a valuable source of food for fishes and human beings and is abundantly available in local water sources. METHODS Prawns were collected from Ramgarh Lake, which is situated south-east to Gorakhpur University, and acclimatized for three days to the laboratory condi- tions prior to the experiments. The insecticides used were (i) an organochlorine, endosulfan (Thiodan, 35 E. C . ), (ii) an organophosphate, methyldemeton (Metasystox, 25 E.C.), and (iii) an organocarbamate, carbaryl (Sevin, 50 W.P.), procured from local markets. Stock solutions of the insec- ticides were prepared on the day of exposure by dissolving the insecticides separately (0.1 g endosulfan (a. i. ), 1.0 g methyl-demeton (a. i . ), and 1.0 g car- baryl (a.i.), respectively) in 1.0 ml acetone and 1.0 litre dechlorinated tap- water. A series of six concentrations of each insecticide (toxic range determined by exploratory tests) were prepared from the stock in ten litres of dechlorinated tapwater contained in a plastic container of fifteen litre capacity. The tapwater was analysed for its physico-chemical characteristics after APHA et al., 19711 284 (pH, 7.5 ± 0.2; dissolved oxygen, 8.2 ± 0.25 mg/l; total hardness as CaC03, 110.35 t 2.5 mg/l and temperature, 25 t 1 °C). Ten healthy prawns of the size 60-65 mm and weight 1.2-1.5 g were transferred carefully from the acclimatization tank to the test solutions. Compressed air was supplied con- tinuously during acclimatization and test periods. Three replicates of each set were performed and controls were also set for each series by adding 1.0 ml acetone in ten litres of dechlorinated tapwater. The mortality was recorded after 24, 48, 72 and 96 h and dead animals were removed regularly. The obtained data were subjected to statistical analysis (Goulden, 1959). RESULTS Prawns are active animals, so the symptoms of insecticidal stress are easily detectable. The animals were restless in the test solutions of each insecticide, earlier in higher concentrations than in lower ones. The first indication of insecticide poisoning was irritability, followed by impaired locomotion, restriction of appendage movement and finally death occurred. Symptoms of poisoning were somewhat similar for all insecticides. It is evident from LC50 values (table I) that the organochlorine insecticide endosulfan is highly toxic, viz., 568.5 times more than the organophosphate methyl-demeton, which is least toxic to this species and the carbamate carbaryl is intermediate in toxicity. Thus they exhibit toxicity in the order: endo- sulfan > carbaryl > methyl-demeton. During the course of present study, it was observed that in 0.002, 0.5 and 0.0175 mg/1 solutions of endosulfan, methyl-demeton and carbaryl respective- ly, no mortality occurred in 96 h while in 0.0075, 6.0 and 0.065 mg/l solutions of endosulfan, methyl-demeton and carbaryl respectively, 100% mortality occurred within 24 h. DISCUSSION It is obvious from results (table I) that these insecticides are toxic at very low concentrations, even traces of them can cause considerable damage to the prawns. Further, it was observed that exposure time plays an important role in the LC5p values. The LC5p value decreases with increase in exposure time. A decrease in LC50 values of endosulfan (0.00534-0.00352 mg/1), methyl- demeton (4.156-1.999 mg/1) and carbaryl (0.0489-0.0326 mg/1) were observed with increase in exposure time from 24-96 h. The LC5p values of endosulfan and carbaryl for Gammarus lacustris Sars (am- phipod crustacean) were 9.2 tg/I and 40 ttgll respectively at 21°C (Sanders, 1969). The LC50 values of carbaryl reported for other crustaceans were 0.029 mg/l for Gammarus pulex (L.) (cf. Bluzat & Seuge, 1979); 5.0 mg/l for crayfish Procambarus clarkii (Girard) (cf. Muncy & Oliver, 1963); 3.02 mg/l for crayfish P. simulans (Faxon) and 0.42 mg/l for grass shrimp Palaemonetes kadiakensis .