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V- SUMMARY AND CONCLUSIONThe three organophosphorus insecticides malathion , profenofos , and chlorpyrifos were tested to determine their acute toxicity to the four larval instars of the mosquito, Culex pipiens L. , the adult stage of the water flea, Daphnia magna (Straus) and two ages of the Nile Bolti, Oreochromis niloticus (one –and seven -days – old ) . The holding period for all insecticides was 24 h . Residues of one of the three preceding insecticides, malathion , were analyzed on and in squash fruits chemically , using the colorimetric and gas chromatography methods , simultaneously with aquatic bioassay using the three tested organisms as biomarkers .Regarding the acute toxicity, it was found that mosquito larvae exhibited the highest sensitivity to all insecticides tested ; 24h lc50 values of malathion were 0.078 , 0.230 ,0.026 , 0.0557 ug / l . for the first , second , third , and the fourth larval instars , respectively. The corresponding figures at the LC90 level were 2.210 ,8.791 , 0.775 , and 26.050 ug /L . These figures show that the third instar was the most susceptible one .With profenofos , the LC50 and LC90 values ranged between 0.013 to 0.044 , 1.689 to 7.979 ug/L , respectively . The obtained figures of toxicity showed that the last larval instar (4th) was the most sensitive instar . The four larval instars tolerated much higher levels of the third insecticide chlorpyrifos. LC50 and LC90 values ranged between 0.047 to 0.086 and 0.880 to 3.494 ug/L , respectively . The second and the first instars exhibited the highest toxicity to chlorpyrifos , depending on the level of toxicity evaluation .The other two organisms tolerated much amounts of the three used insecticides ranged between 15 to 32038 times that of the most sensitive larval instar of mosquitoes . This wide range in susceptibility depended on the insecticide as well as the used organism . It was found that the older fingerling (7- days – old ) was much more sensitive , to the action of the three used insecticides , than the younger one (1 – day - old ) . The susceptibility of the water flea and fish differed greatly and depended upon the age of fingerlings and the used insecticide .To throw some light on the complementary of chemical analysis and aquatic bioassay using the preceding three organisms. Squash plants were sprayed with one of the used insecticide ( malathion ) in the field using the recommended rate of active ingredient per fed . The residues were determined chemically using the colorimetric and gas chromatography methods as well as biologically. Malathion - contaminated squash fruits were taken 1h , 1 , 3 , 7 , 10 , 14 and 21 days after spraying .Residues were assayed in unwashed and tap water – washed contaminated squash to detect the role of washing in decreasing the amount of malathion residues . In this set of experiments the residues were determined based on the parent compound .The initial deposit of malathion in unwashed malathion –contaminated squash fruit was 2.08 mg /kg then decreased to 1.64 ,1.38 ,0.67 ,0.50 ,0.31 ,and 0.02 mg /kg after 1, 3, 7 ,10 ,14 ,and 21 days after spraying , using the colorimetric method . These figures of malathion residues was much higher with using GC method during the first four samples . The amounts of residues were 4.94 , 3.81 ,3.10 ,1.50 ,0.52 ,0.30 mg/kg , and ND , 1h , and 1 ,3 ,7 ,10 , 14 , and 21 days posttreatment . The corresponding amounts in washed squash fruits were 3.44 , 2.80 ,2.54 , 1.35 , 0.50 , 0.30 , mg / kg , and ND . These figures of residues shows that washing fruits with tap water removed reasonable amounts of malathion residues especially with the high levels of residues . Loss percentages were 30.36 , 26.51 ,18.06 ,11.11 ,3.84 ,0.00 and Ca 0.00 in the samples mentioned before . The half lives in the colorimetric and GC methods were found to be 0.81 and 0.6 days , respectively .The initial deposits of malathion , determined biologically , were 3.55 , 5.55 , and 6.66 mg/ kg with using T. Nilotica , D . magna , and c . pipiens , respectively. Again, and as has been found with chemical analysis sharp decrease in the amounts of residues took place in the subsequent samples . The corresponding half lives periods were 0.24 , 0.75 and 0.22 day .In conclusion , it could be stated that the sensitive aquatic organisms may be used as biomarhers for detecting insecticide residues in the environment. In other words integration between chemical analysis and aquatic bioassay must be taken into consideration in this respect .
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