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Abstract (synthetic pyrethroids) insecticides and pyriproxyfen (insect growth regulator). The insecticide pressure was performed for I 0 generations. All experiments were maintained at laboratory conditions of 27± 2 and 80±5 %R.H. !-Build up of susceptible-laboratory strain. The obtained data revealed that the LC50 values of the two investigated insecticides were decreased by succession of generations. A sharp decrease in LCso values was noticed at F 18 and F21 with both insecticides. The recorded values of fenitrothion were 0.052, 0.0056, 0.0037 and 0.0013 ppm for parent, Fl5, Fl8 and F21 respectively, while these values were 0.035, 0.011, 0.0056 and 0.0011 ppm for cyphenothrin. It is clearly evident to notice that the LC50 of F21 line was steeper than other lines representing the forward generations. This means that there is a high degree of strain homogeneity toward susceptibility to both offenitrothion and cyphenothrin. 2-Development of resistance to fenitrothion. Data indicated that the homogeneity of individuals toward resistance to fenitrothion was increased pronounccdly by generations progression. The LC5o values were 0.052, 0.497, 7.332, 10.52 and 12.148 ppm with P, Fl, F4, F7 and FlO, respectively. The slope values of regression lines were increased in F l and F4 generations, while clear decrease was observed in F7 and FlO, but continued higher than parent strain. The slope values were 1.365, 2.476, 2.105, 1.966, and 1.875 for P, Fl, F4, F7, FlO, respectively. On the other hand, the level of resistance was increased from 40 fold at parent generation to 382.3, 5640, 8100 and 9372.3 fold at Fl, F4, F7 and FlO generations, respectively. from these data one may notice clear development of fenitrothion resistance. 3-Development of resistance to cyphenothrin. The LC50 values of cyphenothrin-tested generation were increased when compared with parent strains, it reached 0.035, 0.383, 5.643, 7.913 and 10.272 ppm for parent, Fl, F4, F7 and FlO, respectively. In addition the slope values of ld-p lines of selected successive generation increased gradually than the parent line till the I O’h generation. The recorded slope values were 1.83, 1.98, 1.96 and 1.99 for Fl, F4, F7 and FlO, respectively, while the slope values for parent was 1.71. The calculated resistance ratio confirmed the development of resistance across the selective generation, thus the level of resistance increased progressively as the generation proceeded. The recorded values were 31.8, 348.2, 5138, 7193.6 and 9358.2 fold for parent, Fl, F4, F7 and F I 0, respectively. 4-Development of resistance to pyriproxyfen. The obtained data revealed that the LC50 value for pyriproxyfen was 0.0091ppm compared with 0.0009ppm for susceptible strain. The slope values reached 0.98 and 3.83 for parent and susceptible strain, respectively. It was noticed that the treatment of 4’h instar larvae of parent colony with pyriproxyfen at LC30 as a selective pressure agent caused I 00% mortality of pupal stage resulted from survived larvae. On the other hand, the larvae failed to complete their life cycle. This finding could be attributed to the latent effect of this insect growth regulator. Thus we could not obtain resistance strain from these parent colony of culex pipi ens by exposure of larval strain to selective pressure of pyriproxyfen. 5-Cross-resistance spectrum in fenitrothion and cyphenothrin-selected strains. 1- fcnitrothion-sclcctcd strain. The obtained results revealed that the (JHM), pyriproxyfen and the two pyrethroids deltamthrin and cyphenothrin were more toxic to the 41h instar larvae offenitrothion-resistant strain than the other tested op’s, malathion and diazinon. The LC50 values were 0.853, 0.861, 1.00 I, 7.62 and 9.00 ppm, receptively. However the LC50 of fenitrothion was 12.184 ppm. The rate of toxicity indicated that the (JHM) pyriproxyfen was more toxic by 14.28 times. Deltamthrin showed the almost the same degree of toxicity as pyriproxyfen, followed by cyphenothrin, malathion and diazinon, showing 14.15, 12.17, 1.59 and 1.35 times as fenitrothion. The data further provided evidence that fenitrothion-resistant strain showed wide cross-resistance with all of the tested insecticides. But the pyrethroids and JHM recorded about tenth the level of resistance of fenitrothion. 2- cyphcnothrin-sclcctcd strain. The obtained results revealed that the (JHM) pyriproxyfen was the most toxic compound against the larvae of cyphenothrin-resistant strain of Culex pipiens (L.). The tested organophosphorus diazinon, malathion and fenitrothion showed almost similar LC5o values and followed pyriproxyfen activity. The LC50 values were 0.80 I, 0.991, 1.00I and 1.118 ppm for pyriproxyfen, diazinon, malathion and fenitrothion respectively. The pyrethoid deltamthrin showed lowest toxic effect than the other tested insecticides. The recorded LC5o was 8.302 ppm, while LC50 of cyphenothrin was 10.272 ppm. There was no significant difference between these values. Data also indicated that there was a significant difference between LC50 of pyrethroid group, JHM and organophosphorus compounds. The rate of toxicity indicated that pyriproxyfen was more toxic by 12.82 times. Diazinon showed almost the same degree of toxicity as pyriproxyfen, followed by malathion, fenitrothion and deltamethrin, showing 10.37, 10.27, 9.19 and 1.24 times as cyphenothrin, respectively. The recorded resistance level values were 860, 888.8, 910, 940 and 9224.4 fold for fenitrothion, pyriproxyfen, malathion, diazinon and deltamethrin, respectively. While the level of cyphenothrin-resistant strain was 9338.2 folds. This data indicated that the developed resistance strain showed wide resistance spectrum to the tested insecticides. 6-Resistance spectrum of certain Field population of Culex pipiens larval stage. The susceptibility of 41 instar larval stage collected from Shalakan (Qalubiya govemerate) to certain insecticides was investigated. It could be seen that the JHM pyriproxyfen was the highest effective compared with the other tested insecticides. Dcltamcthrin showed almost the same degree of toxicity as pyriproxyfen. Statistical analysis revealed that there was insignificant difference between the LC50 values of pyriproxyfen and deltamethrin, i.e. 0.008 and 0.0099 ppm, respectively. Cyphenothrin and malathion showed almost the same LC50 values and followed pyriproxyfen and deltamethrin. The LC50 ’s were 0. l 02 and 0. l 03 ppm for cyphenothrin and malathion, respectively. Fenitrothion and diazinon showed almost the same and lowest degree of toxicity. The LC50 for these compounds were 0. l I and 0.12 ppm, respectively. There was insignificant difference between the LC50 of the last four insecticides. The lowest resistance level was recorded with pyriproxyfen (8.8 fold), followed by deltamethrin ( l l fold). However, the other tested insecticides showed high level of resistance, and could be an·anged in ascending order as: fenitrothion (84.6 fold), cyphenothrin (92.7 fold), malathion (93.6 fold) and diazinon ( 120 fold). The sensitivity of larval stage of Culex pipiens collected from EL-Suff was studied. Cyphenothrin proved to be the lowest toxic compound than the other insecticides. Pyriproxyfen proved to be the most effective insecticides followed by deltamethrin, malathion, fenitrothion, diazinon and cyphenothrin, showing LC50 ’s of0.009, 0.011, 0.1 00, 0.1 07, 0.112 and 0.122 ppm, respectively. ?-Susceptibility of certain field population and resistant strains of Culex pipiens females to tested insecticides. The sensitivities of Shobra EL-Khema populations, fenitrothion R and cyphenothrin-R strains of Culex pipiens females to malathion, fenitrothion, diazinon, cyphenothrin and deltamethrin were measured. The standard discriminating dosage and exposure time according to the protocol suggested by WHO (I998) was followed in this investigation. The sensitivity of susceptible strain was taken as a reference for companson. Data revealed that deltamethrin and cyphenothrin (pyrcthroids) recorded the lowest knock down effects’ i.e. 8 and 17.5 for deltamethrin and cyphenothrin, respectively. On the other hand, these compounds recorded in general the highest killing effects, showing 95 and 79% mortality for these two insecticides, respectively in Shobra EL-Khema. The knock down effect of the tested organophosphorus compounds could be arranged in ascending order as fenitrothion (20%), followed by malathion (25%) and diazinon (35%). However the killing effect of these compound were arranged ascending as fenitrothion (72%), malathion (75%) and diazinon (80%). Data in concern of the response of cyphenothrin-resistant strain to tested insecticides. The deltamethrin achieved the lowest knock down effect (I 0%), followed by cyphenothrin ( 15%), diazinon (25%), |