الفهرس 
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Abstract
Culex pipiens (L.) (Diptera: Culicidae) is considered as a major vector of pathogens worldwide. This study was evaluated the toxicity of three insecticides (two biocides and Chlorpyrifos ethyl) against the field and laboratory individuals of 3rd instar larvae of Culex pipiens (C. pipiens) larvae at different concentrations after 24, 48 and 72 hours using the conventional methods recommended by the WHO organizations. Moreover, biochemical assay, the activity of acetylcholinesterase (AChE), the Glutathione S transferase (GST), α, β esterases (ESts), total protein, invertase and protease were determined of three insecticides for the field and laboratory strains. The Results showed the laboratory strain higher susceptibility to the tested insecticides than field strains collected from Arab El Raml, Al Menoufia Governorate and the mortality percentage was increased gradually with increasing the insecticide concentrations. The biochemical effects showed that most of insecticides were increased the activity levels of both acetyl cholinesterase (AChE), the glutathione S transferase (GST), α, β esterases (ESts), in the field strain compared with laboratory strain, indicated that this strain had the highest resistance. While, total protein, invertase and protease were decreased after treatment with insecticides. In general, the activities of detoxifying enzymes increased gradually with raising generation numbers, which indicates that the increased resistance is likely to be associated with the increased activity of target and metabolic enzyme systems.