Responses of whitefly, Bemisia tabaci (Gennadius), adults to several classes of insecticides were determined using two different bioassays. The toxicity data showed that adult-vial bioassay was more sensitive technique than leaf-residue bioassay.  LC₅₀ values of most insecticides tested were < 1.0 µg/vial. The neonicotinoid imidacloprid was the most toxic insecticide (0.11 µg/vial) followed by buprofezin, abamectin, thiamethoxam and pyriproxyfen, whereas methomyl had the lowest toxicity. Status of insecticide resistance levels in three field strains of whitefly was reported using leaf-residue technique. The resistance levels to most selected insecticides were lower especially in BNS-strain indicating RR < 2.1-fold compared to the Lab-SUS strain, whereas slight increases in resistance levels ranged from 3.0- to 5.4- fold for the conventional insecticides tested were detected in all three field strains. The acetylcholinesterase (AChE), non-specific esterases (EST), and phosphatase activities in the tested field strains and susceptible laboratory strain were examined to better understand biochemical mechanisms of resistance. Highest esterase activity was observed for the MNF and FYM strains in comparison with that for the Lab-SUS. AChE activity was also significantly higher in all field strains tested by ~ 5.3-7.6-fold more than that in the SUS strain. In addition, significant increases in the activities of both acid- and alkaline-phosphatase were reported for MNF and FYM strains than these in the SUS strain. These results provide baseline information for further research on the involvement of esterases in the resistance mechanisms of field populations of the whitefly.