The cotton aphid, Aphis gossypii (Glover) (Hemiptera: Aphididae), is a key pest of cucurbits in Fars Province, Southern Iran and is managed with repetitive applications of insecticides such as imidacloprid. Recently, reports of insecticide control failures have increased, particularly with imidacloprid. In present work susceptibility of two A. gossypii populations to imidacloprid and effects of possible synergist, Triphenyl phosphate (TPP), Diethyl maleate (DEM) and Piperonyl butoxide (PBO) were checked using micro-applicator bioassay. The resistant population was collected from cucumber host plant in greenhouse and susceptible population had been reared under greenhouse conditions on Cucumis sativus L. cv. Negin (Cucurbitaceae) for two years. A resistance to imidacloprid with resistance ratio of 11.24 was found in resistant population compare with the susceptible population. Combination of in vivo differential synergism studies and biochemical assays (esterase, GSTs, cytochrome P450 monooxygenase and heme peroxidase assay) suggest that the enhanced cytochrome P450 activity is the primary mechanism of increased resistance in these populations.

The cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), is a key cucurbits pest in Iran and is managed with repeated insecticide applications. Reports of insecticide control failures have recently increased, particularly with imidacloprid. To quantify resistance to imidacloprid in cotton aphid, seven populations were collected from 7 different places in South of Iran (Shiraz, Jahrom, Saadatshahr, Marvdasht, Kavar, Sadra1 and Sadra2, all in Fars province). To estimate the response of 5 days old A. gossypii populations to imidacloprid, leaf dip bioassays were performed in the laboratory. Lethal concentrations at 50% (LC50) values were estimated by probit analysis and used to calculate the resistance ratios (RR). The bioassay results showed significant discrepancy in susceptibility to imidacloprid among the populations. The lowest and highest LD50 were estimated for Shiraz population with 37.09 and Sadra1 with 636.80 mg mL -1 respectively. The highest levels of resistance to imidacloprid were detected for Sadra1 (RR=17.17 fold). In the other populations some levels of resistance were detected. In Jahrom, Kavar, Marvdasht and SaadatShahr populations the RRs were from 3.85 to 7.11. As emphasized by the slope of responses and after a comparison of RRs with other studies it is supposed that resistant populations appear in first stages of development and have the ability to become more resistant with age.