Resistance to imidacloprid in different field populations of Aphis gossypii Glover (Hem.: Aphididae) in South of Iran

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 µg 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.