Abstract:In order to examine the effects of commonly used pesticides on the insect resistance of cotton, the effects on the contents of condensed tannin, one of the insect resistance materials of cotton, in cotton seedlings were determined after hydroponically treatment with methomyl and imidacloprid. The 3-leaf stage cotton seedlings were treated with 25 and 50mg/L methomyl, 25 and 50mg/L of imidacloprid respectively for 15 days, at different treating time, the cotton leaves were harvested for determination of the contents of condensed tannin by vanillin assay. The results showed that the contents of condensed tannin in cotton seedlings increased significantly after treated with 25mg/L methomyl for 6, 9, 12 days and 15 days and with 50mg/L of methomyl for 6 days and 15 days, with the condensed tannin in cotton seedlings increased 74% than that in untreated control cotton seedlings. After treated with 50mg/L of imidacloprid for 2 days, the condensed tannin in cotton seedlings increased 45% than that in untreated control cotton seedlings, while the contents of condensed tannin in cotton seedlings did not show significantly differences between the other treatments of imidacloprid and the control, even no significant differences were observed at the other treating time. So, the effects on the contents of condensed tannin in cotton seedlings were closely related to the insecticides used, and methomyl application has a serious impact on contents of condensed tannins in cotton seedlings, but imidacloprid does not. The effects of the concentration of insecticides and the treating time on the contents of condensed tannin in cotton seedlings were observed.