Abstract: 【Objective】 This project aims to illustrate the effects of infested cotton, roots and stems were chewed by Agrotis segetum larvae, on population fitness of Tetranychus truncates, Tetranychus turkestani and Tetranychus dunhuangensis on cotton leaves. 【Methods】 Based on cotton cultivar CCRI 49, the effects of cotton plant infested by A. segetum larvae on growth, development and reproduction of T. truncates, T. turkestani and T. dunhuangensis were investigated in laboratory. 【Result】 The developmental duration of T. truncates nymph in infested cotton plant was significantly prolonged compared with the control, but there were no significant differences in larva survival rate, nymph survival rate, developmental duration of larva, pre-oviposition period, fecundity and female adult longevity of T. truncates between the infested plants and the healthy ones. The number of effective eggs per female of T. turkestani in the chewed cotton was substantially reduced compared with the control. The infection of A. segetum larvae on roots and stems did not signi ficantly affect the larva and nymph survival rate, developmental duration of larva and nymph, pre-oviposition period, as well as female adult longevity of T. turkestani on cotton leaves. The developmental duration of larva and nymph of T. dunhuangensis in infested ones was significantly prolonged. No significant differences in larva survival rate, nymph survival rate, pre-oviposition period, fecundity and female adult longevity of T. dunhuangensis between the infested cottons and the healthy ones were found. 【Conclusion】 Cotton roots and stems infested by A. segetum larvae made no difference on the population fitness of T. truncates, T. turkestani and T. dunhuangensis.