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BACKGROUND The western flower thrips Frankliniella occidentalis, a worldwide agricultural pest, has developed resistance to an array of insecticides. Spinosad resistance confer an apparent fitness cost in F. occidentalis. In the present study, we compared the reproductive capacities, ovary development, and the expression of the vitellogenin (Vg) gene in spinosad-susceptible (Ivf03) and -resistant (NIL-R) near iso-genetic lines of F. occidentalis in order to clarify the reason of the fitness cost in spinosad resistance. RESULTS The NIL-R strain exhibited a 17.9% decrease in fecundity (eggs laid per female) as compared to the Ivf03 strain, and the ovariole was significantly shortened by 2.8% in the NIL-R strain than in the Ivf03 strain. Compared to the Ivf03 strain, the expression levels of Vg mRNA and protein were down-regulated by 33.7% and 32.9% in the NIL-R strain, respectively. Moreover, interference with Vg gene significantly reduced the expression levels of Vg mRNA and protein, and decreased ovariole length, survival rates, and the fecundity of both of the two strains. CONCLUSION The results indicate that the down-regulated expression of Vg may contribute to the reduction of ovariole length and consequently to a fitness cost in the spinosad-resistant F. occidentalis. The results not only increase our understanding of the evolution of insecticide resistance but could also contribute to the formulation of control strategy of F. occidentalis. This article is protected by copyright. All rights reserved.