Simple SummaryAcyrthosiphon gossypiiis a sap-sucking pest that causes direct cotton damage by sucking sap and indirect damage by honeydew contamination and induced fungal growth. Chemical pesticides are widely used to combat the damage caused byA. gossypii. However, the escalating use of pesticides has had a notable impact on the resurgence and resistance of pests. In addition, microbes that live in insects as symbionts have recently been found to protect their hosts against toxins. In this study, we determined the toxicity and sublethal effects of the neonicotinoid insecticide imidacloprid onA. gossypiiand further determined the effects of imidacloprid on symbiotic bacteria. The sublethal concentrations of imidacloprid had a negative effect on the reproduction and longevity of the G0 generation ofA. gossypii, increased the population growth parameters of the G1 and G2 generations, but not those of the G3 generation, and altered the abundance of symbiotic bacteria. Our results revealed the transgenerational and sublethal effects of imidacloprid onA. gossypii, which may have implications for the endosymbiont-insecticide of the major pest.AbstractSymbiotic bacteria and hormesis in aphids are the driving forces for pesticide resistance. However, the mechanism remains unclear. In this study, the effects of imidacloprid on the population growth parameters and symbiotic bacterial communities of three successive generations ofAcyrthosiphon gossypiiwere investigated. The bioassay results showed that imidacloprid had high toxicity toA. gossypiiwith an LC50of 1.46 mg·L−1. The fecundity and longevity of the G0 generation ofA. gossypiidecreased when exposed to the LC15of imidacloprid. The net reproductive rate (R0), intrinsic rate of increase (rm), finite rate of increase (λ), and total reproductive rate (GRR) of G1 and G2 offspring were significantly increased, but those of the control and G3 offspring were not. In addition, sequencing data showed that the symbiotic bacteria ofA. gossypiimainly belonged to Proteobacteria, with a relative abundance of 98.68%. The dominant genera of the symbiotic bacterial community wereBuchneraandArsenophonus. After treatment with the LC15of imidacloprid, the diversity and species number of bacterial communities ofA. gossypiidecreased for G1–G3 and the abundance ofCandidatus-Hamiltonelladecreased, butBuchneraincreased. These results provide insight into the resistance mechanism of insecticides and the stress adaptation between symbiotic bacteria and aphids.