Cotton bollworm (<i>Helicoverpa armigera</i>) is a Lepidopteran noctuid pest with a global distribution. It has a wide range of host plants and can harm cotton, tomato, tobacco, and corn, as well as other crops. <i>H. armigera</i> larvae damage the flower buds, flowers, and fruits of tomato and cause serious losses to tomato production. Tomato uses the allelochemical 2-tridecanone to defend against this damage. So far, there have been no reports on whether the adaptation of <i>H. armigera</i> to 2-tridecanone is related to its symbiotic microorganisms. Our study found that <i>Corynebacterium</i> sp. 2-TD, symbiotic bacteria in <i>H. armigera</i>, mediates the toxicity of the 2-tridecanone to <i>H. armigera</i>. <i>Corynebacterium</i> sp. 2-TD, which was identified by 16S rDNA gene sequence analysis, was screened out using a basal salt medium containing a unique carbon source of 2-tridecanone. Then, <i>Corynebacterium</i> sp. 2-TD was confirmed to be distributed in the gut of <i>H. armigera</i> by quantitative PCR (qPCR) and fluorescence in situ hybridization (FISH). The survival rate of <i>H. armigera</i> increased by 38.3% under 2-tridecanone stress after inoculation with <i>Corynebacterium</i> sp. 2-TD. The degradation effect of <i>Corynebacterium</i> sp. 2-TD on 2-tridecanone was verified by ultra-high-performance liquid chromatography (UPLC). Our study is the first to report the isolation of gut bacteria that degrade 2-tridecanone from the important agricultural pest <i>H. armigera</i> and to confirm bacterial involvement in host adaptation to 2-tridecanone, which provides new insights into the adaptive mechanism of agricultural pests to host plants.