Simple SummaryThe oriental fruit fly,Bactrocera dorsalis, is a highly significant invasive pest in agriculture, causing damage to over 600 species of fruits and vegetables. To gain a better understanding of its host adaptation and rapid spread, it is crucial to investigate the effects of changes in nutrient content on the phenotype and gene expression ofB. dorsalis. In this study, we examined the developmental duration, adult fecundity, insecticide susceptibility, and gene expression patterns ofB. dorsalisby varying the concentration of sucrose in its larval diet. Our findings indicate that a high-sucrose diet during larval stages resulted in a longer developmental period, higher adult fecundity, and greater tolerance to malathion. In contrast, a low-sucrose diet led to smaller body size, shorter developmental duration, and higher sensitivity to beta-cypermethrin. Additionally, we identified differentially expressed genes associated with various metabolisms, hormone synthesis and signaling, and immune-related pathways under different sucrose concentrations in the larval diet. These results suggest that dietary sucrose plays a significant role in phenotypic adjustments and gene expression patterns inB. dorsalis.AbstractBactrocera dorsalis(Hendel) (Diptera: Tephritidae) is one of the broad host ranges and economically-important insect pests in tropical and subtropical areas. A wide range of hosts means they have strong adaptation ability to changes in dietary macronutrients (e.g., sucrose and protein). However, the effects of dietary conditions on the phenotypes and genotypes ofB. dorsalisare still unclear. In this study, we aimed to investigate the effects of larval dietary sucrose on the life history traits and stress tolerance ofB. dorsalis, and its defense response at the molecular level. The results showed that low-sucrose (LS) induced decreased body size, shortened developmental duration, and enhanced sensitivity to beta-cypermethrin. Otherwise, high-sucrose (HS) diet increased developmental duration, adult fecundity, and tolerance to malathion. Based on transcriptome data, 258 and 904 differentially expressed genes (DEGs) were identified in the NS (control) versus LS groups, and NS versus HS groups, respectively. These yielded DEGs were relevant to multiple specific metabolisms, hormone synthesis and signaling, and immune-related pathways. Our study will provide biological and molecular perspective to understand phenotypic adjustments to diets and the strong host adaptability in oriental fruit flies.