The genus Diadegma is a well-known parasitoid group and some are known to have symbiotic virus, polydnavirus (PDV). A novel IV was discovered from the calyx of Diadegma fenestrate female and sequenced its genome. Diadegma fenestrate has more than two hosts, including potato tuber moth (PTM) and diamondback moth (DBM). Diadegma fenestrate preferred PTM to DBM as hosts based on the oviposition and survival rate. Nevertheless, the developmental period and morphology of Diadegma fenestrate were not significantly different between PTM and DBM. We compared Df1V gene expression patterns between PTM and DBM under various conditions to understand the phenomena. DfIV genes were more widely expressed in PTM with large numbers than in DBM after parasitized by Diadegma fenestrate, particularly at the initial point. They showed differential expression patterns between two lepidopteran hosts. This DfIV gene expression plasticity showed a dependency on the lepidopteran host species and parasitization time, suggesting that it may contribute to increase the parasitoid survival rate. This might be one of the key elements that determine the symbiotic relationship between PDV and parasitoid. (C) 2015 Elsevier Inc. All rights reserved.