Insulin/insulin-like peptide-binding protein (IBP) is abundantly found in venom of the solitary hunting wasp, Eumenes pomiformis (Hymenoptera: Eumenidae). E. pomiformis IBP (EpIBP) is most similar to insect IBP-like proteins that are known to inhibit insect growth and insulin signaling. To investigate the toxicity and target protein, EpIBP was in vivo expressed by Escherichia coli. Spodoptera exigua (Lepidoptera: Noctuidae) larvae injected with EpIBP showed a 20% lower pupation rate than the control larvae, although their body weight was not significantly different from the control when the larvae were provided artificial diet after the injection. EpIBP extended the larval stage without inducing paralysis of S. exigua larvae. To investigate the effects of EpIBP on caterpillar under a starvation condition, survivorship and body weight of the EpIBP-injected were evaluated without providing artificial diet until all the larvae died. The survivorship of the EpIBP-injected larvae was 24-36% higher than the control larvae at 4-5 d post-injection. The body weight of the control larvae reduced to 59% that is approximately 10% lower than the body weight of the EpIBP-injected larvae. These results suggest that EpIBP might inhibit the metabolism of the caterpillars, which is likely related with insulin-like peptide signaling pathway, suppress the loss of body weight and eventually extend the larval stage. An EpIBP-binding protein (EpIBPBP) isolated by immunoprecipitation was matched with a coiled-coil domain-containing protein of the fruit fly. The full-length sequence analysis of EpIBPBP is in progress.