12-Oxo-phytodienoic acid (OPDA) plays unique roles in plant defenses against biotic and abiotic stresses. In the current study, we infested two resistant wheat (Triticum aestivum L.) cultivars, ‘Molly’ and ‘Iris’, with an avirulent Hessian fly population and determined the impact of exogenous OPDA application on wheat resistance to the insect under heat stress. We observed that Molly and Iris treated with OPDA solution prior to the heat treatment exhibited significantly enhanced insect resistance. We also measured OPDA concentrations at Hessian fly feeding sites in Molly infested with Hessian flies. We found that exogenous application of OPDA resulted in increased abundance of endogenous OPDA in Molly seedlings and that OPDA abundance in plants treated with the combination of heat and OPDA was similar to that of plants in the incompatible interaction. Our results suggest that high abundance of endogenous OPDA may be necessary for wheat under heat stress to resist to Hessian fly infestation.