Like pathogens, galling insects deliver effectors into plant tissues that induce gall formation. The gall midge Mayetiola destructor, also called Hessian fly, can convert a whole wheat seedling into a gall by inducing the formation of nutritive cells at the feeding site, inhibiting wheat growth, and reprogramming metabolic pathways of the attacked plants. Here we demonstrated the identification of a secreted Hessian fly protein, the nucleoside diphosphate kinase (NDK), in infested wheat plants through liquid chromatography-tandem mass spectrometry (LC-MS/MS) and western blots. In association with the NDK presence, enzymatic activity of NDK increased significantly in wheat tissues at the feeding site. In addition, there was a sudden increase in ATP abundance at the feeding site of infested susceptible wheat seedlings 24 h following Hessian fly larval infestation. Even though a direct link between cause and effect remains to be established, our data points to the direction that Hessian fly larvae injected NDK into wheat tissues as an effector protein, which plays a role in manipulating host plants and converting the plants into galls.