This dissertation explores the chemically-mediated mechanisms for host plant discrimination in specialist phytophagous insects using the grape berry moth (GBM; Paralobesia viteana)-grape, Vitis spp. complex as a model. The GBM–grape complex represents an excellent system to explore the chemically-mediated mechanisms of host plant location because the GBM is an ovipositional specialist, meaning gravid females discriminate between host and non-host plants. Additionally, synthetic blends of host plant volatiles have already been shown to attract GBM females in the flight tunnel (Cha et al. 2008). Therefore the GBM-grape complex was used to test the existing theories regarding host plant location. I used flight tunnel assays to observe GBM responses to host and non-host odor sources (cut shoots, extracts, and synthetic blends), and isolate and identify the volatiles that elicit the observed behavior. All antennally active compounds found in grape shoots were also present in non-host plants. Moths displayed higher levels of upwind flight than expected to non-host sources, suggesting discrimination is not occurring at a distance. I used additional flight tunnel assays, to investigate cues necessary to elicit landing on an odor source (water vapor and visual cues). Individual and paired stimuli did not elicit landing, and landing only occurred when plant volatiles, a visual cue, and water vapor were all present, suggesting the cues have a synergistic effect. Interestingly, moths flew upwind a low percentage of the time in response to water vapor alone suggesting the moths can use nonspecific cues to locate a host. In a final study, I explored whether microorganisms living on the surface of plant shoots produced the behaviorally active compounds. Volatile collections of surface sterilized plant shoots indicated the surface microorganisms did not significantly contribute to the volatile profile of the grape shoots as all of the peaks in the volatile profile of sanitized shoots were found in the profile of control shoots. In flight tunnel assays, moths responded similarly to sanitized shoots as they did to control shoots suggesting surface microorganisms did not play a significant role in the production of previously identified blend of behaviorally active volatiles.