The recent increase of ground-level ozone (O₃) may affect the quality of leaves and consequently affect the degree of grazing by insects. Using a free-air O₃ enrichment system, we found lower grazing damages on leaves of Japanese white birch by alder leaf beetle under elevated O₃. Contrary to the result found in the field, choice and non-choice style of feeding experiments showed that leaves exposed to elevated O₃ were preferred by the leaf beetles due to lower defensive chemical production. To clarify this contradictory phenomenon, we pointed out the possibility of BVOC (Biogenic Volatile Organic Compounds) working as signal chemicals for insects to locate their host plants. In this research, two groups of BVOC, monoterpene (MT, C₁₀H₁₆) and sesquiterpene (SQT, C₁₅H₂₄) were of interest. Although many compounds were emitted from leaves of the birch, both MT and SQT had no significant difference in the basal emission rate regardless of O₃ exposure. Therefore, BVOC emission in itself does not work as a factor that changes grazing activity of the leaf beetles, however, the blend ratio of BVOC should be considered. Under elevated O₃, BVOC dynamics should be studied considering the reaction with O₃ after emitted into the atmosphere.