Maize plants attacked by lepidopteran larvae emit a volatile mixture that consists mostly of the sesquiterpene olefins, (E)-alpha-bergamotene and (E)-beta-farnesene. These volatiles are produced by the herbivore-induced terpene synthase TPS10 and attract natural enemies to the damaged plants. A survey of volatiles in maize lines and species of teosinte showed that the TPS10 products (E)-alpha-bergamotene and (E)-beta-farnesene are consistently induced by herbivory, indicating that release of TPS10 volatiles is a defense trait conserved among maize and its wild relatives. Sequence comparison of TPS10 from maize and its apparent orthologs from four teosinte species demonstrated stabilizing selection on this defense trait. The teosinte volatiles and the enzymatic activity of the apparent TPS10 orthologs were not completely uniform but varied in the ratio of (E)-alpha-bergamotene to (E)-beta-farnesene products formed. We identified a single amino acid in the active center which determines the ratio of (E)-alpha-bergamotene to (E)-beta-farnesene and has changed during the evolution of maize and teosinte species. Feeding experiments with the substrate (Z,E)-farnesyl diphosphate revealed that this amino acid controls the rate of isomerization of the (E,E)-farnesyl carbocation intermediate to the (Z,E)-configuration.