We utilize the model legume species Medicago truncatula to assess molecular events that occur in plants in response to damage from chewing insects. Changes in transcript profiles were carried out through analysis of expressed sequence tag libraries and oligonucleotide microarrays. In response to insect herbivory, plants synthesize and release volatile terpenoids that can act as attractants to natural enemies of phytophagous insects. We have focused on expression patterns of key regulatory genes in terpenoid biosynthetic pathways in M. truncatula in response to larval herbivory by the beet armyworm, Spodoptera exigua. Transcript levels of genes encoding enzymes at early steps in both terpenoid pathways were lower in caterpillar-damaged plants. Insect larvae treated to prevent the release of labial gland saliva were used to examine the role of the salivary elicitors in modulating gene expression. Results suggest that glucose oxidase activity from insect saliva serves as an active factor in decreasing transcript levels following herbivory. The data suggest that insect oral factors counteract plant defenses and repress induction of some genes involved in terpenoid biosynthesis. In contrast, transcript levels of genes encoding terpene synthases late in terpenoid biosynthetic pathways, are strongly induced by herbivory. We have identified and characterized expression patterns for several genes predicted to encode terpene synthase proteins.