Significance Whether insect sesquiterpenes are synthesized de novo, derived from plant precursors, or produced by symbionts is often unknown. We identified an evolutionarily novel terpene synthase gene family in the striped flea beetle, a notorious pest of Brassica crops in North America and Asia, and one of these genes was shown to be directly involved in the biosynthesis of the male-specific sesquiterpene aggregation pheromone. Phylogenetic and gene structure analyses indicate that an expansion of the trans -isoprenyl diphosphate synthase gene family in the ancestor of the subfamily Galerucinae enabled functional diversification toward this terpene synthase gene family. These insights into how flea beetles synthesize their aggregation pheromones may lead to new approaches for pest management.