Sirex noctilio, the European horntail woodwasp, is an insect pest that attacks pines and other conifers by ovipositing in the xylem where, in association with its eggs, it deposits a symbiotic fungal pathogen and a venom that causes a number of physiological changes in the host tree. RNA-Seq analysis of the woodwasp venom gland produced an abundance catalog of sequences that include the proteinaceous factors affecting pine defense responses. Among the abundant transcripts we identified one appearing to encode a laccase-like multicopper oxidase (LMCO), as well as others likely encoding enzymes whose activities were previously reported in S. noctilio venom (e.g. esterases, lipases, phosphatases and proteases). The seven most abundant sequences, on the other hand, returned no annotations. Noctilisin, an 11-amino acid peptide isolated from venom, was encoded by the most abundant transcript. This peptide is the heat-stable factor from S. noctilio venom that moves via the transpirational stream from the site of oviposition to the tree crown where it causes rapid wilt (flagging) of needles. Structural characterization of noctilisin identified several post-translational modifications, including the addition of two unusual O-linked sugar residues whose presence was essential for bioactivity. This is the first characterization of a bioactive peptide from Hymenopteran venom that has evolved specifically for its activity in plants.