During oviposition, female Sirex noctilio (F.) (Siricidae) woodwasps inject their conifer hosts with a venom gland secretion. The secretion induces a variety of host physiological changes that facilitate subsequent lethal infection by a symbiotic fungus. A heat-stable factor that can migrate from the site of oviposition in the trunk through the xylem to needles in the crown of attacked pines was purified by size-fractionation and reversed-phase—high-performance liquid chromatography using activity assays based on defense gene induction as well as the needle wilt response in pine shoot explants. An 11-amino acid, posttranslationally modified peptide (SEGPROGTKRP) encoded by the most abundant transcript recovered from S. noctilio venom gland tissue comprised the backbone of the 1,850 Da active factor. Posttranslational modifications included hydroxylation of a Pro residue at position 6 as well as O-glycosylation of Ser and Thr residues at positions 1 and 8, respectively. The O-linked sugars were identical α-linked N-acetylgalactosamine residues modified at the C6 position by addition of phosphoethanolamine. In contrast to the native peptide, a synthetic version of the hydroxylated peptide backbone lacking the glycosyl side chains failed to induce pine defense genes or cause needle wilt in excised shoots. This peptide, hereafter called noctilisin, is related to the O-glycosylated short-chain proline-rich antimicrobial peptides exemplified by drosocin. The noctilisin structure contains motifs which may explain how it avoids detection by pine defense systems.