Pin oak, Quercus palustris Muenchhausen, is the primary host for the gall wasp Callirhytis cornigera (Osten Sacken). Woody stem galls formed by C. cornigera may be infested by the dogwood borer, Synanthedon scitula (Harris), an important pest of flowering dogwood, Cornus florida L. Previous research has shown that S. scitula has a bimodal seasonal flight pattern, with peaks in late spring and midsummer. We tested the hypothesis that moths emerging from dogwoods largely account for the first flight pulse, whereas emergence from stem galls contributes disproportionately to the second pulse. Seasonal flight activity of S. scitula was monitored with pheromone traps baited with Z,Z-3,13-octadecadien-1-ol acetate. Traps were hung near plantings of dogwoods in suburban landscapes or near heavily galled pin oaks. Borer emergence from dogwood was monitored by sampling infested trees for pupal exuviae, and from galls that were collected and held in outdoor rearing cages. The impact of S. scitula on C. cornigera larvae was assessed by weighing, measuring, and dissecting galls. Flight activity of S. scitula began on 5 May and ended on 13 October 1999, with peaks in late May and in late July to early August. The flight pattern was similar for the two types of trapping sites, and moths emerged from both hosts during both flight periods. Proportionately more moths emerged from dogwoods during the first flight pulse than during the second, but emergence from galls was nearly evenly divided between the two flight peaks. We therefore reject the hypothesis that emergence of borers from galls contributes disproportionately to the second flight period. Approximately 12–15% of stem galls (2–3 yr old) contained S. scitula larvae. Feeding and tunneling by borers contributed to gall desiccation and reduced horn development, but rarely killed C. cornigera larvae. This study has implications for management of S. scitula because borers emerging from horned oak galls may represent a threat to dogwood.