Sap often exudes from the trunks of the oak, Quercus acutissima. Carpenterworms (Lepidoptera, Cossidae) are frequently observed at these sap sites and believed to be involved in exudation. Field observations from 2002 to 2004 showed that 20–40% of all patches (exuding spots on trees) harbored these insects and that roughly 20% of all patches had only their nests. The peak period of carpenterworm abundance corresponded to the peak number of patches harboring them in all 3 years (August 2002, September 2003, and July 2004). Patches with carpenterworms or their nests had a wider surface area and exuded more sap than patches without them or their nests. Moreover, their experimental removal resulted in decreased exudate quantity, indicating that these insects promote exudation. These results suggest that carpenterworms contribute greatly to sap exudation through wood boring and that their distribution and abundance affect sap resources.