Gypsy moth management is divided into three components: eradication, suppression, and transition zone management. All three components require knowledge of the boundaries that delimit these areas. Additional interest is also placed on the relationship between population spread and defoliation to prepare for the gypsy moth advance in new areas and minimize its impact. We developed relationships between advancing population boundaries, which were estimated using an algorithm implemented under the USDA Forest Service Gypsy Moth Slow-the-Spread Project and defoliation records collected by State and Federal agencies. We used current data from Wisconsin, West Virginia, and Virginia and historical data from the lower peninsula of Michigan. We observed that in West Virginia, Virginia, and Michigan, defoliation generally occurred in areas where moth abundance exceeded 300 male moths per pheromone-baited trap (i.e., the 300-moth population boundary), whereas in Wisconsin, it generally occurred between the 100- and 300-moth population boundaries. We also detected temporal changes in Michigan in the relationship between boundaries and defoliation, where the transition time between the 10-moth population boundary and defoliation was 4–5 yr. Recent data from Wisconsin suggest a similar transitional time, whereas recent data from West Virginia and Virginia do not seem to contradict an earlier study suggesting a transition time of roughly 8 yr.