When attempts to eradicate an introduced pest species fail and it becomes: established, barrier zones are often used to stop or to slow the spread of the population into uninfested areas. The U.S. Forest Service is currently conducting a Slow-the-Spread (STS) pilot project to evaluate the feasibility of slowing the spread of the gypsy moth (Lymantria dispar L.) in several areas along the population front. To predict the effect of barrier zones on the rate of gypsy moth spread we developed a model that assumes establishment of isolated colonies beyond the expanding population front. These colonies grow, coalesce, and thereby contribute to the movement of the population front. The model estimates the rate of spread from two functions: (1) colonization rate as a function of the distance from the population front and (2) population numbers in a colony as a function of colony age. Eradication of isolated colonies in a barrier zone was simulated by truncating the colonization rate function beyond a specific distance from the population front. The model predicts that eradication of isolated colonies in the STS project will result in a 54% reduction of spread rate. The actual rate of spread in the Appalachian Mountains has declined by 59% since 1990 when eradication of isolated colonies started. Thus, model predictions were close to the observed reduction in the rate of population spread.