dc.description.abstract Exotic pests are serious threats to North American ecosystems; thus, economic analysis of decisions about eradication, stopping, or slowing their spread may be critical to ecosystem management. We present a model to analyze costs and benefits of altering the spread rates of invading organisms. The target rate of population expansion (which may be positive or negative) is considered as a control function, and the present value of net benefits from managing population spread is the criterion that is maximized. Two local maxima of the present value of net benefits are possible: one for eradication and another for slowing the spread. If both maxima are present, their heights are compared, and the strategy that corresponds to a higher value is selected. The optimal strategy changes from eradication to slowing the spread to finally doing nothing, as the area occupied by the species increases, the negative impact of the pest per unit area decreases, or the discount rate increases. The model shows that slowing population spread is a viable strategy of pest control even when a relatively small area remains uninfested. Stopping population spread is not an optimal strategy unless natural barriers to population spread exist. The model is applied to managing the spread of gypsy moth (Lymantria dispar) populations in the United States.