A computer simulation model was constructed to organize existing data and explore possible results of actions taken to manage colonizing populations ofLymantria dispar(L.). The model consisted of a forest submodel, a gypsy moth population sub model and a tree mortality submodel. Various control policies were examined on poor, medium and good forest sites over an 80-yr period. Current eradication policies fail owing to low density survival of portions of the populations. Periodic management of moderate to high gypsy moth populations is effective and can be accomplished at lower cost due to fewer sprays being required over time. Trees growing on poor sites are most subject to mortality from defoliation due to less foliage surface area available for gypsy moth feeding.