A model (BIOCONTROL-PARASITE) that simulates host and parasitoid population interactions, parasitism rates, and plant damage is described. BIOCONTROL-PARASITE can simulate many different species of phytophagous insects, parasitoids, and plants because specifics of the insect and plant biology are entered through menus at the beginning of a simulation. Augmentative releases of egg and nymphal parasitoids also can be simulated. The accuracy of BIOCONTROL-PARASITE predictions was tested by comparing them with field estimates ofLygus hesperus(Knight), in an alfalfa agroecosystem withAnaphes iole(Girault), andLeiophron uniformis(Gahan) as egg and nymphal parasitoids, respectively. BIOCONTROL-PARASITE predictedL. hesperusnymphal and adult populations, egg and nymphal parasitism rates, and plant state with reasonable accuracy. Simulations with and without parasitoids predicted that egg and nymphal parasitoid populations created by way of augmentative releases would reduce theL. hesperuspopulation, but would not greatly decrease feeding damage and subsequent loss of alfalfa seed. Simulations predicted that releasing both egg and nymphal parasitoids would decreaseL. hesperusegg, nymphal, and adult populations slightly below levels achieved by releasing only egg parasitoids. Estimates of seed yields from simulations with and withoutL. hesperusindicated that about one-third of the alfalfa seed crop would be eaten byL. hesperus.Using BIOCONTROL-PARASITE as a tool for identifying parasitoid species that might be candidates for mass rearing and release as biocontrol agents and to test the applicability and realism of data collected from laboratory experiments of host-parasitoid interactions is discussed. The assumptions and limitations of the model also are described.