A population of 570 adult Cyrtobagous sp. (Coleoptera: Curculionidae) released into two fields cages containing the host plant S. molesta Mitchell declined to about 40 adults over nearly 7 mo. or about 4 generations. Eleven weeks after adding another 592 adults to the cages, and starting weekly application of urea fertilizer, there were approximately 3000 adults present. A field enclosure experiment in which insects were placed on unsprayed control plants, on plants sprayed with N fertilizer and on plants sprayed with N + P fertilizer produced additional evidence that the rate of insect population increase could be manipulated by application of N to the host plant. On release from cages, and in the unsprayed control treatment, populations of the insect increased exponentially despite there being very low concentrations of N in the host plant. Evidence is presented which suggests that by damaging its host, the insect can increase the quality of the host as food. It is suggested that there might be a ''critical'' population density of the insect for particular population densities and low N-contents of the host, below which a population of the insect will decline and above which it will increase. Slow dispersal by the insect, despite its ability to fly, is described for several sites and may be the result of selection in favor of populations remaining above ''critical'' densities. The dispersal behavior contrasts markedly with that of insects suspected of responding to induced defenses in their host plants. Some practical implications of the results for biological control of weeds and management of crop pests are discussed.