The European corn borer, Ostrinia nubilalis (Hübner), is a major pest of US corn, Zea mays L. Genetically-modified corn hybrids (Bt-corn) are increasingly used for the control of O. nubilalis, but development of insect resistance to Bt-corn is a potential concern. To sustain the use of Bt-corn, the US Environmental Protection Agency requires expression of a high dose of Bt toxin in the plants and provision of refugia. Success of the current Insect Resistance Management (IRM)-strategy depends on resistant moths emerging from Bt-corn mating with susceptible moths emerging from refugia. A significant proportion of mating is presumed to occur within grassy aggregation sites. Understanding this behavior will have implications for IRM because adult dispersal patterns directly affect patterns of mating and gene flow. The objective of this study was to examine the influence of pheromone lure, plant density, and distance from adult-eclosion site on O. nubilalis density among artificial aggregation sites of spring wheat, Triticum aestivum L., Siberian foxtail millet, Setaria italica (L.) Beauvois, and oats, Avena sativa (L.). Laboratory-reared moths that were fed meridic diet and dye were marked and released as pupae from corn near the aggregation sites. Although feral moths were collected in the aggregation sites in relative abundance, the marked moths were not found frequently. Our data indicate that a significant proportion of newly-emerged adults were not colonizing the aggregation sites, suggesting that recently-eclosed adults leave their natal field or point of adult eclosion and do not settle in the first acceptable grassy sites encountered.