Many bioassays of insect species are dependent on the use of laboratory-reared insects. If the purpose of the research is to assess the genetic variance present for an insect trait, e.g., insecticide-resistance monitoring, it is imperative to understand the potential mating dynamics and genetic contributions of adults to the larvae evaluated in bioassays. We report the results of a study utilizing a laboratory-reared colony of Ostrinia nubilalis (Hübner). The changes in the population dynamics (e.g., numbers of males, females, fertile egg masses, mated females) were evaluated. Although the numbers of emerging females, living females, mated females and fertile egg masses changed during the experiment, the percentage of total females that were mated did not change (∼54%). The first of the females to emerge were beginning to die as later-emerging females were mating. Results suggest that experimental designs that rely on laboratory-reared O. nubilalis will need to test larvae from several nights of oviposition to better ensure that the total genetic composition of the population is sampled.