Neonates collected as eggs from North Carolina in 1999 were subjected to a 24 h feeding disruption bioassay. The bioassay uses a diagnostic concentration of Bt toxin (0.04 g MVP per ml diet) to simultaneously monitor for the presence of Bt-resistance in tobacco budworm and for the differentiation of species in the budworm-bollworm complex. The assay proved highly effective at identifying the species of these young larvae, which are not practically distinguishable otherwise. With the implementation of an additional dose of 400 g MVP per ml diet for bollworm resistance monitoring, no Bt-resistant insects of either species were identified in these field collections. In order to illustrate the predictive value of this technology, 95% confidence intervals were generated for population estimates based on the bioassay response of different sample sizes.