In Michigan, 27–70% of the prepupae ofHeterocampa manteo(Dbldy) remained in diapause longer than 1 yr and up to 31% diapaused for 2 yr. Diapause was facultative but the factors of development were not determined. Prolonged prepupal diapause ofH. manteois believed to contribute to cyclical outbreaks and provides a means of avoiding population collapse due to starvation and parasitism. Application of insecticides over extensive areas could, by depressing univoltine parasites ofH. manteo,reduce the parasitism rates of thoseH. manteoin prolonged diapause and emerging the year subsequent to insecticide applications.