Each summer the true armyworm, Pseudaletia unipuncta (Haw.), like many other noctuid species, establishes temporary populations in habitats considerably north of areas where permanent populations persist. Data are presented in support of the hypothesis that P. unipuncta is a seasonal migrant and not a victim of an agricultural “Pied Piper” phenomenon. It is proposed that the spring northward migration occurs to avoid the deleterious effects of summer temperatures and the southward fall one to escape severe winter conditions. It is also argued that both spring and fall migrations are initiated by similar short-day, low temperature conditions, which serve as predictable environmental cues for an impending habitat deterioration.
