The geomagnetic field affects all living organisms on the Earth. In this study we investigated the developmental and behavioral effects of rearing Mythimna separata in a near-zero magnetic field (<500 nT) compared to the local geomagnetic field (approximately 50 µT). The near-zero magnetic field produced by a Helmholtz coil system significantly lengthened larval and pupal development durations, increased male longevity, and reduced pupal weight, female reproduction, and the relative expression level of the vitellogenin (Vg) gene in newly emerged females. Moreover, the near-zero magnetic field had a considerable negative effect on the mating ratio of M. separata adults. In addition, the moths in the near-zero magnetic field displayed less flight activity late in the night than those in the Earth's normal geomagnetic field, indicating that the flight rhythm of M. separata may be affected by the near-zero magnetic field. Reduction in magnetic field intensity may have negative effects on the development and flight of oriental armyworm, with consequent additional effects on its migration.