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.

The armyworm Mythimna roseilinea (Walker) is a major pest of grain crops in South China.  So far little is known about its basic biology and ecology, making prediction of population dynamics difficult.  This study examined the relationships of individual development and population growth with temperature based on an age-stage, two-sex life table of M. roseilinea reared on maize in the laboratory at 18, 21, 24, 27 and 30°C.  The highest values of net reproductive rate (R₀) and fecundity were observed at 21 and 24°C, respectively.  Both the intrinsic rate of increase (r) and finite rate of increase (λ) increased significantly and mean generation time (T) decreased significantly with increasing temperature.  M. roseilinea was able to develop, survive and lay eggs at all temperature regimes tested.  Development rates of the egg, larval, pupal, as well as the whole pre-oviposition stages had a positive linear relationship with temperature.  The calculated development threshold temperatures of egg, larval, pupal, pre-oviposition and total pre-oviposition stages were 13.29, 8.39, 14.35, 7.42, and 12.24°C, respectively, and their effective accumulated temperatures were 63.59, 445.00, 211.11, 89.02, and 698.95 degree-days, respectively.  These results provide insight into temperature-based phenology and population ecology of this insect pest and will allow population prediction and management available in the field.