Cutworms, semiloopers, budworms and armyworms in the genera Agrotis, Chrysodeixis, Heliothis, Mythimna, Persectania and Spodoptera are important pasture and crop pests in Australia. They comprise both cosmopolitan species, such as A. ipsilon, H. armigera, M. separata and S. exempta and endemic species such as A. infusa, H. punctigera, M. convecta, P. ewingii and P. dyscrita. Although the cosmopolitan species are major pests of parts of Asia and Africa, they are, with the exception of H. armigera, less important agricultural pests in Australia than their endemic counterparts. The latter are widely distributed outside cropping areas, because they breed on a wide range of native host plants as well as on introduced crops and pastures and also have the potential to invade cropping areas from native habitats. The cosmopolitan species are largely confined to tropical and subtropical summer crops and improved pastures of north and east Australia where chronic infestations often develop although major outbreaks and migration out of cropping areas are rare. Periodic outbreaks of endemic species result from an unusually favourable growth of vegetation in early autumn, following drought-breaking rains in the inland. Moths arrive in the rain-affected areas as a result of migration and concentration by rain-bearing troughs and depressions. Southward migration of their progeny occurs in spring on warm northerly airflows produced ahead of eastward-moving cold fronts and results in invasions of temperate crops and pastures. Migration also occurs in anticyclonic conditions, resulting in an extensive redistribution of populations and is adapted to the erratic distribution of rainfall in inland Australia. Among cosmopolitan species, only Spodoptera spp. cause outbreaks which are infrequent and occur with the onset of summer rains in coastal and sub-coastal areas. Most movements of cosmopolitan species are confined within cropping areas although migration from coastal to inland crops, under the influence of prevailing SE winds, and in frontal systems, troughs and storm outflows, is also suspected.