Adaptations of locusts and grasshoppers to the low and variable rainfall of Australia

In Australia, where approximately 80% of the land area is arid or semiarid, rainfall is the major factor limiting acridid populations. Rainfall is not only limiting in terms of quantity but also in being highly variable, both temporally and spatially. In this paper, the main adaptations seen in Australian Acrididae to overcome limiting rainfall are discussed with special reference to economically important species. In the arid to semiarid subtropics (lat 23-33°S) rainfall is slightly summer-dominant but extended dry periods can occur in any season. Chortoicetes terminifera, the main pest species, avoids dry periods through embryonic diapause or survives dry periods as quiescent eggs or adults. Migration is critical for survival as it allows locusts to locate areas of localized rainfall. Outbreaks are frequent and develop when enough rain falls to allow continuous breeding over three to four generations. In temperate areas of subcoastal southern Australia, summers are dry and most rain falls in winter or early spring. Austroicetes cruciata, a univoltine pest species, avoids the dry summers by having an embryonic diapause between summer and early winter. In the tropical north (lat 13-23°S), rainfall is strongly summer-dominant and Austracris guttulosa, another univoltine species, survives the dry winter as immature adults in reproductive diapause. Adults mature after feeding on the green vegetation present following early rains of the wet season. But the early rains are often localized and adults migrate until they encounter these localized areas of rain. Adults then mature and lay, but the survival of their offspring eggs and very young nymphs is assured only if there is further rain within 6 weeks. Outbreaks develop only when there are several years of regular rains both in outbreak areas and adjacent areas of the arid zone. Locusta migratoria does not have a stage that can survive extended dry periods. Consequently, it is mainly restricted to subcoastal areas of moderate rainfall. In the main outbreak areas of subtropical Queensland, populations often decline during the commonly dry winters but outbreaks develop when good rain falls in all seasons.