Introduction

There is now strong evidence that the arid zones of Australia, the driest continent on Earth, play a vital role in the annual life cycle of several of the country's most significant insect pests. This chapter provides an overview of recent findings on the migratory behaviour of the Common Army worm Mythimna convecta, and of how it leads to outbreaks in the agricultural zones following population buildup in the dry inland. The following two chapters provide similar treatments for the Native Budworm Helicoverpa punctigera and the Australian Plague Locust Chortoicetes terminifera.

Australia's climate is characterised by relatively low and irregular rainfall: more than 80% of the continent receives less than 600 mm of rain annually (Fig. 7.1a,b). There are major interannual fluctuations in rainfall across the continent which are attributable to the El Niño / Southern Oscillation (ENSO) phenomenon (Nicholls, 1991). The most predictable rainfalls occur in the coastal areas, and regularity and quantity diminish rapidly inland (Division of National Mapping, 1982). The southern part of the continent receives most of its rain in winter in conditions associated with mid-latitude frontal depressions whose centres move eastwards at a latitude of about 40° S. The tropical and subtropical climate zones cover the northern half of Australia and are characterised by rainfall occurring mainly in summer. However, much of the inland of the continent is arid. It is neither far enough north to receive regular monsoon rainfall, nor sufficiently far south to be often visited by rainbearing cold fronts.