Population Dynamics of the Australian Plague Locust, Chortoicetes Terminifera (Walker), in Central Western New South Wales. I. Reproduction and Migration in Relation to Weather.

Abstract

On three adjacent experimental plots in central western New South Wales, from 1971 to 1974, population fluctuations of the Australian plague locust were interpreted in terms of the interactions between reproductive rates (Ro) between generations and net migration losses or gains (M plus or minus ) within generations. Changes due to variations in net migration fluctuated over a greater range than those due to variations in reproductive rates, and were the major cause of the population fluctuations during the 16 generations studied. Variations in rainfall during breeding accounted for nearly 90% of the variance in reproductive rates, provided rain followed relatively dry periods. Changes caused by migration resulted from episodic, often long-range and generally southward displacements of adults at night in the gradient wind during disturbed weather. The frequency and intensity of migration depended primarily on the availability of potential emigrants and secondarily on the incidence of weather favourable for takeoff and prolonged flight. The incidence of rainfall before periods of migration was higher than expected. Adults thus migrated into areas already favourable for successful breeding. Long-term median rainfall during breeding at Trangie gave mean reproductive rates greater than 1 per generation. Hence, population surpluses tend to be produced and generally disperse south to areas less favourable for breeding. There was close correspondence between the occurrence of heavy drought-breaking rains, leading to substantial population increases and migrations, and the observed incidence of outbreaks in this area. Changes in the relative importance of reproduction and migration in the drier breeding areas further inland are discussed in relation to historical changes in the distribution and abundance of the plague locust.