An analysis of the outbreaks of the Australian Plague Locust, Chortoicetes terminifera (Walker) during the seasons 1940-41 to 1944-45.

Abstract

Outbreaks of the Australian plague locust, Chortoicetes terminifera (Walker), occurred each locus: season from 1940-41 to 1944-45. The largest outbreak, that of 1942-43, was less severe than those recorded in 1937-38, 1938-39, and 1939-40. Consideration of the course of these recent outbreaks in relation to temperature and moisture has again demonstrated the importance of these factors as determinants of the regional and seasonal occurrence of swarms. The curve of limiting conditions for the persistence of swarm populations, obtained by Key (1942), was a useful guide for the interpretation of events in the present analysis. However, a number of discrepancies occurred. These were due probably to a variety of causes, e.g, the kind of infestation concerned, the crudenessof the monthly Meyer ratio as an index of soil moisture and pasture conditions, and regional differences in the upper moisture limits at high temperatures. The results of the present investigation suggest that:(a) The limiting Meyer ratios for the persistence or development of a spring infestation of small, weakly gregarious swarms at temperatures between 60 and 70¦F. exceed 4-5; either when the September Meyer ratio is 5 or less, or when the spring is preceded by a severe winter drought.( b ) At temperatures around 75'F., the maximum Meyer ratio at which swarms can exist is approximately 20. (c) At temperatures near the estimated hatching threshold of 57.5'F., swarm hatchings can occur at Meyer ratios as low as 4-5. The collapse of the 1942-43 outbreak in part of the main area infested was due apparently to parasitism by an unidentified fly. Similarly, a biotic factor, possibly the same parasite, was largely responsible for the decline of the 1943-44 outbreak during the late spring and early summer.