Rainfall and changes in abundance of Aphodius tasmaniae Hope (Coleoptera: Scarabaeidae) in the lower south-east of South Australia.

Abstract

This paper discusses the causes of three "outbreaks" of the univoltine species, A. tasmaniae, that occurred in the lower south-east of South Australia between 1927 and 1958. Since experimental data had suggested that moisture was the major factor influencing fluctuations in numbers, a comparison was made of the abundance of A. tasmaniae and the "wetness" in each year during three postulated critical periods, namely (1) January-March (summer) when a deficiency of rain could result in a low birth rate; (2) March-May (autumn) when drought could result in a high mortality of first instar larvae; and (3) July-September (winter), when excess rainfall could drown the larvae or promote infection with the pathogenic fungus Cordyceps aphodii Mathieson. The comparison suggested that a marked reduction in numbers followed a long autumn drought on one occasion, and followed a summer with little effective rain on three other occasions; but a reduction in numbers could not be associated clearly with even one wet winter. On the other hand, the greatest increases in numbers followed winters that were drier than average, provided that the weather was not too unfavourable at other times of the year; and the periods of 4 years before each outbreak were noticeable for the unusual prevalence of dry winters and wet summers. It is concluded that the fluctuations in numbers of A. tasmaniae between 1927 and 1958 have been determined largely by weather and by Cordyceps infection.