The ecology of Oncopera fasciculata (Walker) (Lepidoptera: Hepialidae) in South Australia. 4. Fluctuations in numbers during 1948-1955 and an analysis of their causes.

Abstract

The development of the pastoral industry in County Grey has increased the numbers of Oncopera fasciculata (Walker) in the area. Even during unfavourable periods between outbreaks the numbers are greater than they used to be. Although the outbreak of 1948-50 was the greatest that has been recorded, not all of the suitable places were occupied during the outbreak, nor was all of the food eaten except in restricted areas. With food and shelter in excess of needs, the numbers reached during 1948-50 depended mainly on weather. Predators have very little influence on the numbers of 0. fasciculata except when unfavourable wet weather forces larvae, prepupae, and pupae from their subterranean burrows. No parasites were found during this study. The evidence suggests that an increase in the numbers of 0. fasciculata in any one generation may be expected when there are fewer than about 45 days without rain during the late egg and early larval stages (October-December). Extensive damage to pastures may be expected when there is a series of such years; this has happened only once during the past 95 years (1947-49), and was followed by the outbreak of 1948-50. The numbers of 0. fasciculata on the poorly drained soils are likely to increase whenever a dry, late winter (fewer than about 2 or 3 days of waterlogging during July-September) is followed by a wet, late spring. Changes in the distribution and abundance of 0. fasciculata during 1948-55 are described and a probable explanation is put forward.