A study of the ecology of the adult bogong moth, Agrotis Infusa (Boisd) (Lepidoptera: Noctuidae), with special reference to its behaviour during migration and aestivation.

Abstract

Observations have been made during three summers on the ecology and behaviour of the bogong moth, Agrotis infusa (Boisd.) (Lepidoptera: Noctuidae), which occurs in large assemblages at altitudes above about 4000 ft in the Australian Alps. Moths of the spring generation migrate to the, mountains, where they aestivate gregariously in crevices and small caves in certain rock outcrops at or near the mountain summits. Many of these moth "camps" are occupied annually from early November until early April. In the late summer and autumn, the moths migrate back to their breeding grounds. A small proportion of the aestivating moths became intensely active for a period of about an hour just after sunset and just before sunrise each day, when they indulged in a random flight over the outcrops. Activity appeared to be initiated by rapidly changing light intensity, and was accompanied by pigment migration in the eyes of the moths. Food was not sought when the moths were active, although the feeding response was sometinles elicited if they happened to settle on sugary food. However, moisture from rain or dew was apparently ingested as required. Without exception the aestivating moths had not copulated and their ovaries were immature. Their fat-body was well developed and a series of estimations during the summer and autumn showed that the average fat content of the abdomens of males exceeded 61 per cent, and of females 51 per cent, of their dry weight. Attacks by various predators on the aestivating moths seemed to be unimportant, but a mermithid nematode caused some mortality when its larvae emerged from parasitized moths in midsummer. This parasite appeared to be confined to the moth camps, where its life cycle has been adapted to the aestivating habit of its host. Mature larvae of A. infusa were collected during the late winter and early spring of 1952 in pastures over a wide area of New South Wales. They were most abundant on heavy soils on the Western Slopes and Plains and caused serious damage to linseed crops in the north of the State and in southern Queensland. The main areas of heavy, self-mulching soils, where the large moth populations are believed to originate, have been broadly defined. Although A. infusa is a multivoltine species, there is evidence that a facultative diapause occurs in the adults of the spring generation. This, together with migration, enables most of the adult population to avoid the breeding grounds during the summer, when the pastures are dominated by perennial grasses unpalatable to the larvae. The same individuals return to the breeding grounds in the late summer and autumn, when the dicotyledonous annuals upon which the larvae feed have germinated.