Geographical and Temporal Distribution of the Common Armyworm, Mythimna Convecta (Lepidoptera: Noctuidae), in Eastern Australia: Larval Habitats and Outbreaks

Surveys for juvenile Mythimna convecta throughout the agricultural and arid regions of eastern Australia were conducted from 1986 to 1989. Armyworm populations north of 33 degrees S were generally dominated by M. convecta, and further south by Persectania ewingii. M. convecta was most widely distributed in spring. Incidence during autumn and winter ranged from very low in Victoria to high in northern New South Wales and southern Queensland. Summer infestations were found mostly on the south-east coast where favourable habitats were abundant. Colonised habitats included extremely arid regions, where small numbers of larvae were associated with grasses in temporary watercourses, and the higher-rainfall, eastern regions. The largest infestations occurred in south-east Queensland and north central and north-east New South Wales, particularly after heavy autumn rains. There appeared to be two generations of M. convecta over the autumn/winter period: the first a synchronised event starting on the autumn rains and the second commencing in June/July and comprising a wide spread in age distribution. The progeny of the winter generation are probably the source of most economic outbreaks. Mythimna convecta larvae were collected from subtropical and temperate grasses. In the former, most larvae were found in tussocks, particularly of Dichanthium sericeum and Chloris truncata, which provided a dense, fine-leaf crown and canopy. After good autumn rainfall and vegetative growth, the wiry-stemmed tussocks, including Astrebla spp. and C. ciliaris, were also common hosts. The temperate grasses, particularly Avena fatua and Hordeum leporinum, were the main winter hosts although the greatest densities were found only in thick swards of growth, particularly those that contained dried grass. Two of the largest surveys, in autumn 1987 and 1988, followed periods of heavy rain and provided strongly contrasting results. The 1987 survey of central and south-west Queensland located no M. convecta larvae, indicating that densities were below detection thresholds. The paucity of larvae was attributed to lack of suitable atmospheric conditions to assist moth immigrations and absence of adequate populations in potential source areas. The 1988 survey revealed a major outbreak of M. convecta larvae in south-east Queensland and northern New South Wales. The area received record rains during early April of that year, and the outbreak probably arose from moth migrations from the east and south-east coast. An outbreak of similar scale occurred after further heavy autumn rains in 1989.