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. A rmyworm 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 ha bitats 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, par ticularly after heavy autumn rains. There appeared to be two generatio ns of M. convecta over the autumn/winter period: the first a synchroni sed event starting on the autumn rains and the second commencing in Ju ne/July and comprising a wide spread in age distribution. The progeny of the winter generation are probably the source of most economic outb reaks. Mythimna convecta larvae were collected from subtropical and te mperate grasses. In the former, most larvae were found in tussocks, pa rticularly of Dichanthium sericeum and Chloris truncata, which provide d 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, partic ularly Avena fatua and Hordeum leporinum, were the main winter hosts a lthough the greatest densities were found only in thick swards of grow th, particularly those that contained dried grass. Two of the largest surveys, in autumn 1987 and 1988, followed periods of heavy rain and p rovided strongly contrasting results. The 1987 survey of central and s outh-west Queensland located no M. convecta larvae, indicating that de nsities were below detection thresholds. The paucity of larvae was att ributed to lack of suitable atmospheric conditions to assist moth immi grations and absence of adequate populations in potential source areas . The 1988 survey revealed a major outbreak of M. convecta larvae in s outh-east Queensland and northern New South Wales. The area received r ecord rains during early April of that year, and the outbreak probably arose from moth migrations from the east and south-east coast. An out break of similar scale occurred after further heavy autumn rains in 19 89.