Over-wintering ecology of Lucilia cuprina in south-eastern Australia

Flystrike is a major problem for the Australian sheep industry, estimated to cost at least $280 m annually. A review of blowfly strike and the ecology of the primary sheep blowfly (Lucilia cuprina) in south-eastern Australia revealed that some aspects of the biology of L.cuprina were still poorly understood. In addition, no recent studies had been conducted in Western Victoria, a high winter rainfall area with a large population of Merino sheep.\ud \ud Consequently, a 2-year study on the seasonal timing of larval over-wintering and spring emergence of flies was undertaken on a farm near Ballarat, in Western Victoria, during 2005 and 2006. Replicated cohorts of postfeeding larvae of L.cuprina were deposited regularly, and the daily emergence of flies and meteorological observations were recorded.\ud \ud Larvae deposited during spring, summer and early autumn developed rapidly, with the time to median emergence of flies taking 30 days in spring, decreasing to 10 days as soil temperatures increased in summer. A transitional phase of larval development was observed during mid-autumn of both years (11-26 April), slightly later than in a previous study at Canberra. Some larvae deposited in this period pupated immediately, whereas others entered an arrested development, emerging as flies the following spring. Induction of this arrested development was associated with sustained low soil temperatures (≤ 10°C). In both years, over-wintering larvae resumed their development in late winter after soil temperatures exhibited a rising pattern or consistently stayed above 11°C. The mean dates for the first and median emergence of flies in spring were 4 and 21 October in 2005, and 1 and 12 October in 2006, respectively. This emergence of flies from over-wintering larvae was synchronous, regardless of the date larvae were deposited. Mortality of over-wintering larvae was high, although quite variable between deposits, being 95% in 2005 and 68% in 2006.\ud Sequential sampling of larvae deposited in May 2006 indicated that pupation of over-wintering larvae occurred between 29 August and 14 September. Trapping of free-ranging flies at the site found that fly numbers followed a bimodal pattern, with a large peak in November and a smaller peak in early March.\ud \ud Validation of six predictive models of L.cuprina development, using data from this study, showed none could predict the last generation of flies in autumn, the time when larvae entered arrested development, or the occurrence of a split emergence. A simple linear model ('Temsum'), using actual soil temperatures and 1 July as start date, was best able to predict the first generation of flies in spring.\ud \ud Ecological studies such as this will help to refine Integrated Parasite Management (IPM) programs for the control of flystrike. A number of aspects of L.cuprina biology were identified that deserve further study, in particular the development of immature stages at low temperatures. This information would also support the development of more complex models simulating the population dynamics of L.cuprina