Distribution and Seasonal Movements of Culicoides brevitarsis Kieffer (Diptera: Ceratopogonidae) at the Southern Limits of its Distribution in New South Wales and their Correlation with Arboviruses Affecting Livestock

The distribution and seasonal movements of C. brevitarsis and the viruses it transmits were studied at the southern limits of the distribution of the vector in the seasons 1990 to 1993. Distinct patterns of spread of the vector from the Manning region to the Hunter Valley and down the southern coastal plain were observed each season. All adult stages of C. brevitarsis apparently were involved in rhese movements. Distributions were compared by averaging data from sites within six zones (starting at I with the Manning Region) separated by the monrh that C. brevitarsis was recorded at a site for the first time. The species was most abundant in autumn and numbers were progressively lower away from the Manning. The number of generations in each zone ranged from seven (zone I) to zero (zone 6) and was dependent on the month that the species was first recorded in a zone. Average generation time was 4.13 0.71 weeks ranging from 5 weeks (November- December) to 3 weeks (March-April). Fewer males than females were recorded, although, within zones, sex ratios were constant. Differences in sex ratios were recorded between the zones. Parous ratios were related to temperature and an increase in parous ratio in summer preceded maximum numbers in autumn. Temperature affected population growth and was suitable for C. brevirarsis activity for varying periods. Wind patterns that were consistent with the observed direction of spread occurred during most of the season. There were no clearly defined effects of rainfall on spread although, once established, numbers increased in autumn following the summer rains. A high incidence of infection of cattle with Akabane and Palyam viruses was recorded in 1990191 and 1992193. Initial levels of seroconversions were related to the population size of vectors which ranged from 2 to >2,000 (as recorded by light traps) in the month prior to infection. The first occurrence of the virus was positively related to the time of first occurrence of the vector with delays in the appearance of the virus of 4 and 3 months in 1990191 and 1992193, respectively.