Distribution, seasonality and relative abundance of Stomoxys calcitrans (stablefly) (Diptera: Muscidae) in New Zealand

AIMS: To determine the current distribution, seasonality and relative abundance of Stomoxys calcitrans in New Zealand in order to provide information that could be used to assess risks of transmission of equine infectious anaemia (EIA). METHODS: Adhesive yellow traps were distributed to schools throughout New Zealand and used to detect the presence of S. calcitrans between November 1999 and April 2000 at sites considered likely to be a focus for S. calcitrans breeding and activity. In addition, researchers undertook monthly trapping at six other sites between August 1999 and June 2000 to measure the duration and seasonal periodicity of S. calcitrans flight activity. Veterinary practices and farmers were also surveyed to provide anecdotal evidence of the presence or absence of S. calcitrans, particularly in areas where no flies were trapped by schools. RESULTS: Stomoxys calcitrans was found to occur in both North and South Islands, but principally in locations where dairy farming occurred. The fly was active during most months of the year, except perhaps during July and August, and was particularly active in warmer North Island districts such as the Waikato. Peak activity was recorded from January to May. The fly was more abundant in Northland, Auckland, Waikato, the Marlborough Sounds and Nelson than in other districts, as determined by the number of occasions flies were caught relative to the number of traps set. There are only a few areas, such as around Taupo, the Otago Lakes, Central Otago and the Mackenzie Basin, where S. calcitrans was not trapped or, on the basis of anecdotal evidence, thought not to occur. CONCLUSIONS: Given that other potential vectors of EIA are absent from New Zealand, Taupo, the Otago Lakes, Central Otago and the Mackenzie Basin districts could be considered areas of low risk for EIA transmission to horses. In addition, teachers and school children were capable, in most instances, of supporting a nationwide survey providing the methods were simple and the aims few.