Assessing the relative culpability of Simulium (Diptera: Simuliidae) species in recent black fly outbreaks along the middle Orange River, South Africa

Black fly along the Orange River are major pests of livestock and labour-intensive agriculture, causing annual estimated industry losses in excess of US$30 million. The problem is attributed to winter high flows, with the main pest species being Simulium chutteri Lewis, 1965, although Simulium damnosum Theobald, 1903 and Simulium impukane de Meillon, 1936 may also be periodically problematic. During 2011, black fly outbreaks along the middle Orange River were perceived by farmers to have worsened and attributed to S. impukane. Here, we investigate the likelihood of this being the case, using a weight-of-evidence approach incorporating ecohydrological data. Results showed that it is unlikely that the 2011 outbreaks were caused by S. impukane, and more likely that the main outbreak cause remains S. chutteri. Sustained high flows and turbidity levels favour S. chutteri species over the other species of black fly, while flow conditions for a species such as S. impukane were favourable for 1% of the time only. However, during periods of lower flow and lower turbidity, other species of black fly may be favoured and contribute towards periodic outbreaks. We conclude that black fly control should focus on management issues around the control programme.