FRESHWATER RESEARCH CENTRE 
      
       SPECIALISTS IN AQUATIC RESEARCH AND TRAINING

Water Research Commission Project K8/1004: Pilot study on investigating the current status and changes in the pest blackfly (Diptera: Simuliidae) problem on the Orange River 

Project leader: Dr Nick Rivers-Moore ( blackfly1@vodamail.co.za)

Project Duration: January 2012 to December 2012 - Completed

Project team: Dr Nick Rivers-Moore (FRC), Dr Helen Dallas, Dr Rob Palmer (Nepid Consulting)

Blackfly along the middle and lower reaches of the Orange River are major pests of livestock and labour-intensive agriculture, with losses to the industry calculated to exceed R300 million per annum based on current meat prices.  The problem is attributed to winter high flows, with the main pest species being Simulium chutteri Lewis although there are times when S. damnosum s.l. Theobald and S. impukane de Meillon are problematic.  During 2011, blackfly outbreaks in the middle and lower reaches of the Orange River were noted to have worsened.  It was proposed that current reduced success in controlling S. chutteri using larvicides could be one of the reasons for this.  Alternatively, or additionally, outbreaks may have been due to other species of blackfly not being targeted for control, particularly S. impukane.  The aim of this study was to investigate the likelihoods of these competing reasons, and to propose control options. 

A weight-of-evidence approach was used to assess the most likely causes of the recent problems.  The approach included a comprehensive literature review of the hydraulic preferences of both species, complemented by hydrological analyses of flow patterns from relevant gauging weirs along the middle Orange River to assess available habitat.  A field survey of sites covering fast- and slower-flower habitats in single-channel and anastomosing reaches of the Orange River between Upington and the downstream town of Keimos was undertaken during November 2012.  All data were related back to the classification of Palmer and Craig (2000) which classifies blackfly larvae based on seston concentration and flow velocity preferences.  This was used as a predictive framework for assessing what the most likely species responsible for the outbreaks was likely to be based on flow and habitat conditions.  A third dimension of thermal preferences was considered, based on preliminary laboratory experiments to establish thermal thresholds (LT 50 values).  This was included to test the hypothesis that a species preferring higher flows was more likely to have a lower LT 50 than a species which occurred in lower flow habitats.

A total of six species of Simulium were recorded: S. adersi; S. chutteri; S. damnosum; S. mcmahoni; S. medusaeforme; S. ruficorne, with no individuals of S. impukane found.  S. chutteri was the most abundant and widespread species, while S. ruficorne was restricted to the slow-flowing, highly saline agricultural return flow channels.  Notably, this was the first time that S. medusaeforme has been recorded from the Orange River (Palmer, 2012, pers. comm.).  The blackfly species sampled showed distinct flow velocity preferences, with S. chutteri being the only species preferring flows > 1ms -1, while the remaining species showed preferences for lower flows across a spectrum from 0.35-0.68ms -1.  Analyses of flow data confirmed that flow conditions for a species such as S. impukane are only favourable for 1% of the time for flows of up to 30m 3s -1, although this may be higher in the anastomosed sections. 

Based on the combined evidence, it is unlikely that the outbreaks experienced during 2011 were the result of another species of blackfly additional to S. chutteri.  Hydrologically, it is an anomaly that S. impukane is a problem in the Orange River.  Given its larval habitat preferences, it was noted that there should always be some habitat for S. impukane along certain reaches of the Orange River.  Optimal habitat conditions would be during low-flow periods of clear water.  It is therefore most likely that the main cause of the blackfly problem remains S. chutteri, where sustained high flow volumes and turbidity levels favour this species over the other species of blackfly.  However, during periods of lower flow and lower turbidity, other species of blackfly may be favoured and contribute towards periodic outbreaks.  Additionally, anasotomosing reaches of the Orange River, such as near Keimoes, which are difficult to apply larvicides to because of limited downstream carry, may act as reservoirs of various species of blackfly.  This may contribute towards periodic outbreaks of pest blackfly, caused by multiple blackfly species including S. damnosum and S. impukane

Ongoing monitoring is recommended, where species are recorded, as well as turbidity levels.  What this study has highlighted is that the best avenues for improved blackfly control should focus on the following two areas:

 

  • Management issues around the control programme itself, as previously highlighted by Palmer et al. (2007)
  • Hydraulic studies linked to investigations of downstream carry of larvicides in the anastomosing sections, which are likely to be the least well controlled areas on the middle Orange River.  However, while it is unfortunate that these sections are associated with human settlement areas, it is also important to bear in mind that residual populations of different species of blackfly should be maintained in the range river as parts of its natural ecological functioning, and that the anastomosing sections could be serving a role as refugia.

 

Rivers-Moore NA, Palmer RW & Dallas HF. 2014. Assessing the relative culpability of Simulium (Diptera: Simuliidae) species in recent black fly outbreaks along the middle Orange River, South Africa. Can. J. Zool. 92: 505–513 (2014) dx.doi.org/10.1139/cjz-2014-0008.