Review of Blackfly (Diptera: Simuliidae) Control in South Africa
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Onderstepoort Journal of Veterinary Research, 70:307–317 (2003) Review of blackfly (Diptera: Simuliidae) control in South Africa E. MYBURGH1 and E.M. NEVILL2 ABSTRACT MYBURGH, E. & NEVILL, E.M. 2003. Review of blackfly (Diptera: Simuliidae) control in South Africa. Onderstepoort Journal of Veterinary Research, 70:307–317 The medical, veterinary and economic importance of blackflies in South Africa, and the historical development of blackfly control programmes in various South African rivers, are reviewed in this paper. In 1996 it was estimated that blackflies can cause more than R 88 million damages per annum along the middle and lower Orange River where Simulium chutteri is considered the main pest species. A clear link between the construction of dams and the spread of the blackfly problem was shown. Four phases characterize the development of blackfly control in South Africa: (1) during the 1960s blackflies in the Vaal River were controlled with DDT; (2), during the 1970s and into the 1980s blackflies were controlled using water-flow manipulation; (3) when used at strategic times, water-flow manipulation could be used to enhance the effect of natural predator populations; and (4) during the 1990s the organophosphate temephos and toxins produced by the bacterium Bacillus thuringiensis var. israelensis were tested for their efficacy against blackflies. The larvicides temephos and B. thuringiensis proved to be effective and are still used in several control programmes. The lat- est research focuses on the factors that influence adult blackfly survival and annoyance, as well as the development of methods that can be used to protect sheep from blackfly attacks. Keywords: Bacillus thuringiensis var. israelensis, blackfly, economic importance, Orange River, Simulium, temephos, water manipulation INTRODUCTION material and a more constant seasonal flow of water, which in turn creates ideal conditions for immature Blackflies were not considered significant pests in blackflies (Howell & Holmes 1969; Car 1983; Nevill South Africa but developed pest status along many 1988) to increase in numbers in rapids downstream rivers following the building of dams, canals, irriga- of such structures (Chutter 1963; Palmer 1991). tion schemes or hydro-electrical plants along those Moreover, these structures change many of the rivers (Nevill 1988). This is because impoundments rivers from seasonal to perennial and thus create promote the development of suspended organic ideal and continuous breeding conditions for simuli- ids (Myburgh 1999a; b). At present, blackflies are common and significant pests along the Orange, Vaal, Great Fish, Sundays 1 ARC-Onderstepoort Veterinary Institute, Blackfly Field Sta- tion, Upington, South Africa. Present address: P.O. Box 835, and Gamtoos Rivers. Periodic outbreaks are also Duiwelskloof, 0835 South Africa. E-mail: [email protected] experienced along the Eerste, Olifants and Berg 2 ARC Onderstepoort Veterinary Institute, Private Bag X5, Rivers (Edwardes & Palmer 1994; Palmer 1997) Onderstepoort, 0110 South Africa (Fig. 1). It is expected that blackflies may acquire Accepted for publication 12 June 2003—Editor pest status along the Liebenbergsvlei River follow- 307 Review of blackfly (Diptera: Simuliidae) control in South Africa N W E S Pretoria Johannesburg Vaal Barrage Vaal Dam Parys Bloemhof Dam Vaal River Liebenbergvlei Vaalharts Weir Wilge River Upington River Onseepkans Caledon River Tugela River Douglas Kakamas Kimberley Orange River Bloemfontein Pietermaritzburg Prieska Hopetown Gariep Durban Van der Kloof Dam Dam Orange-Fish Tunnel Orange River Clanwilliam Cradock Buffalo Great Fish River Olifants River Sundays River River Fish-Sunday Tunnel East London Berg River Cape Town Gamtoos River Grahamstown Eerste River Port Elizabeth Blackfly outbreaks River / dam Tunnel FIG. 1 Known blackfly problem areas in South Africa ing the completion of the Lesotho Highlands Water Medical, veterinary and economic importance Scheme, although so far this has not happened (Fig. of blackflies 1). Currently 39 blackfly species are known to occur in southern Africa (Palmer 1997; Palmer & De Moor In addition to feeding on sugar, which is used as a 1998). These include five members of the genus fuel for flight (Hocking 1953; Davies, Peterson & Paracnephia and 34 Simulium species (Palmer Wood 1962; Hunter 1977; Sutcliffe 1986), anauto- 1997). The latter genus includes the mammalian genous adult female blackflies also require a blood pests Simulium chutteri and Simulium damnosum meal (Welton, Bass, Ladle & Merritt 1987; Palmer sensu latu and the avian pest species Simulium 1997; Gibson & Torr 1999) for ovarian development adersi and Simulium nigritarse sensu latu. (Davies & Peterson 1956; Peterson 1959; Cross- key 1990). Because of their blood-feeding activity The ARC-Onderstepoort Veterinary Institute (ARC- they are considered ideal disease vectors (Cross- OVI) has been involved in the development and key 1990) and are probably best known for trans- implementation of blackfly control programmes for mitting the filarial nematode Onchocerca volvulus more than three decades. In this paper, the to humans (Nelson 1991; Davies 1994; Hougard, advances made towards the control of blackflies in Yaméogo, Sékétéli, Boatin & Dadzie 1997; Gibson South Africa are reviewed, the medical, veterinary & Torr 1999). The resulting disease known as and economic importance of the pest is outlined onchocercosis or “river blindness” has left more and current and future blackfly research by the than 20 million people infected and millions blind in ARC-OVI is discussed. West Africa and South America (Rodriquez-Perez, 308 E. MYBURGH & E.M. NEVILL Reyes-Villanueva & Rodriquez 1995; Samba 1995; reduced efficiency of agricultural and industrial Hougard et al. 1997; Molyneux & Davies, 1997). workers, interference in recreation, and reduced Furthermore, in humans, the bites of some blackfly real estate values (Mason & Shemanschuk 1990). species can cause allergic reactions known as “blackfly fever” or simuliotoxicosis (Crosskey 1990; Although there are numerous reports of blackfly Palmer 1997). This condition is characterized by epidemics in South Africa, only Steenkamp (1972) swelling, itching, haemorrhage and oedema (De has made a detailed study of the economic impact Villiers 1987) which, in severe cases, requires med- of the pest. He reported physical destruction of the ical attention (Mason & Schemanschuk 1990). teats of some cows and a reduction in milk produc- tion of up to 35 kg milk per week per cow (30–50% In animals, blackflies have been implicated in the reduction). In poultry he found a 10–15% reduction spread of leucocytozoonosis (Anderson & Voskuil in egg production. Other reports confirm that cows 1963; Crosskey 1993), bovine onchocercosis, can loose their udders and sheep their ears as a (Crosskey 1990; Hadi & Takaoka 1995), the cyto- result of secondary infections that develop because plasmic polyhedrosis virus, the iridescent virus, of blackfly wounds (De Moor 1986). During a more vesicular stomatitis virus (Bernardo & Cupp 1986; recent, though smaller-scale survey along the Vaal Bridges, McCluskey, Salman, Hurd & Dick 1997; River, farmers reported that blackflies killed lambs, Maré 1998), avian trypanosomes (Crosskey 1993), caused losses of 60% in total farm stock production myxomatosis (Williams & Williams 1966; Kettle and reduced milk production by as much as 55 l per 1984) and Dirofilaria species (Simmons, Edman & cow per week (O’Keeffe 1985). In 1996 the North- Bennett 1989). It has also been shown that allergic ern Cape Agricultural Union estimated that black- reactions to blackfly bites, similar to that described flies, if left uncontrolled, can cause potential losses in humans, can lead to the death of cattle (Mason of more than R88 million per annum to the stock & Schemanschuk 1990). In South Africa, simuliids industry along the Orange River (Palmer 1997). have been implicated in the spread of two pathogens This figure is based on a 25 % reduction in lamb to animals namely a Chlamydophila (previously production and excludes other figures such as land Chlamydia) sp., that causes blindness in sheep and depreciation and tax losses to the State. abortion in cattle (De Moor 1982a), and Rift Valley Fever virus. Mechanical transmission of this virus, Today S. chutteri is considered to be the most by S. chutteri, may have contributed to the major important blackfly pest species in South Africa. This Rift Valley Fever outbreak between Prieska and species occurs along the Orange, Vaal and Great Groblershoop in 1975 (McIntosh, Jupp, Dos Santos Fish Rivers, and, to a lesser extent, the Sundays & Barnard 1980). River, but it is most abundant and causes the largest economic problems along the middle and Blackflies readily attack exposed parts of the bod- lower Orange River (Palmer 1997). Simulium chut- ies of livestock, such as the eyes, ears and teats teri is a large-river species endemic to southern (Anderson & Voskuil 1963), and the resulting Africa which, under favourable conditions, can wounds are prone to secondary infections which become the most abundant blackfly species in this sometimes lead to the death of animals (Palmer region with larval densities exceeding 500 000/m2 1997). Blackflies also cause considerable irritation (Palmer & De Moor 1998). It is a multivoltine spe- (annoyance) to livestock (Anderson & Voskuil 1963; cies with 11–13 generations per annum (Palmer, Crosskey 1990; Kok, Fourie & Oberem 1994). In Edwardes & Nevill