The efficacy of Bacillus thuringiensis var. israelensis (B.t.i.) and temephos in controlling the pest blackfly
Simulium chutteri Lewis along the middle Orange River between 1990 and 1995, was assessed.
Larvicides were applied by helicopter to rapids and riffles between Hopetown and Onseepkans, a river
distance of 807 km. Larvicidal efficacy was based on the change in larval abundance at selected sites
before and after each treatment. The success of the control programme was assessed independently
by local farmers, who ranked adult blackfly annoyance on a 4-point scale.
Before treatment, blackfly annoyance showed consistent peaks in spring, and sometimes in autumn,
and levels were unacceptably high for between 17 and 36 weeks of the year. After treatment started,
blackfly annoyance levels were reduced significantly. The number of annual treatments necessary to
reduce blackfly annoyance to acceptable levels was highly variable (3-13), and depended on river conditions,
as well as the efficacy and timing of each treatment. During low-flow conditions (<50 mᵌ/s), applications
became increasingly difficult in braided sections of the river, and dosage calculations were inaccurate
because of local abstraction and return flows. Both larvicides worked well in winter (water
temperature 11-13 °C). Control of the spring outbreak can be planned well in advance, with the first
treatment starting in mid July. A flexible protocol is required to control outbreaks at other times of the
year. We recommended the use of B.t.i. for most applications, with increased dosages during algal blooms
(> 1 500 cells/ml). The use of temephos in the Orange River should be considered only during algal
blooms or when flows exceed 300 m³/s. We conclude that helicopter application of larvicides is an effective
method of controlling pest blackflies along the middle Orange River.
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