Species composition and cyclical changes in numbers of savanna blackflies (Diptera: Simuliidae) caught by suction traps in the Onchocerciasis Control Programme area of West Africa

AbstractTen large suction traps were operated continuously, day and night, throughout the wet season of 1977 at four widely separated places within the area of the Onchocerciasis Control Programme in the Volta River Basin (OCP) in West Africa. At least ten species of Simulium s.l. were trapped and the daily catches were analysed qualitatively and quantitatively. In the total catch of 9189 flies there were 187 males and 9002 females; among the latter only six were gravid and only five contained a full blood-meal. Cyclical changes in numbers, with periodicities ranging from 10 to 20 days, were analysed in series of overlapping Gaussian distributions, each possibly reflecting the rise and fall of a distinct population. Similar patterns in the sequence of cycles were shown by S. adersi Pomeroy, S. ruficorne Macq., S. evillense Fain, Hallot & Bafort and other species at a particular trapping site. Patterns differed between catching sites, except in the general tendency for population maxima to increase during the season. In some species, notably S. adersi, populations decreased in late July and early August before increasing greatly in September (end of the rains) prior to the October onset of the dry season. Among the species caught, S. evillense was present at all four trapping sites and particularly abundant at two of them, although the species has never been recorded before in West Africa and its early stages remain undiscovered there despite special searches for them since the capture of adult flies. By contrast, only four specimens of S. hargreavesi Gibbins were trapped although this species is abundant in local breeding sites. Diel flight periodicity was recorded over a one-week period at the extreme end of the wet season and showed that most flies were caught in the daytime; S. ruficorne showed a greater tendency to a unimodal periodicity than other species. The behaviour of flies at the time of capture is discussed, including a consideration of the local wind speeds and the air speed of Simulium s.l.

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The pattern and speed of displacement of females of Simulium damnosum Theobald s.l. (Diptera: Simuliidae) across the Onchocerciasis Control Programme area of West Africa in 1977 and 1978

Bulletin of Entomological Research ◽

10.1017/s0007485300014188 ◽

1985 ◽

Vol 75 (1) ◽

pp. 73-92 ◽

Cited By ~ 7

Author(s):

C. G. Johnson ◽

J. F. Walsh ◽

J. B. Davies ◽

S. J. Clark ◽

J. N. Perry

Keyword(s):

West Africa ◽

Control Programme ◽

The South ◽

Wet Season ◽

Breeding Sites ◽

Simulium Damnosum ◽

Biting Rate ◽

Western Border ◽

Alternative Sources ◽

Onchocerciasis Control

AbstractBreeding of Simulium damnosum Theobald s.1., the vector of Onchocerca volvulus, had been eliminated by 1977 from about 654 000 km2 of the Onchocerciasis Control Programme in the Volta River Basin Area (OCP) of West Africa. Nevertheless, migrating adult females continually invaded the controlled area, being blown on the prevailing south-westerly winds from uncontrolled breeding sites beyond the south-western border of the OCP area. Graphs of numbers of females caught per man per day (the daily biting rate) throughout the wet season, March to October, at 16 sites in 1977 within the OCP controlled area were remarkably similar in pattern from site to site over a range of about 500 km downwind. In 1978, only seven sites within the OCP area were similarly monitored, and the graphs were different in pattern from those in 1977, when they each consisted of three or four well-defined waves or cycles of daily biting rate which could be easily identified and traced across country, their times of occurrence lagging increasingly as the distance of the site from the south-western border of the OCP area increased. Four methods were used to demonstrate and estimate the lag: visual comparison of seasonal graphs; comparison of the mean dates of cycles at different sites and the regression of these dates on distance from the south-western border; the dates when particular cumulative percentages of the total season's catch occurred at each site and their regression on distance; and principal coordinate analysis of the data and its relation to distance from the south-western border. Statistically significant lags were demonstrated and averaged one day for every 10–30 km from the border in 1977, which indicated an average speed of migration across country. In 1978, a rate of one day per 7–35 km was indicated. Where some graphs at outlying sites were anomalous, possible alternative sources of immigrant flies are considered. The possible behaviour of flies in causing the lag is discussed.

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Progress in controlling the reinvasion of windborne vectors into the western area of the Onchocerciasis Control Programme in West Africa

Philosophical Transactions of the Royal Society of London Series B Biological Sciences ◽

10.1098/rstb.1990.0141 ◽

1990 ◽

Vol 328 (1251) ◽

pp. 731-750 ◽

Cited By ~ 42

Keyword(s):

West Africa ◽

Vector Control ◽

Sierra Leone ◽

Control Programme ◽

West African ◽

Breeding Sites ◽

Western Area ◽

Monsoon Winds ◽

First Time ◽

Onchocerciasis Control

Since vector control began in 1975, waves of Simulium sirbanum and S. damnosum s.str ., the principal vectors of severe blinding onchocerciasis in the West African savannas, have reinvaded treated rivers inside the original boundaries of the Onchocerciasis Control Programme in West Africa. Larviciding of potential source breeding sites has shown that these ‘savanna’ species are capable of travelling and carrying Onchocerca infection for at least 500 km northeastwards with the monsoon winds in the early rainy season. Vector control has, therefore, been extended progressively westwards. In 1984 the Programme embarked on a major western extension into Guinea, Sierra Leone, western Mali, Senegal and Guinea-Bissau. The transmission resulting from the reinvasion of northern Cote d’Ivoire and Burkina Faso has been reduced by over 95%, but eastern Mali has proved more difficult to protect because of sources in both Guinea and Sierra Leone. Rivers in Sierra Leone were treated for the first time in 1989 and biting and transmission rates in Sierra Leone and Guinea fell by over 90%. Because of treatment problems in some complex rapids and mountainous areas, flies still reinvaded Mali, though biting rates were approximately 70% lower than those recorded before anti-reinvasion treatments started. It was concluded that transmission in eastern Mali has now been reduced to the levels required to control onchocerciasis.

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