A field investigation of short-range dispersal by female Simulium damnosum s.l.

Female Simulium damnosum s.l. were caught at a site in a savannah zone beside the Mono River in Togo and at varying distances westwards perpendicular to it in an experiment to investigate short-range dispersal by the flies. The numbers of flies caught and the percentages that were parous decreased with increasing distance from the river, the latter suggesting that nulliparous flies disperse on average further than parous flies. The decreases were quantified using statistically significant multiple regressions involving distances from the river and the days since the start of the experiment because there was much day-to-day variation, probably attributable to the flies' gonotrophic cycles. For future modelling purposes a relationship between numbers caught and distance alone was also estimated for both numbers caught and parous rates. Of the different members of the S. damnosum species complex identified in larval samples, S. damnosum s.str. predominated (66.7%), with S. squamosum accounting for 25.5% and the Beffa form of S. soubrense for 7.8%, proportions that were not significantly different from those of adults identified at the river and 10 km away. A small sub-sample of dissected parous flies showed that transmission was occurring at the riverside and at 10 km away from the river.

Automated summary

Female Simulium damnosum s.l. were captured in a savannah zone beside the Mono River in Togo and their short-range dispersal behavior was investigated. The number of flies caught and the proportion of parous flies decreased as the distance from the river increased. Statistical analysis showed that nulliparous flies tended to disperse further than parous flies. The study also identified different members of the S. damnosum species complex and found transmission of the disease occurred both at the riverside and 10 km away.