Abstract

<jats:p>
During the rainy season of 1980 the southeastern flank of the W.H.O. Onchocerciasis Control Programme was extended to include potential sources of
<jats:italic>Simulium squamosum</jats:italic>
,
<jats:italic>S. damnosum s.s.</jats:italic>
and
<jats:italic>S. sirbanum</jats:italic>
, vectors of onchocerciasis that were reinfesting the controlled zone in Togo. The experimental extension with a radius of 100 km from the reinvaded sites only partly reduced the scale of the reinfestations. To examine whether some
<jats:italic>S. squamosum</jats:italic>
, the principal species involved in Togo, were travelling more than 100 km, a larger experimental extension was treated in 1981, which further reduced the numbers reinvading. The results of these trials and data on the ages and species compositions of the reinfesting flies, their likely sources and the meteorological conditions in the area are discussed. It was concluded that
<jats:italic>S. squamosum</jats:italic>
may travel for 150 km or more in Togo and that
<jats:italic>S. damnosum</jats:italic>
and
<jats:italic>S. sirbanum</jats:italic>
travel similar distances in both Togo and Benin.
</jats:p>