Two field trials of obstruction (‘ obstructive clearing ’) as a means of control of Glossina palpalis (R.-D.) in locations representative of the Northern and Southern Guinea Savannah zones of Nigeria, are described.

In neither case was complete eradication achieved, pockets of G. palpalis remaining from the beginning at certain points on the main stream. Greater success was obtained in the smaller tributaries, which lent themselves more to obstruction. There was little penetration of G. palpalis for any great distance into obstructed reaches from natural ‘ reservoirs ’ left abutting on to the obstruction, but a trial of the ‘ protective ’ value of obstruction failed, owing to complete penetration in some strength of 300 yd. of obstructed stream on either side of the point to be protected. In neither case was immigration of flies from elsewhere into the trial areas an important factor. Although, at first, the surviving fly population was very localised, there were later indications that dispersal was beginning to take place.

Some accidents which may happen to obstruction are described, with their effects on its ultimate appearance. Consequent to these, a number of factors which limit the wide application of obstruction are stated. The more important of these are: the dimensions of the river-bed, which may be too large to permit of successful obstruction; the presence of wide swampy forest through which G. palpalis ranges freely; alternatively, the presence of shallow rocky stretches with low eroded banks that may be devoid of all but certain characteristic trees, where adequate obstruction is impossible; the rate of run-off of water in the catchment area, rapid run-off leading to spates which disrupt the obstruction; human interference with the obstruction in quest of firewood.

No technical difficulties were encountered in carrying out obstruction, and costs were from 40 to 50 per cent, cheaper than comparable partial clearing.

Considering the subsequent appearance of originally obstructed stream, and its frequent resemblance to partial clearing, the suggestion of ‘ destructive ’ clearing is put forward, in which the top canopy would be destroyed, but no effort made either to create obstruction or to clear away the fallen trees. Instead, reliance would be placed on the action of the various agencies encountered in these trials, which brought about the disruption of the obstruction, to produce the desired end-result.

It is concluded that successful obstruction depends too much on specialised conditions, difficult to fulfil in large-scale tsetse control schemes, and that it is unlikely to become a normal method of control of G. palpalis in the savannah zones of Nigeria.