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Movement of mosquitoes belonging to the Anopheles gambiae complex (mixed wild populations of An.arabiensis, An.gambiae and An.melas) between three neighbouring rural villages in The Gambia was investigated by mark-release-recapture. A total of 12,872 mosquitoes were collected in bednets, marked with a magenta fluorescent powder and released over a 15-day period in one of the villages. A further 15,507 mosquitoes were collected in exit traps, marked with a yellow powder and released over the same period. Mosquitoes were captured daily in all three villages using pyrethrum spray catches, as well as bednet and exit trap catches. The catching period extended for 6 days after the last day of release. Of the mosquitoes released, 372 (1.3%) were recaptured 2-21 days later. Of these recaptures, 272 were caught in the release village, and 98 were caught in other villages situated 1-1.4 km away. The 'movement index' between villages was calculated as 17.2% (12.2-22.4% confidence limits) for mosquitoes released after feeding and 20.1% (14.7-25.3%) for those released unfed. These results suggest that movement of mosquitoes between neighbouring villages in The Gambia seriously affects the entomological evaluation of pyrethroid-impregnated bednet programmes in areas where treated and untreated villages are interspersed.
Thomson et al. (Sun,) studied this question.