Population Size and Migration of Anopheles gambiae in the Bancoumana Region of Mali and Their Significance for Efficient Vector Control

We present results of two intensive mark-release-recapture surveys conducted during the wet and dry seasons of 2008 in the villages of Fourda and Kenieroba, Mali. The former is a small fishing village by the Niger River with a moderate to high densities of Anopheles gambiae Giles s.s. (Diptera: Culicidae) throughout the year, while the latter is a large agricultural community 2 km inland that experiences strong seasonal fluctuation in An. gambiae densities. We estimate the population size of female An. gambiae in Fourda to be in less than 3,000 during the dry season. We found evidence of large population size and migration from Fourda in Kenieroba during the wet season, but very low numbers and no sign of migrants during the dry season. We suggest that malaria vector control measures aimed at adult mosquitoes might be made more efficient in this region and other seasonal riparian habitats by targeting disruption of mosquito populations by the river during the dry season. This would decrease the size of an already small population, and would be likely to delay the explosive growth in vector numbers in the larger inland villages as rainfall increases

Estimated population sizes for Fourda using the joint hypergeometric maximum likelihood estimator from the program NOREMARK for varying daily survival rates.

a<p>CI, confidence interval.</p

Instantaneous Population Size Estimated via Lincoln Index.^a,b

a<p>Lincoln index with small <i>r</i> (<20) adjustment was used.</p>b<p><i>P</i>, population size; <i>a</i>, number marked and released the previous day; <i>n</i>, number captured; <i>r</i>, number captured that were marked the previous day (recaptures).</p

Species and Molecular Form Composition in Fourda, Bozokin, and Kenieroba.^a,b

a<p>Number of individuals of each type from samples collected in March and July 2008.</p>b<p>M, M molecular form; S, S molecular form; R, <i>Anopheles arabiensis</i>; H, M-S molecular form hybrids.</p

Lincoln index and Fisher-Ford estimates of population size in Fourda over time with varying survivorship.

<p>Lincoln index estimates (horizontal lines) are based on recaptures of marked mosquitoes released the previous day only. Fisher-Ford estimates per day are based on recaptures on all days prior to the estimate with an assumed daily survival rate (<i>s</i>): Triangles, <i>s</i> = 0.6; Squares, <i>s</i> = 0.8; Circles, <i>s</i> = 0.9. Black symbols and lines, dry season; Red symbols and lines, wet season.</p

Sampling and recapture locations in Fourda and Kenieroba with breeding sites.

<p>Maps show Fourda (to the South) and Kenieroba (North) in March (a) and July (b). The arrow indicates the marked mosquito release site. Note that the compounds in the northern half of Fourda had much higher numbers of mosquitoes overall, which correlated with higher numbers of recaptures (results not shown). See text for details on sampling regime.</p

Fisher-Ford trellis plots of mark-release-recapture results obtained in Fourda.

<p>For each plot, dates are given along the top of the plot; numbers of mosquitoes marked and released for each date (follow diagonally) are shown along the right edge; numbers of mosquitoes captured are along the left edge. Numbers between these two edges represent the number of marked mosquitoes recaptured, which can be identified by cohort. See the text for further details. (a) Dry season; (b) Wet season. * Landing catch was conducted in Fourda the evening of March 20 with four collectors; four marked <i>An. gambiae</i> were recaptured, all having been released on March 17. These are not included in the analysis.</p