Estimates of the population size and dispersal range of Anopheles arabiensis in Northern KwaZulu-Natal, South Africa: implications for a planned pilot programme to release sterile male mosquitoes

Abstract

Background: Anopheles arabiensis is a major malaria vector, recently implicated as contributing to ongoing residual malaria transmission in South Africa, which feeds and rests both indoors and outdoors. This species is, therefore, not effectively targeted using core malaria vector control interventions alone. Additionally, increasing resistance to available insecticides necessitates investigations into complementary non-insecticide-based vector control methods for outdoor-resting mosquitoes. The feasibility of the sterile insect technique (SIT) as a complementary vector control intervention is being investigated in South Africa. Successful implementation of an SIT programme largely depends on inundating a target insect population with sterilized laboratory-bred males. Therefore, knowledge of the native population size and dispersal ability of released sterile laboratory-reared males is critical. In this study, we estimated the male An. arabiensis population size and the dispersal of released males in an area targeted for a pilot sterile male release programme.

Methods: Three separate releases were performed within a 2-year period. Approximately 5000-15,000 laboratory-reared male An. arabiensis (KWAG) were produced and marked for mark-release-recapture experiments. To recapture released mosquitoes, cloth tubes were deployed in widening concentric circles. The average dispersal distance of released males was calculated and the wild male An. arabiensis population size was estimated using two Lincoln index formulae. The natural population was sampled concurrently and Anopheles species diversity examined.

Results: The Anopheles gambiae complex and An. funestus group species made up the majority of wild collections along with other anophelines. The An. arabiensis population size was estimated to be between 550 and 9500 males per hectare depending on time of year, weather conditions and method used. Average dispersal distance of marked males ranged from 58 to 86 m. Marked males were found in swarms with wild males, indicating that laboratory-reared males are able to locate and participate in mating swarms.

Conclusions: It was logistically feasible to conduct mark-release-recapture studies at the current scale. The population size estimates obtained may provide a guideline for the initial number of males to use for a pending SIT pilot trial. It is promising for future SIT trials that laboratory-reared marked males participated in natural swarms, appearing at the right place at the right time.

Keywords: Malaria elimination; Malaria vector control; Male mosquitoes; Over-flooding ratio; Population size; Sterile insect technique.

Fig. 1

a Location of the study site in KwaZulu-Natal Province, South Africa. b View of the Jozini Municipality, showing the location of Mamfene. c Study site showing the target site and the two control sites. Control site one is adjacent to the target site and control site two is approximately 8 km away. The area of each section is: target site, 0.48 km²; control site one, 0.70 km²; control site two, 0.26 km²

Fig. 2

a Cloth tubes used during mark–release–recapture experiments strung onto a pole for collection and deployment of multiple tubes at once. b, c Typical placement in the field. Note the netting sleeves (one closed and one open) used to close the tube for more rapid collection

Fig. 3

Cloth tube set-up around the central release site (C) in the mark–release–recapture (MRR) pilot trial (MRR^PT). Cloth tubes were placed in concentric circles around C at radii of 50 m (red), 100 m (blue), 200 m (orange) and 400 m (pale green). Insert shows the land cover type in the study area

Fig. 4

Distribution of cloth tubes in the mark–release–recapture (MRR) one (MRRI) experiment. Cloth tubes were placed in concentric circles around the central (C) and edge (E) release points at radii of 25 m (pink), 50 m (red), 75 m (beige), 100 m (blue),125 m (green) and 150 m (burgundy). Swarm locations are also indicated. Swarms and tubes with marked males are indicated with the colour of marked males (yellow released from C and orange released from E)

Fig. 5

Distribution of cloth tubes in the mark–release–recapture (MMR) two (MRRII) experiment. Cloth tubes were placed in concentric circles around the central (C) release point at radii of 25 m (pink), 50 m (red), 75 m (beige), 100 m (blue), 125 m (green) and 150 m (burgundy). Tubes with marked males are indicated