Participation of irradiated Anopheles arabiensis males in swarms following field release in Sudan

Abstract

Background: The success of the sterile insect technique (SIT) depends the release of large numbers of sterile males, which are able to compete for mates with the wild male population within the target area. Unfortunately, the processes of colonisation, mass production and irradiation may reduce the competitiveness of sterile males through genetic selection, loss of natural traits and somatic damage. In this context, the capacity of released sterile Anopheles arabiensis males to survive, disperse and participate in swarms at occurring at varying distances from the release site was studied using mark-release-recapture (MRR) techniques.

Methods: In order to assess their participation in swarms, irradiated and marked laboratory-reared male mosquitoes were released 50, 100 or 200 m from the known site of a large swarm on three consecutive nights. Males were collected from this large swarm on subsequent nights. Over the three days a total of 8,100 males were released. Mean distance travelled (MDT), daily probability of survival and estimated population size were calculated from the recapture data. An effect of male age at the time of release on these parameters was observed.

Results: Five per cent of the males released over three days were recaptured. In two-, three- and four-day-old males, MDT was 118, 178 and 170 m, and the daily survival probability 0.95, 0.90 and 0.75, respectively. From the recapture data on the first day following each release, the Lincoln index gives an estimation of 32,546 males in the natural population.

Discussion: Sterile An. arabiensis males released into the field were able to find and participate in existing swarms, and possibly even initiate swarms. The survival probability decreased with the age of male on release but the swarm participation and the distance travelled by older males seemed higher than for younger males. The inclusion of a pre-release period may thus be beneficial to male competitiveness and increase the attractiveness of adult sexing techniques, such as blood spiking.

Figure 1

The area of the Northern State in which an SIT feasibility pilot study is proposed. In preparation for this pilot study Anopheles arabiensis swarms were surveyed at Merowe West, Hamadab Village, Nuri area and an abandoned and flooded brickworks southwest of Nuri in September 2012. In November 2012, swarms were only found in in Nuri and an MRR study was conducted to investigate participation of sterile males in swarms in the field.

Figure 2

Geographical layout of the mark-release-recapture study, conducted to investigate swarm participation in sterile irradiated Anopheles arabiensis males in Nuri, Sudan. The letter A indicates the position of a large swarm, and B and C indicate two small swarms. The circles, squares and triangles indicate release points located 50, 100, 200 m from the site of the large swarm, respectively (Google maps).

Figure 3

Release-recapture setup for calculation of MDT. Annuli (75, 150, 225 m) are used to calculate MDT. A represents the classical MDT set up. The black circle indicates the release point and white points indicate the recapture points. B, C and D represent the actual release set up in Sudan (see Figure 2) and the representation of the annuli for the modified MDT calculation. B represents the three annuli 75 m from the three release points and the letter A represents the large swarm, considered as one recapture point for each release point. The circles, squares, and triangles indicate release points located 50, 100, 200 m from the large swarm site, respectively.

Figure 4

Regression lines of recaptures (expressed as log number of released males recaptured + 1) of cohorts of Anopheles arabiensis released at two, three and four days of age. The antilog of the slopes of regressions lines gives the daily survival probability.