NEA carried out Phase 1 small-scale releases in Braddell Heights (landed residential area) and Tampines West and Nee Soon East (HDB). These sites are representative of housing estates in Singapore, and had previously experienced dengue outbreaks and high Aedes aegypti populations. Prior to Phase 1, NEA had been monitoring mosquito populations in these sites for up to three years, thereby establishing a baseline against which to compare the impact of releases.
Phase 1 findings
Phase 1 gave us important information on the behaviour of male Wolbachia-Aedes mosquitoes in our urban environment, which then guided the design of subsequent release strategies.
- Flight range: 90% of male Wolbachia-Aedes mosquitoes were caught within 40m from the release point. Male Wolbachia-Aedes mosquitoes released on the ground floor of HDB blocks could be caught on higher levels, even on top floors.
- Longevity: 50% of male Wolbachia-Aedes mosquitoes survived to four days after release.
- Potential to suppress urban Aedes aegypti populations: At the end of Phase 1, egg hatch rates and adult Aedes aegypti populations in release sites were reduced by 50% compared to sites with no releases. This suggested that released males successfully mated with urban females; optimisation of release methods is now needed to achieve better suppression.
Phase 1 also helped us identify the following challenges:
- Migration of females into release sites: In Singapore’s dense urban environment, Aedes aegypti mosquitoes from adjacent areas can easily move into the release sites, thus reducing the impact of releases.
- High-rise mosquito breeding: Ground-floor releases were not effective at reducing Aedes aegypti numbers on the upper floors of HDB blocks.
- Release of small numbers of Wolbachia-Aedes females: Although only a very small number of female Wolbachia-Aedes mosquitoes slip through the sorting process, successive releases over time may result in establishment of Wolbachia in the urban Aedes aegypti population. This would reduce the effectiveness of the Wolbachia-Aedes suppression strategy, which relies on incompatible matings between Wolbachia-carrying males and urban females.