Readers should now realise that to date there have been no operational-level mosquito SIT programmes. However, the intensity of prior scientific research has produced valuable information for future activities, but it is evident that much still needs to be done to determine if and how well SIT will meet the needs of major disease management programmes. The SIT experimentation and preparation process tends to be long-term and expensive.
Nevertheless, one should not expect that research in itself will answer every question related to mosquito SIT. There is no complete guarantee of operational success regardless of the amount of research conducted. Decision makers will need to interpret the findings of the research community and determine the likelihood of achieving the desired objectives. The challenge for researchers will be to develop sufficient information for decision makers to rationally weigh the benefits of SIT against the risks and cost of implementation. However, it should be assumed that some important questions regarding each specific SIT programme will not be answered until operational level programmes are underway. In the following sections some of the more important, but perhaps less apparent, aspects of SIT implementation are addressed.
Surveillance
Surveillance is an essential component of SIT. The ecology and biology of the target species throughout a proposed release area must be well understood. Seasonal patterns of mosquito distribution and density estimates are integral parts of the database that is used to plan strategies and initiate actions. Surveillance must continue throughout the programme and for an unspecified duration after the wild population can no longer be detected. This activity provides the data required to determine if and when releases should be terminated. A significant portion of the budget will be dedicated to surveillance for the purpose of documenting the extent and nature of the pre-release populations, monitoring progress during the programme and confirming the status in the post-release phase.
Population dynamics
Determining population trends and establishing appropriate over-flooding ratios is not an exact science. This is because R0 (rate of increase, explained previously) changes seasonally, geographically, and in response to a variety of abiotic and biotic factors. Since sterile male efficacy is in part dependent on the over-flooding ratio achieved in each generation, estimates of changes in mosquito abundance are critical components of planning and execution. As an example, in the Lake Apastapeque study, R0 was found to range from 0.4-4.8 [19]. SIT is more efficient when R0 is small than when it is large because fewer released insects are required to achieve the same result. In that study, releases were initiated just before the end of the dry season when the wild mosquito R0 and abundance were low in order to optimize the impact of the sterile males that were available for release and to avoid the need for a prior suppression effort. The timing of this action prevented the rapid increase in vector density that was expected to result soon because of the onset of a period of large R0 values.
Density dependent survival has been observed in experimental mosquito SIT projects. This phenomenon occurs when population density drops sufficiently to alleviate conditions that normally restrict population growth. For example, when Culex larval habitats become overcrowded and an external stress factor (including SIT-induced sterility) causes a rapid decline in density, the R0 may respond by exceeding - even doubling - its normal maximum. An increase in R0 to 10 (normal maximum = 5x) was observed in an experiment with Cx. quinquefasciatus in Florida [20], but this density dependent phenomenon was not observed in the An. albimanus population at Lake Apastapeque [19]. However, when it does happen, increased numbers of sterile male may be required.
When population levels are low, SIT is most efficient because it is easier to achieve the over-flooding ratios necessary to initiate population decline. Thus, it is advantageous to use alternative control options to reduce mosquito density to levels that are compatible with the numerical release potential, and it is most beneficial to do so when the natural R0 of the target population is at its lowest. Fortunately, current mosquito management practices are compatible with SIT, especially those that do not specifically target the released males.
Competitiveness
When released insects are not fully competitive, the release numbers must increase to compensate for the deficiency. For example, when competitiveness is 0.5, twice as many sterile males are needed as when competitiveness is 1.0. Competitiveness estimates are usually derived in laboratory or field cages by introducing known numbers of sterile and fertile males to compete for virgin females. A standard calculation method is used to determine competitiveness [21]. Competitiveness values range from zero to one, and they are a measure of the ability of the sterile males, in competition with wild males, to successfully mate with wild females. Experiments to measure competitiveness can be conducted in natural mosquito habitats when wild mosquito density is low by releasing known numbers of each male type along with marked virgin colony or wild females. Fertility of recaptured marked females indicates the level of competitiveness. In the case of the MACHO strain, the observed competitiveness in this type of field study was high - 0.78-0.80 [22].
In practice, effectiveness of the released male involves much more than competitiveness. Population reduction is a function of the average effective released male:wild male ratio. This is influenced by, e.g., frequency and distribution of releases, effect of release techniques on competitiveness, longevity of the released males and their ability to disperse and locate mates, distribution of the wild population and losses to predation. These parameters are not easily measured. Managers need to custom-fit the release to match known habitat preferences, adjust for heterogeneous distribution of the wild population and other factors or simply release more sterile insects than are calculated to meet a specified ratio. As an example of the discrepancy between estimates of competitiveness and release requirements, Patterson [9] reported a competitiveness value of 0.75 in a cage study for Cx. quinquefasciatus males sterilised as adults, but after release the competitiveness value was estimated as 0.25-0.33. The 0.25-0.33 estimate from the field study is an indication of the released male's average effectiveness, indicating that it actually took 3-4 released males (rather than 1.3) to compete with one wild male. To achieve an effective 10:1 ratio under this condition would require a release rate of at least 30:1.
Rearing and handling
Proven rearing methodologies exist for the daily production of millions of some mosquito species, e.g., Cx. quinquefasciatus, Ae. aegypti and An. albimanus [23–25]. Yet, there is a continuing need to develop methods of rapid colonisation that 1) minimize the adverse effects of selection [26], 2) minimize space and personnel requirements, 3) maximize the yield of hardy insects, and 4) minimize the release of female mosquitoes. Mosquito production for the WHO/ICMR and Lake Apastapeque experimental releases focused on the need to exclude females. In the WHO/ICMR project, separation of Cx. quinquefasciatus pupae by size yielded releases that were > 99% males [13] at the expense of considerable wastage of the males produced. In the Lake Apastapeque programme, 14% of the released insects were females, but no estimates are available for male losses during size separation. However, male production per tray was doubled, and male losses were minimized (ca 10%) after adopting the MACHO genetic sexing strain.
Although the MACHO strain made it possible to increase male releases by 3-4 fold and release extremely low numbers of females, management of the rearing process became more complex [24]. Egg viability of the strain was only 50% because of the translocation so the number of cages holding females for egg production was doubled. A separate colony had to be maintained to ensure the purity of the brood stock used for release production because of genetic recombination (0.1-0.2% per generation). Without informed management, this loss of linkage of insecticide resistance with the Y chromosome would gradually result in an increase in recombinant resistant females and susceptible males. It was necessary to purge the recombinants and replace the stock at regular intervals despite the stabilizing inversion that had been incorporated into the strain [27].
Sterilisation methodology
Excessive levels of sterility in released males could reduce their effectiveness as mosquitoes suffer somatic damage as a result of exposure to radiation. Achieving 100% sterility might be counter-productive if it results in a substantial loss of male competitiveness (see [28] for a detailed discussion). The competitive sterile males released at Lake Apastapeque were 99.8% sterile after exposure to bisazir (females, 96.6%) [16], which was adequate to achieve control. Sterility requirements are generally related to the biotic potential of the target species. If there are data on this potential and on mating competitiveness in the field, the optimum male sterility level could be calculated. Sterility levels in females need to be considered, but in general, radiation levels that sterilise male mosquitoes also sterilise the females. Chemosterilant levels that completely sterilise males produce high levels of sterility in females. This sterility usually is permanent, lasting for the lifetime of the insect.
Radiation sterilisation and chemosterilisation offer straightforward methods to produce sterile male mosquitoes. However, ionizing radiation is known to cause somatic damage and severely reduce competitiveness when pupae are exposed to sterilising doses, but older pupae are not as sensitive. Although mosquito males irradiated as young adults have often displayed lowered competitiveness in past field trials, recent laboratory findings suggest that somatic damage may be avoidable when newly emerged adults are irradiated [28]. Chemosterilisation appears to provide the option of sterilisation without somatic damage. Concerns about the environmental fate of chemosterilant residues highlighted by laboratory bioassay of non-target predators [29] may have been answered by findings of extremely low initial residues, virtually complete degradation within 24 h post-treatment and simple bulk detoxification methods [30]. Using modern equipment and personal protection, workers involved with the sterilisation process can be protected from the hazards of radiation and chemosterilants.
Packaging, transport, release mechanisms and strategies
Sterile mosquito releases conducted to date have relied on ground release. Relatively simple packaging, transport methodology, release containers and shelters have been devised for pupal and adult releases [31], but no work has been initiated on methods of aerial distribution. Certainly, in urban programmes ground release might suffice, but the availability of satisfactory aerial release methods could provide timelier and more effective distribution with reduced opportunity for pre-release damage to the sterile males. Production and release of millions per day will demand expedited delivery mechanisms to prevent losses in quality and competitiveness.
Scheduling of releases can be very complex, even with the availability of computerized models that incorporate geographical information systems. Continuous adult emergence is characteristic of some mosquito species most likely to be considered for SIT but little work has been conducted on determining appropriate release intervals. An important factor for scheduling release intervals is the likelihood that the average lifespan of sterile males may be shorter than wild males as has been documented with irradiated mosquitoes [8, 9]. This aspect may influence release distribution patterns as much as wild mosquito density, aggregation, and released male dispersal parameters. Distribution sequence will also be dictated by geographic characteristics, with release patterns for urban settings perhaps differing from those of rural locations.
SIT programmes could be faced with the options of area-wide or selective suppression of the wild mosquito population prior to initiating releases. It is conceivable that major sectors of the target area may not breed mosquitoes and that infestation foci are well defined, which could lead to selective release coverage based on the extent of the individual infestations with generous buffer zones to ensure disruption of the potential impact by immigration of fertilized mosquitoes. Alternatively, the programme could advance along a common front as population control is achieved in the area designated for the initial releases (Figure 1).
Quality control
During the implementation of SIT programmes, there are procedures, equipment and facilities that must operate according to pre-determined standards. These standards are maintained by those responsible for the specific activities, but there also needs to be an oversight component to ensure that the standards are understood and faithfully observed. This task falls to quality control (QC). Examples of standards to monitor are: mean larval, pupal and adult weights, pupae produced/standard rearing container, sex ratio, adult longevity and sexual aggressiveness of pre and post release males, eggs per colony female, blood and food quality. To ensure that QC acts independently and is free from the influence of those groups that it oversees, QC is routinely placed between Management and Operations in the organizational hierarchy. Leadership of the QC team should have management-level authority in order to ensure objectivity and preserve the ability to act independently. Based on its findings, QC advises Operations and reports directly to the Director.
Minimal attention has been given to the possibility of mosquito escape from most SIT research-level rearing facilities, whereas in existing operational programmes located in disinfested zones, e.g., screwworm and fruit fly, facility quarantine is a major consideration. Unintentional mosquito release from either research or operational activities not only could jeopardize the success of the programme, but it could also cause significant public relations and public health problems. Fail-safe procedures are necessary components of rearing facility management to prevent unintentional escape and to ensure that intentionally released insects are sterile.
Research
Research does not end when operational activities begin as there is a continued demand for problem solving and advance preparation. Major SIT programmes require an independent on-site research component acting in support of the programme. Problem-solving and development of supporting technology should not be the responsibility of production or release staff, although they should be active contributors to the research programme.
Key issues
In light of the progress that has already been documented, perhaps the more pressing issues that need to be addressed in depth are:
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Stable genetic sexing mechanisms, possibly transferable to multiple species
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Optimization of radiation sterilisation, especially in the pupal stage
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Development of aerial release capability
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Optimization of rearing technology for species under consideration
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Realistic decision making by administrators with awareness of research limitations