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How to calculate the number of sterile insects that must be released in SIT?

How to calculate the number of sterile insects that must be released in SIT?


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The sterile insect technique (SIT) is used to control insect pests through overwhelming the population in sterile males. The increase in sterile males released into an environment causes the increase in infertile matings. This, in theory, would reduce the population.

However, how would you calculate the number of sterile flies needed to be released in order to be effective? It would be dependent on the reproduction rate of the insect, initial density, and sex ratio in the young, but is there an equation to express this? Or is there simply a generic ratio of the target population that is used?

I'm looking at the example of the New World Screwworm fly, and the eradication in Central America.


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Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK

Present address: Centre for Ecology & Hydrology, Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK.

Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109, USA

Center for the Study of Complex Systems, University of Michigan, Ann Arbor, MI 48109, USA

Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA

Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK

Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK

Present address: Centre for Ecology & Hydrology, Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK.

Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109, USA

Center for the Study of Complex Systems, University of Michigan, Ann Arbor, MI 48109, USA

Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA

Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK


The sterile insect technique, commonly known as SIT, was initiated in the 1930s. Its first successful use was to control screwworm fly, a devastating cattle pest, on the island of Curacao in 1953. Subsequently it was used to eradicate screwworm from the USA and Mexico.

Since then SIT has been further developed to suppress more than 20 insects including fruit flies and other key agricultural pests.

Sterile insect technique for Queensland fruit fly was first trialled in New South Wales from 1962 to 1965.

The first use of sterile insect technique in Western Australia was against Mediterranean fruit fly (Medfly) in Carnarvon in 1978. Sterile insects combined with baiting successfully eradicated Medfly from the Carnarvon area by 1984. Unfortunately, lack of quarantine barriers meant it soon re-invaded.

In 1989 a special factory was built to produce sterile Queensland fruit flies to fight a large outbreak in Perth. This was successful and by 1991 Queensland fruit fly had been eradicated from Western Australia.

From 2001 to 2013 the same factory was used to rear sterile Mediterranean fruit fies under contract to South Australia. When not used to fight outbreaks in that state, the sterile flies were used for research in WA.

For many years a sterile insect release program combined with protein baiting was used successfully to manage fruit flies at Katanning. Sterile pupae were shipped to Katanning where they emerged and were released by staff from the Shire of Katanning.

Sterile Medfly bred in Western Australia have been used to eradicate eight fruit fly outbreaks in South Australia since 2001. In some countries, sterile fruit flies are released continually to prevent establishment of wild flies.


Chapter 3The sterile insect technique

The sterile insect technique (SIT) involves sustained, systematic releases of sterile insects among the indigenous target population. When female flies are mated by sterile male flies, the females become infertile for the remainder of their life spans. The insects to be released are propagated at special large-scale rearing facilities. Males are sterilized by radiation at the appropriate stage and then taken to the selected area and released. Distribution of the sterile insects can be optimized by aerial release. By continually releasing sterile males in quantities and over a time span that is sufficient to cover several generations of the target population, its reproductive capacity and, hence, the fertile population are progressively reduced. Eventually, so few fertile insects remain that fertile matings do not occur and the population is eliminated.

For maximum effectiveness, the sterile males released must outnumber the fertile, native male flies by a considerable margin. In order to reduce populations when conditions are highly favourable for fly reproduction, the ratio of released sterile males to native males should be at least 2 to 1 (Knipling, 1955) and may, in certain circumstances, have to be as high 15 to 1. It therefore follows that SIT is most cost-effective when the target population is low. On the other hand, insecticide applications cost the same, regardless of the insect population density and are, therefore, most cost-effective when the target population density is high. This suggests that the phased and complementary use of both "conventional" methods and SIT would result in maximum efficiency throughout the phase of intervention (Figure 1). For some populations, and for species of insect pests that have strong seasonal fluctuations, SIT releases may be initiated and the number of sterile males released cause efficient "overflooding" ratios even when native populations have not been suppressed by conventional means prior to the SIT release. Contrary

to the conventional integrated pest management (IPM) concept, which suggests interventions should only be made after a pest population has exceeded the economic threshold level, SIT is initiated when the target pest population reaches its seasonal minimum, for example, at the end of the winter, long before it starts increasing again.

FIGURE 1
Optimizing the efficiency of an insect pest intervention campaign by using conventional control and SIT in an integrated, phased approach


Modeling resistance to genetic control of insects

The sterile insect technique is an area-wide pest control method that reduces pest populations by releasing mass-reared sterile insects which compete for mates with wild insects. Modern molecular tools have created possibilities for improving and extending the sterile insect technique. As with any new insect control method, questions arise about potential resistance. Genetic RIDL ® 1 (Release of Insects carrying a Dominant Lethal) technology is a proposed modification of the technique, releasing insects that are homozygous for a repressible dominant lethal genetic construct rather than being sterilized by irradiation. Hypothetical resistance to the lethal mechanism is a potential threat to RIDL strategies' effectiveness. Using population genetic and population dynamic models, we assess the circumstances under which monogenic biochemically based resistance could have a significant impact on the effectiveness of releases for population control. We assume that released insects would be homozygous susceptible to the lethal genetic construct and therefore releases would have a built-in element of resistance dilution. We find that this effect could prevent or limit the spread of resistance to RIDL constructs the outcomes are subject to competing selective forces deriving from the fitness properties of resistance and the release ratio. Resistance that is spreading and capable of having a significant detrimental impact on population reduction is identifiable, signaling in advance a need for mitigating action.


Background

The Mediterranean fruit fly (medfly), Ceratitis capitata (Wiedemann Diptera: Tephritidae), is one of the most devastating and economically important insect pests [1]. An effective biological and environment-friendly control of this pest is the sterile insect technique (SIT) [2]. The SIT reduces a pest population by mass release of reproductively sterile male insects into a wildtype (WT) population of the same species. This leads to the decrease of progeny by competition of sterilized males with WT males for WT females [3]. Thus, the sterilization of the pest species in SIT programs is of major importance and is commonly induced by radiation. However, the sterility and competitiveness are indirectly correlated [4]. In some programs therefore lower doses of radiation are used to generate lines which are more competitive even though only partially sterile. In preventional release programs, completely sterile flies are released into pest-free areas to avoid the establishment of invasive fruit flies and to control the constant problem of re-infestation [5]. These programs have to use 100% sterile flies to avoid a novel introduction of insect pests. However, the competitiveness of such flies is reduced due to the high dose of radiation required for complete lethality, which results in the expensive need of high numbers of males per field-release and a high frequency of such releases.

A first approach to cause reproductive sterility by transgene-based embryonic lethality without the need of radiation has been successfully shown in the non-pest insect Drosophila melanogaster [6]. The system is based on the transmission of a transgene combination that causes embryo-specific lethality in the progeny. To limit the effect of the transgenes to the embryonic stage, promoter/enhancers (P/Es) from cellularization-specifically-expressed genes drive the expression of the tetracycline-controlled transactivator (tTA). The expressed heterologous transactivator then activates the expression of the lethal effector gene hid Ala5 [7] and leads to complete embryonic lethality in D. melanogaster. To generate suppressible, dominant lethality in medfly and at the same time restrict the effects of lethality to embryos, a direct transfer of the sterility system from D. melanogaster [6] to medfly was previously tried. The genomic integration of the driver construct carrying the sry α P/E from D. melanogaster into medfly was successful, but none of the transgene insertions expressed the system activator tTA at a detectible level [8]. We therefore concluded that the cellularization-specific P/E from D. melanogaster is not functional in medfly and that endogenous P/Es have to be used to generate such a system.

Here, we report the development of the first transgenic embryonic lethality system for medfly using an early embryonic lethal transgene combination. When transgenic males carrying this system are mated to WT females, all progeny die during embryogenesis without the need of radiation. Due to the complete lethality in embryonic stages no fruit damage from developing larvae would occur from progeny of WT females mated to transgenic males and no transgenes would ingress into the wild population. Moreover, males carrying this system are highly competitive in laboratory and field cage tests. After successful evaluation, a combination of this new embryonic lethal medfly system with a sexing system will become a powerful tool to improve SIT programs.


Discussion

The 137 Cesium irradiation of pupae from Saint Benoit (Reunion Island) strain of Ae. albopictus males induced a similar decrease of fertility levels as reported by Balestrino et al. [27] on the Rimini (Italy) strain of the same species. The sterility induced by a 40 Gy irradiation was permanent as successive matings and a resting period did not show any recovery of fertility in males. The fertility was even reduced, as all the females inseminated during the second mating period were completely sterile. Radiation damage is higher on the earlier stages of spermatogenesis than on mature cells [28] assuming that the sperm used during the second mating period originated from sperm cells that were immature during the irradiation process, and hence more radiosensitive, it is likely that this would result in the total sterility of the sperm cells. Patterson et al. [29] also showed permanent sterility in radio-sterilized males Cx. quinquefasciatus Say during 2 weeks. However, our results are different from those obtained in Italy with the Rimini strain where male Ae. albopictus showed a slight but non significant increase in fertility with male age [30].

Sterile male mating vigour as shown by the mating success tests was not significantly affected during the first week of adulthood, but the males became less efficient thereafter. Although the male mosquitoes inseminated fewer females each day, the differences were significant only from day 9 onwards. After that period, untreated males inseminated 50% more females than sterile ones. As the irradiation may interfere with the maturation of new sperm cells, it seems likely that sterile males might have fewer successful matings (i.e. leading to insemination), therefore a lower daily mating success might be expected after several matings. However, using a 1∶5 male-female ratio during 4 h, Balestrino et al. [27] observed no effect of a 40 Gy irradiation on the mating performances of male Ae. albopictus, but they reported fewer successful matings for 50-Gy-irradiated males. Grover et al. [31] reported that chemosterilization slightly reduced the insemination rates of male Ae. aegypti for periods of 24 h or 48 h over 10 days on the other hand, radiosterilized males An. pharoensis Theoblad [32] or Cx. Quinquefasciatus Say [29] did not show a different mating success compared with laboratory males.

We observed that when sterile and untreated males were offered new receptive females daily, creating intensive mating opportunities, most of the females had only one spermatheca filled. This pattern was also reported by Boyer et al. [33] for untreated male Ae. albopictus from another strain of Reunion Island. However, we showed that when a male was enclosed with the same 10 females for a longer period or when several males and females were caged together, females had mostly two spermathecae filled. We hypothesized that males from this strain might inseminate most of the time only one spermatheca per mating attempt and when kept with the same females for several days they might have more opportunities to transfer more sperm to the same female thus filling two spermathecae. The filling of only one spermatheca should not prevent female egg laying. Contrary to anopheline [34], the oviposition behaviour in Ae. albopictus and Ae. aegypti females would not be dependent on the spermathecae containing sperm but would rather be triggered by proteins from the male accessory gland (MAG) secretion that are transferred together with the sperm in the bursa inseminalis [35], [36], [37]. In addition, as the MAG substance would diminish the female propensity to be inseminated by another male [22], a female with only one spermatheca filled should therefore not have a higher probability of multiple-insemination. However, this would need further investigation as recent studies indicated the occurrence of multiple matings in the field for Ae. albopictus [38] and in semi-field condition for Ae. aegypti [39]. In the case of the SIT, multiple inseminations would not be prejudicial if sterile and wild males are equally involved, and when the sterile males outnumber the wild ones. The conditions and duration of mating success tests in laboratory might affect the possibility to highlight differences or not between sterile and untreated males, while carrying out these tests in a semi-field environment can help getting a realistic value of the males’ sexual performance and capacity as outlined by Huho et al. [40].

In order to perform unbiased competitiveness tests, the time of sexual maturation of both untreated and sterile males has to be similar so that neither one would have the opportunity to mate with females earlier. The genitalia rotation of males was overall not greatly affected by irradiation, although rotation was slightly slower for sterile males between 15 to 19 h post-emergence. Similarly, sterile males inseminated fewer females during the first 20 h but this difference was no longer visible after 25 h post-emergence. However, this difference observed between untreated and sterile males is only of concern for the establishment of competitiveness tests protocols.

Assuming that 40 Gy-irradiated males were equally competitive with untreated males, the fertility of the caged population should have averaged 49% (as the fertility of wild and sterile control males was 93 and 5% respectively). However, sterile one-day-old males had a low competitiveness index when competing in an equal ratio with wild males, and the wild female population fertility was only reduced with 10%. In Italy, competitiveness studies on Ae. albopictus indicated a good performance of sterile males irradiated at 30 Gy with a competitiveness index equal to 1.00±0.66 a high variability among replicates and between years was however reported [30]. A different strain, experimental setting and the diverse environmental conditions might affect the behaviour and survival of the mosquitoes, and could explain the differences observed between this study and ours. We observed that maintaining males for five days after emergence in the laboratory before the release greatly improved their competitiveness, and allowed the decrease of the semi-field cage wild population’s fertility to 62%, which indicated a nearly equal participation of both groups of males for the inseminations of females. The age of females differed between the experimental tests 1 and 2, as they were the same age as the males. However, females are already fully receptive to mating when two-days-old, and the receptivity to males and ability to retain semen should not differ with age as mentioned by Spielman et al. [41] for Ae. aegypti females. This almost four-fold improvement of the competitiveness value of the five-day-old males does not seem to be due to an increase of intrinsic male mating ability since, in laboratory conditions, a difference in male age did not affect the number of females inseminated. The time spent with an easily reachable sugar source in the insectarium during the pre-release period could have increased males nutritional status and thus improved some of their traits such as survival and flight capacity. A similar age effect on the mating competitiveness of sterile males Culex pipiens fatigans Wiedemann was reported by Krishnamurthy et al. [42] who observed that 36–60 h old males from a highly sterile male-linked translocated strain competed almost equally, in semi-field cages, against same age males from an untreated strain, whereas 12–36 h old males had a reduced competitiveness. Similarly, 36–60 h old chemosterilized Ae. aegypti males were competing better, in semi-field, against wild males than did 7–8 days-old males [31].We reported that a five-fold ratio of sterile to wild males allowed a reduction of almost 50% of the wild females natural fertility, suggesting that a 10-fold ratio could bring total sterility in a wild population and continuous releases might have a rapid efficient impact on the reduction of vectors density in the field. Releases of sterile males were usually performed with “over flooding ratios” so that the impact on the wild population would be faster [43], [44]. Laboratory studies on radiosterilized male An. quadrimaculatus Say allowed a reduction of 80% of the fertility of wild females when released in a ratio higher than 6∶1:1 (sterile males : wild males : wild females), but no reduction was observed at a ratio equal or less than 4∶1:1 [45]. A reduction of 95% in the fertility was possible in laboratory experiments with irradiated males An. pharaoensis Theobald competing in a ratio 10∶1:1 [46]. More recently, an average 5∶1 overflooding ratio of engineered sterile male Ae. aegypti occasioned an 80% reduction of a wild local population in the Cayman Islands over a 23-week period [47].

The results of this study suggest that a 5∶1 or higher sterile to wild male ratio should be combined with a pre-release period in an insectary to ensure the efficiency of a sterile male Ae. albopictus release. The Ae. albopictus density on Reunion Island is high and covers a wide range of habitats although the ravines may be less easily accessible, releases near habitation gardens and parks should be straightforward. Releases at the pupal stage are often considered as more convenient, but it may be conceivable to use the emergence cage where the sterile males would be maintained during the pre-release period to bring them to the various release areas. Provided suitable aerial release systems can be developed and the surface of the treated area is large enough, aerial releases would ensure a cost-effective area-wide coverage. Furthermore, if the release at the adult stage is selected, it might then be of interest to irradiate males as adult in order to reduce the radiation induced somatic damages and thus improve their competitiveness [8]. The pre-release period may turn out to be an important cost factor in a mass-rearing facility further studies should determine the minimal period required before release. The balance between sterility level and competitiveness of the sterile males is a major question for such programmes [48]. As in this study we chose a lower radiation dose ensuring a better competitiveness but not a complete sterility, the question remains whether the use of 5% fertile males is conceivable for a field release?

A reduction of competitiveness of radio-sterilized males is the key argument put forward to support a transgenic approach over classical SIT [49]. However, we showed that the effect of irradiation could be counteracted by adapting the release process, and does not prevent accomplishing an efficient reduction of an Ae. albopictus population’s fertility. In the native habitat, the competitiveness of the released sterile males will also depend on the effect of rearing and handling, the location of the release sites and the distribution of the wild mosquitoes [50] a field trial is therefore now desirable to put sterile males to the test in conditions where they would also have to find sugar sources, locate female mates, and face predation risks.


5 EXPLOITING REPRODUCTIVE INTERFERENCE IN A STERILE INSECT TECHNIQUE PROGRAM

Here we introduce a novel type of program that uses sterile insects to suppress the wild-type population of the same species and also that of a closely related pest species through reproductive interference, which we call ‘sterile interference’ (Fig. 1). Sterile interference can be applied when closely related pest species with similar mating signals are distributed sympatry. The implementation of this strategy, which ‘kills two bugs with one stone’, has several possible benefits for an area-wide IPM program. First, because sterile interference requires only one species to be sterilized, the cost of developing and running a system for the mass production of another species can be saved, even though a larger number of sterile insects must be released than in a standard sterile insect program. Second, even after the targeted species have been eradicated, the continued release of sterile insects can work as a preventive mechanism against the reinvasion of both the primary targeted species and one or more closely related pest species. Finally, the risk of unpredictable agricultural or health hazards arising from the release of insects might be reduced in a pest control program involving the release of fewer species. Therefore, sterile interference can be viewed as a means to add value to a standard program by targeting closely related species to the pest of insect.

We summarized possible cases where sterile interference is effective in the field (Table 1). Similar to sterile insect technique, sterile interference can be used in (i) suppression of the pest species, (ii) complete eradication from the infested area, (iii) containment in the restricted area, and (iv) prevention of (re)introduction for targeting closely related species. Additionally, sterile interference might be applied to (v) host limitation of the generalist pest species, which will in turn become a non-serious pest that specializes on a limited number of host species for example, the host ranges of two closely related generalist species, Bactrocera dorsalis and B. carambolae, largely overlap, 28 while the overlap becomes extremely limited in the sympatric area. 29 A current study strongly suggested unidirectional reproductive interference from B. dorsalis to B. caramborale as the main cause of this resource partitioning. 30 Thus, release of sterile B. dorsalis may exclude B. carambolae from several valuable fruits through sterile interference, as well as controlling wild B. dorsalis through standard sterile insect technique. According to the theoretical work on reproductive interference, 23 the strength of reproductive interference as well as abundance of the interacting species determine the ecological consequences, i.e., species exclusion in the regional scale, parapatric distribution in the two species, and niche partitioning. Even weak reproductive interference would be effective in (iv) prevention and (v) host limitation. Therefore, pest management strategies of sterile interference should be organized based on the strength of the mating interactions and the number of released sterile insects (Fig. 2).

1. Reducing the abundance of more than one pest species with single sterile species

2. Reducing a pest population, when its sterile insects are not available, with sterile heterospecies

  • a Comparable consequences of reproductive interference in natural populations, summarized in Nishida et al. 23 based on the theoretical prediction.
  • b Required relative strength of interfering effect from sterile insect on the targeted species. Note that weaker interfering effect can be compensated by increasing the number of releasing sterile insects (see Fig. 2).
  • c Several strategies can be combined in a single program.

Among the possible cases (Table 1), sterile interference would be most effective for prevention of (re)introduction. In the area with high risk of introduction of several pest species, it is difficult and costly to maintain a sterile program for all possible species. With sterile interference, however, it may be possible to control all the members of the species group by releasing only the strongest species in terms of reproductive interference, even when we cannot predict the identity and the timing of introduced species. Additionally, since the density of invading pests should be low, they can be eradicated even when the effect of sterile interference is relatively weak. Similarly, containment of closely related pest species in the restricted area would also be promising. On the contrary, sterile interference may be unsuitable for complete eradication of closely related species of the main targeted species because it requires strong reproductive interference.

Importantly, sterile species might have already controlled population growth of the other species in the past programs that adopted the standard sterile insect technique. However, the effects of released sterile insects on the heterospecific dynamics are largely unknown, probably because researchers have not focused on such interspecific interactions. Therefore, it should be important to examine and, if any, purposely exploit the underestimated effects of sterile interference which can work even in the existing programs.

If two or more closely related pest species inhabit an area, which species should be sterilized and released to control all of the pest species by sterile interference? The effects of reproductive interference on the interacting species are often asymmetric, probably because of differences in species recognition abilities and in ecological traits such as body size and the timing of reproduction. 17 Therefore, the direction and speed of species exclusion through reproductive interference depends on the specific combination of interacting species. In several Tephritid species, for example, there is a clear hierarchical pattern in species displacement: specifically, (i) the displacement is always unidirectional for a given pair of species and (ii) the effect is transitive for any combinations of fly species that is, if species A outcompetes species B and species B outcompetes species C, then species A always outcompetes species C (Fig. 4). 31 Therefore, if these displacement patterns are caused by reproductive interference, the most efficient strategy for suppressing the fruit fly community with sterile interference is to choose for sterilization the ‘strongest’ species, in terms of reproductive interference, among the pest species inhabiting the focal area. Among Bactrocera Tephritids, this could be the Oriental fruit fly, B. dorsalis (Fig. 4) however, more studies are required to support this prediction because multiple biotic and abiotic factors can shape and even alter the dominance among species. A sophisticated sterile interference program that incorporates knowledge about reproductive interference, such as about its asymmetric properties, will have greater practical application.


DISCUSSION

Before the release experiments, the Moscamed team was engaged in several public relations activities in the release area, which resulted in an overall good acceptance of the drone releases by the general public (see the Supplementary Materials for details). The data of this trial indicate that releasing sterile Aedes mosquitoes from an UAV platform is feasible with a uniform dispersal of sterile males in the field and a homogeneous sterile-to-wild male ratio as a result.

The induced sterility observed in our trial was unexpectedly high considering the number of sterile males released (

5000 per hectare per week) and the low sterile-to-wild ratio (<1), indicating the high competitiveness (

0.3) of the 35-Gy irradiated sterile males. This compared favorably with an index of <0.06 that was observed for ground-released RIDL A. aegypti males (14). It is generally assumed that the competitiveness of irradiated sterile male mosquitoes is reduced because irradiation causes somatic damage. Obviously, excessive irradiation will impair competitiveness of any insect, but in general, it is possible to obtain a trade-off between a dose obtaining >99% sterility of the males without substantially affecting their biological quality (29, 30). For example, a competitiveness of 0.7 to 1.0 was observed for irradiated male A. albopictus under semi-field conditions in Italy (31) and of 0.4 to 0.8 in Reunion island (32). Moreover, flight ability of A. albopictus, A. Aegypti, and Anopheles arabiensis was not impaired with radiation doses of up to 40, 90, and 50 Gy, respectively (25, 33). Last, triple Wolbachia-infected male A. albopictus irradiated with 40 Gy showed a competitiveness close to 1 in walk-in field cage studies and of 0.5 to 0.7 in the field. This resulted in successful suppression of two isolated target populations in China (9).

In the present study, the high competitiveness was also possible because good irradiation practices were adhered to by irradiating low amounts of pupae under normoxic conditions. This is very relevant because irradiating large numbers of pupae can lead to anoxia, which increases their radiation resistance, thus making it necessary to increase the dose, which consequently will increase the somatic damage, not to forget that anoxia is in itself damaging (34). It needs to be emphasized that most of the reduction in quality of the irradiated males needed for the SIT is not related to irradiation per se but mostly to the mass-rearing, handling, and release processes of the sterile males (29). This study shows that, when these components are mastered, competitiveness of the released male mosquitoes will be adequate to ensure success in the field.

Successful release of sterile males from a drone is an important outcome, especially in view of the low dispersal capacity of Aedes mosquitoes. To obtain the same coverage using ground releases would have required a release site every 80 m, taking into account the observed median dispersal distance. Releases from the ground in the required 63 release sites would have necessitated two field staff, a vehicle, and 2 hours of work. The UAV release system used in this trial could cover much larger areas by replacing the battery and release cassette more frequently (every 20 to 25 min, given the autonomy of the drone at the speed of 10 m/s used in this study) or by using several UAVs that would fly in an echelon formation. The release system might also be mounted on a motorcycle or a bicycle for ground releases in an urban setting. Further improvements to the system are currently under development i.e., while ensuring the same autonomy, the mosquito load may be doubled (100,000), the total weight would remain below 2 kg, and a parachute could be added to the system to operate safely in urban areas (35). In addition, improvements will be needed with respect to insulation to ensure a stable temperature below 10°C throughout the flight (fig. S10).

The use of a UAV-based system for the aerial release of mosquitoes will substantially reduce the operational release costs. For example, in an IIT-SIT trial against A. albopictus in China, the cost of releasing from the ground was estimated at 20 USD per hectare per week, which could be reduced to an estimated 1 USD per hectare per week using a drone (9). Irrespective of the size of the target areas, UAVs might be a good substitute for ground releases to mitigate some of the limitations of ground releases, i.e., no uniform distribution of the sterile males due to the point releases and accessibility of some sites.

In the future, it might even be envisioned that chilled adult mosquitoes are irradiated when already packed into the release cassettes that could then be shipped using courier services from production to release sites within 48 hours (36). This would make the technology even more cost-effective, because it would abolish the need for costly emergence and release centers in the target areas. The International Atomic Energy Agency (IAEA) and the WHO recently published a guidance framework to assess the feasibility of using the SIT as a mosquito control tool and thus reducing or eliminating Aedes-borne diseases (30). This guidance covers all processes for decision support, including risk assessment, regulatory and technical aspects (e.g., insect mass rearing), entomological and epidemiological indicators, as well as community involvement, cost-effectiveness, and program monitoring and evaluation. These recommendations will be applied in the 34 currently implemented SIT pilot projects against Aedes species worldwide to maximize the chances of success (37).


How to calculate the number of sterile insects that must be released in SIT? - Biology

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Modeling the Sterile Insect Technique for Suppression of Light Brown Apple Moth (Lepidoptera: Tortricidae)

John M. Kean, 1,* David Maxwell Suckling, 2 Lloyd D. Stringer, 2 Bill Woods 3

1 1AgResearch Limited, Private Bag 4749, Christchurch 8140, New Zealand.
2 3The New Zealand Institute for Plant & Food Research Limited, Private Bag 4704, Christchurch 8140, N
3 4Department of Agriculture and Food, Western Australia, Baron-Hay Court, South Perth, WA 6151, Austr

* Corresponding author, e-mail: [email protected]

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A population model was derived for light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), subject to the sterile insect technique (SIT). The model was parameterized from the literature and from recent laboratory studies conducted in New Zealand and Australia. Relationships were fitted for several model parameters that vary with irradiation dose, allowing the model to simulate effectively complete sterility at 300 Gy through inherited sterility occurring from lower doses. At 300 Gy, the model suggests that eventual population extinction is 95% probable when the ratio of released to wild males in monitoring traps exceeds 6.4. Higher overflooding rates would be required to achieve eradication more rapidly. The optimal release interval, in terms of minimizing the required rate of production of factory moths, is approximately weekly. There is little advantage in releasing males only compared with releasing both sexes. Female-only releases are unlikely to be a useful tool for inherited sterility eradication because there is no reduction in the fertility of F1 offspring. The critical release rate required to halt population increase declines with decreasing irradiation dose, but at doses of <171 Gy there is a risk that irradiated-lineage moths may form a self-sustaining population, making eradication by SIT alone impossible. The model suggests that a dose of around 200 Gy may be optimal because the resulting inherited sterility would reduce by a third the number of factory moths required compared with 300 Gy.

© 2011 Entomological Society of America

John M. Kean , David Maxwell Suckling , Lloyd D. Stringer , and Bill Woods "Modeling the Sterile Insect Technique for Suppression of Light Brown Apple Moth (Lepidoptera: Tortricidae)," Journal of Economic Entomology 104(5), 1462-1475, (1 October 2011). https://doi.org/10.1603/EC11086

Received: 21 March 2011 Accepted: 1 July 2011 Published: 1 October 2011


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