Almost a year ago, in June 2019, the first sterile male tiger mosquitoes were released in the Duparc district. A test as part of the TIS (Sterile Insect Technique) project, conducted by the IRD in Reunion, and the results of which have just been made public.
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In the midst of a dengue epidemic in Reunion, it seems even more urgent to work on a long-term method of combating the mosquito. This is precisely what the scientists of the TIS project have been applying for more than 10 years. The results of the first phase, which consists in verifying the survival capacity of sterile male mosquitoes in the wild, have just been made public. And they are conclusive for the implementation of the next phase of the project.
What is the Sterile Insect Technique?
This technique consists of sterilizing male mosquitoes by radiation and releasing them in large quantities in the wild. Females fertilized by these males lay eggs that will never hatch. Used in addition to the “classic” vector control actions, this preventive and non-polluting method makes it possible to reduce mosquito populations and the risk of transmission of dengue or chikungunya.
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On June 15, 2019, 3,000 sterile male tiger mosquitoes found themselves released at Duparc in Sainte-Marie. A desired date in early southern winter, when the mosquito population drops sharply.
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Then followed two other releases: September 7, in winter, when there are the fewest mosquitoes, and November 9, at the beginning of the southern summer, when the number of mosquitoes increases.
These three test releases were not yet intended to “reduce the fertility of wild mosquitoes” as clarified by Louis-Clément Gouagna, medical entomologist at the IRD who coordinates the TIS project in Reunion. “but to assess sterile males in the wild”. Either first observe whether sterile male tiger mosquitoes could survive in the wild, and how they behaved compared to wild mosquitoes.
Released, then recaptured
To be able to make this comparison, the scientists of the TIS project released a batch of 3000 sterile male mosquitoes each time they were released, and another 3000 wild male mosquitoes, marked.
Then, for the next two weeks, around twenty mosquito traps were deployed in the neighborhood (between 25 and 400m around the point of release) to recapture both sterile mosquitoes and wild animals marked by scientists. “The mosquitoes from these recaptures were thus able to be differentiated, in order to assess the proportion of sterile males among the total population captured,” explains the IRD.
Conclusive results
It appears, after studying the recaptures, that a sterile mosquito as a wild mosquito have equivalent probabilities of survival, regardless of the season. They also disperse similarly in nature. “These tests enabled us to note that the sterile males have a behavior and a survival similar to the wild males and an average distance of dispersion of 62 meters (against 54m in the wild)”.
If we now know that these sterile male mosquitoes survive in the wild, it remains for scientists to implement the second phase of the TIS project. And the release strategy will be refined thanks to the present results. This TIS 2B pilot phase has in fact already started since February 2020, funded by the Region as part of the ERDF program and the Directorate General of Health.
More releases coming soon
The IRD and its partners can now envisage future releases this southern winter, taking advantage of a low density of mosquitoes, which should have the effect of “accentuating the male / female imbalance and increasing efficiency”.
It will then be necessary to release many more sterile individuals, because, as Louis-Clément Gouagna specifies, “an effective release must include 5 to 10 times more male mosquitoes than the existing population”.
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It should also be noted that these next releases will be subject to prefectural authorization, like the previous ones. Beyond this prefectural framework, the cowardly tests had also been preceded by an information campaign with residents of the Duparc district, via an information note and a consent form in 84 homes, but also a public meeting . The initiative had received 87% favorable opinions among the residents visited.
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