# Preventing the establishment of invasive exotic mosquitoes

Jérémy Bouyer, Diana P. Iyaloo, Thierry Baldet

► To cite this version:

Jérémy Bouyer, Diana P. Iyaloo, Thierry Baldet. Preventing the establishment of invasive exotic mosquitoes. Trends in Parasitology, 2025, 41 (8), pp.617-619. 10.1016/j.pt.2025.06.001. hal-05249249

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Submitted on 11 Sep 2025

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Forum

Preventing the
establishment of
invasive exotic
mosquitoes

Jérémy Bouyer ID 1,2,3,*,
Diana Iyaloo ID 4, and
Thierry Baldet ID 1,2

<figure><img src="image_2.png" /></figure>

The boosted sterile insect tech-
nique, which involves releasing
sterile males treated with a biocide
such as pyriproxyfen to contami-
nate breeding sites, has been
shown to be effective against
*Aedes* vectors. This approach
could prevent the establishment of
invasive mosquitoes sharing the
same larval habitats, such as
*Anopheles stephensi*.

Threats from invasive mosquito
species

Vector-borne diseases represent a major
global health challenge, accounting for
17% of all infectious diseases and resulting
in more than 1 million deaths annually, as
reported by the World Health Organization
(WHO) [1]. Of these diseases, malaria,
transmitted by *Anopheles* mosquitoes
and arboviral diseases such as dengue,
chikungunya, yellow fever, and Zika, trans-
mitted by *Aedes* mosquitoes, are of partic-
ular concern.

Malaria remains a public health priority
worldwide, with 263 million cases re-
ported by the WHO in 2023, mainly in
tropical Africa. *Anopheles stephensi*,
which was originally an urban vector of
malaria in India, is invading the entire
African continent, raising fears of a resur-
gence of this disease in this already partic-
ularly affected region. Its presence was
confirmed in eight African countries since

2012 [2]. Islands of the Indian Ocean region
are also at a high risk of invasion and in-
clude Mayotte, Comoros, Madagascar
(still endemic for malaria) and Réunion,
Mauritius, and Seychelles (free of the dis-
ease). The risk of invasion is even more
worrying for the region because its recent
establishment in Kenya raises fears of a
probable extension toward Tanzania,
Mozambique, and neighboring countries,
which have active, extensive commercial
and human international links with those
islands, notably with Comoros. Although
several methods could effectively reduce
the density of *An. stephensi* [3], none
would allow its elimination after its introduc-
tion into a new territory.

The incidence of arboviruses is also in-
creasing every year. Europe has largely
been invaded by *Aedes albopictus* during
the 2000s, whereas *Aedes aegypti* re-
mained limited to Madeira and the areas
bordering the Black Sea. The invasion of
Europe by *Aedes aegypti* would be cata-
strophic in terms of public health because
it is the most competent vector for most
arboviruses (dengue, chikungunya, Zika).
However, its invasion of Europe is more
and more likely because of globalization
and climate change, especially because it
is regularly introduced, as, for example, in
Marseille in 2018 [4]. It has also success-
fully established itself in Cyprus since
2021 [5].

These challenges and threats are
prompting health managers, supported
by research teams, to strengthen surveil-
lance systems of these vector-borne dis-
eases and response capacities to their
risk of expansion and resurgence. It is
generally established that preventing a po-
tential invasion far outweighs the curative
cost associated with disease casualties
as well as economic impacts of invasive
mosquito vectors [6,7]. Here, we propose
such a prevention strategy based on the
heterospecific impact of the boosted ster-
ile insect technique (SIT) (Figure 1A).

Lessons from boosted SIT trials

During a case-control field trial carried out
in Valencia, Spain, sterile male Ae.
*albopictus* mosquitoes treated with
pyriproxyfen and released at a density of
1700–2700 sterile males/ha/wk led to a
89% and 98% suppression of Ae.
*albopictus* in two villages. In Reunion Is-
land, sterile males of Ae. *aegypti* treated
with pyriproxyfen and released at a density
of ~600 males/ha every 2 weeks led to a
>90% suppression of adult Ae. *aegypti* in
3 months. This trial also led to a 60% sup-
pression of Ae. *albopictus*, which shares
some of the larval habitats with Ae. *aegypti*
in the release area, demonstrating the
heterospecific impact of this method [8].
Similarly, in China, releasing 15 000
*Wolbachia*-carrying male Ae. *albopictus*
treated with pyriproxyfen per ha per week
resulted in a 72.1% suppression of adult
Ae. *albopictus* [9]. These releases also sig-
nificantly reduced the density of *Culex
quinquefasciatus*, although this species
only occasionally overlaps the breeding
sites of Ae. *albopictus* and has different
dominant habitats and peak seasons.

During a semifield experiment carried out in Reunion [10], we found that pyriproxyfen-treated sterile males of Ae. albopictus are capable of contaminating larval breeding sites even in the absence of females! Actually, the concentration of pyriproxyfen in the breeding sites, represented by ovitraps in this experiment, was similar to the presence or absence of females (Figure 1B), showing direct contamination of the larval habitats by boosted males.

Lessons learned from surveillance
of entry points in Mauritius

Because of its connections with Asia and
Africa, Mauritius is particularly exposed to
the risk of introduction of *An. stephensi*.
A network of 38 artificial breeding sites im-
itating the larval habitats of this species is
therefore regularly monitored in the sea-
port of Mauritius [11]. During 25 succes-
sive surveys carried out from July 2022
<figure><img src="image_3.png" /><figcaption>(A) Release of sterile male mosquitoes sp1 treated with pyriproxyfen e.g., *Aedes albopictus*</figcaption></figure>

*Aedes albopictus*

<figure><img src="image_4.png" /><figcaption>Contamination of larval habitats of mosquito sp1, potential habitats for mosquito sp2 e.g., Ae. aegypti and/or *An. stephensi*</figcaption></figure>

*Aedes albopictus*

<figure><img src="image_5.png" /><figcaption>Suppression of mosquito sp1 + prevention of the establishment of other strains of mosquito sp1 and pathogens transmitted by mosquito sp1</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Prevention of the establishment of mosquito sp 2</figcaption></figure>

*Aedes aegypti*

*Anopheles stephensi*

<figure><img src="image_7.png" /><figcaption>(B)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(C)</figcaption></figure>

<figure><img src="image_9.png" /></figure>

Trends in Parasitology

Figure 1. Application of the boosted sterile insect technique to suppress a local mosquito species while preventing the establishment of exotic species. (A) Principle of the strategy. (B) Amount of pyriproxyfen transferred by boosted sterile male *Aedes albopictus* in ovitrap water from large field cages, conditionally on the presence of untreated females; vertical red dotted lines represent the concentration leading to 10% and 95% inhibition of adult emergence (source: modified from Figure 2 of [10]). (C) Heat map showing the frequency of artificial mosquito breeding sites positive for larvae of *Ae. albopictus* in the seaport of Mauritius; the numbers represent the density of larvae (source: modified from Figure 3 of [11]). Created with BioRender. Abbreviations: CI, confidence interval; IE50, inhibition emergence 50%; IE100, inhibition emergence 100%; Sp, species.

to April 2023, no specimens of *An. stephensi* were detected. However, 85% of these breeding sites (n = 32) were positive for *Ae. albopictus* (Figure 1C), with about one-third of potential breeding sites positive for *Ae. albopictus* during each survey. This ability of this species to occupy the same breeding sites as *An. stephensi* in tropical areas has also been highlighted [12].

## Proposition of a new strategy to prevent mosquito invasions

On the basis of these two results, we propose to use boosted SIT against a local species of major public health interest (e.g., *Ae. albopictus*, the vector of dengue and chikungunya in Reunion and Mauritius) to contaminate the larval habitats that may be shared by other species and that are threatening to invade a given

territory, with pyriproxyfen or other biopesticides active at low concentration (Figure 1A). In this particular case, this method, applied to points of entry, can prevent the introduction of notable species such as *An. stephensi* and *Ae. aegypti* on these two islands. It will also suppress homospecific populations at points of entry, preventing the transmission of pathogens by this species, as well as the introduction of other populations of the same species, which may be more competent or resistant to insecticides than the already established populations. In Europe, this strategy can be applied to prevent the introduction of *Ae. aegypti* using *Ae. albopictus* and the reverse in African, Asian, or American countries not yet colonized by *Ae. albopictus*. It is actually much more difficult to apply genetic control against

these species when they are both present because of the risk of competitive release, where the specific suppression of a target species can favor another one sharing the same ecological niche [13]. A major advantage of the strategy we are proposing is that besides its cost-effectiveness and rapidity of deployment, it circumvents the risk of importing an invasive species into a country it has not yet colonized for its mass production and sterilization. Instead, sterile males of a local vector species of major interest sharing similar breeding sites could be mass produced in existing SIT facilities [14] to counter a potential invasion. This strategy has many advantages over other methods: it is efficient on a low-density target species, it can reach and treat cryptic or inaccessible breeding sites, and it presents no risk of resistance. Research is needed to evaluate in the field the density and frequency of sterile males to be released to achieve a treatment coverage and a concentration of biocide in the larval habitats that will completely inhibit potential invaders at points of entry. Also, it will be important to improve the pyriproxyfen formulation in order to reduce the large variations of the concentration observed during the semifield trial [10], because exposure to sublethal doses may lead to the development of resistance to this molecule or even to cross-resistance to other insecticides [15] in the species used to contaminate the breeding sites. Given that the response to a potential invasion must be rapid and effective, this strategy will require investment in appropriate infrastructure and the strengthening of local capacity to mass produce local species of public health interest as a preventive measure, which is never easy to finance. Also, it will always be necessary to improve surveillance systems in order to be able to detect the presence of an invasive mosquito at an early stage and to monitor the efficiency of this strategy to prevent its establishment and spread.
Finally, some large breeding sites suitable to *An. stephensi* may not be shared with *Aedes* species, and it will be necessary to destroy or treat them with other insecticides (e.g., *Bacillus thuringiensis*) to avoid the selection of different oviposition preference in this species. Fortunately, these breeding sites are much easier to locate and control around entry points that the cryptic ones.

## Acknowledgements

This research work was carried out as part of the One Health Indian Ocean Dispositif en Partenariat (DP) partnership research network. The partnership between Réunion and Mauritius on vector risks is supported by the Interreg PRERISK_OI project funded by Région Réunion and the European Regional Development Fund (ERDF). J.B. and T.B. are also funded by the French Ministry of Higher Education and Research (MESR) through the OPTIS project (Opérationnalisation de la technique de l'insecte stérile contre les *Aedes* vecteurs de dengue à La Réunion). D.I. is also funded by the Ministry of Health and Wellness of Mauritius.

## Declaration of interests

J.B. is cofounder and CEO of MoSITouch, a company offering vector control services, particularly the boosted sterile insect technique.

## References

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<footnote n="1">¹ASTRE, CIRAD, F34398 Montpellier, France</footnote>

<footnote n="2">²ASTRE, Cirad, INRAE, Univ. Montpellier, Plateforme Technologique CYROI, Sainte-Clotilde, La Réunion, France</footnote>

<footnote n="3">³MoSITouch, Le Tampon, 97430, La Réunion, France</footnote>

<footnote n="4">⁴Vector Biology and Control Division, Ministry of Health and Wellness, Curepipe, Mauritius</footnote>

<footnote>*Correspondence:
jeremy.bouyer@cirad.fr (J. Bouyer).</footnote>

<footnote>https://doi.org/10.1016/j.pt.2025.06.001</footnote>

<footnote>© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).</footnote>