Female tiger mosquitoes, vectors of the chikungunya virus and of dengue fever, had been thought to mate only once during their short few weeks of life. They are apparently much less faithful than imagined, however. A team representing the IRD and its partners(1) has discovered that they may in fact mate with several males during their short lives. What is more, the same clutch of eggs can be engendered by different fathers! For their part, the males can mate with over 10 different females.

These recent studies, conducted on the island of Reunion and published notably in the journal PLOS ONE, will make it possible to perfect the "sterile insect technique." This innovative approach to fighting against vectors of disease consists of fooling females into mating with infertile males so as to reduce the population of mosquitoes in the wild. Researchers will, in particular, be able to establish what quantity of sterilised males to release in order to compensate for the infidelities of the females.

Dengue fever and the chikungunya virus continue to take their toll all over the world. In the absence of either a vaccine or effective treatment, attacking the vector remains the sole course of action. If this new method proves promising, it will offer an ecologically acceptable alternative to the use of insecticides.

Female tiger mosquitoes, the vectors of the chikungunya virus, are much less faithful than entomologists had previously thought. A team of researchers from IRD and its partners(1) recently discovered that the ladies of the Aedes albopictus species, to give them their scientific name, mate several times during their short lives. Until now specialists had thought that they copulated with a single male during their few short weeks of existence. According to this generally accepted hypothesis, once they had been impregnated they developed an aversion to any further insemination. The discovery of an unexpected sexual appetite on their part has major implications for the newly-developed "sterile insect technique."

Objective: to fool the females

IRD researchers on the island of Reunion have been working on this antivector strategy for several years. Based on the sterilisation of male mosquitoes, it is intended to reduce the population of vectors found in natural conditions. The method consists of releasing sterilised, and therefore infertile males in order to 'deceive' the females, who had been thought to mate only once, into doing so with these infertile individuals. As shown in previous studies, male mosquitoes sterilised by radiation(2) in the laboratory nevertheless succeeded in mating normally with the females. The latter continue to produce a large number of eggs, but they are non-fertilised and will thus produce no offspring.

One mother, several fathers

To optimise this new antivector technique, a good understanding of biology, ecology and the behaviour and competitiveness of mosquitoes is essential. The researchers thus attempted to determine whether the eggs of the female tiger mosquitoes captured in Reunion actually came from a single male or had involved several male progenitors. Thanks to genetic markers, they were able to demonstrate that the same clutch of eggs could be the result of mating with several different males. The new study, published in PLOS ONE, thus reveals that the females are in reality likely to have recourse to multiple inseminations. This result suggests that the repulsion felt by females of the species for any new union after mating a first time is not systematic.

Highly active males

During a previous study, the research team had also shown that male tiger mosquitoes are very efficient. They remain sexually active for over 14 days. Moreover, scientists have underlined the high number of females impregnated by a single male: the tiger mosquito is capable of mating with over ten females on average during this period. Where does this great reproductive success come from? This fundamental question remains unanswered. Sterilised male mosquitoes do, however, seem a little less competitive.

These results will enable researchers to maximise the effectiveness of the sterile male technique, and in particular to specify how many infertile males should be released in order to increase the probability that a female will mate with one of them and so as to compensate for their slightly lower attractiveness.

Dangerous females

The much-feared tiger mosquito is responsible for the transmission of several diseases throughout the world. It is the main vector of the chikungunya virus and the second of dengue fever -- after another mosquito of the genus Aedes. As in the case of the famous members of the anopheles genus that carry malaria, it is the females that transmit the viruses to men and to animals. They in fact bite the latter in order to feed on their blood and thus ensure the development of their eggs.

Rarely fatal, the chikungunya virus is nevertheless considered a major public health problem in Asia, South America and Africa. In 2006 on the island of Reunion, the infection affected almost a third of the population, equivalent to hundreds of thousands of people in total throughout the Indian Ocean. The most frequent symptoms are fevers, joint pains and headaches. No vaccine or treatment has been developed to date. The only available means of action thus continues to be the antivector strategy.

In light of this, the sterilisation of male mosquitoes could offer an alternative that is more respectful of the environment and of ecosystems than the use of insecticide sprays. If at the end of the laboratory research phase this approach is indeed judged promising, researchers may proceed in the next few years with experimental releases in the field. A significant effort is needed beforehand in order to increase awareness, however, to ensure that the new method is well accepted and supported by the local population.

(1) These studies were conducted in partnership with the Centre for the Study and Monitoring of Emerging Diseases in the Indian Ocean (CRVOI) in Reunion, the Indian Ocean Regional Health Agency and the International Atomic Energy Agency in Austria.

(2) Sterilisation by radiation consists of exposing a body very briefly to a source of radioactive emissions. This procedure is currently used on food and sanitary products, etc.