Mosquitoes hibernate during drought

Malaria mosquitoes in the Western Sahel can survive periods of drought by temporarily hibernating in the summer. It has been known for about ten years that this so-called aestivation occurs, but now an international team of biologists in Mali has also studied how common this strategy is. About 1 in 5 mosquitoes of the species Anopheles coluzi goes into summer sleep, the researchers wrote last week a Ecology and evolution of nature. This knowledge can be applied to fight malaria.

About 94 percent of malaria cases occur in Africa, including the western Sahel region, which includes Senegal, Gambia, Mauritania and Mali. Infectious disease is caused by parasites that can enter the body during a mosquito bite. Most malaria mosquitoes, including the common one Anopheles Gambia in Anopheles arabiensis, do not occur during the dry season, which lasts six to eight months from December to July in the Western Sahel. Mosquitoes themselves live three to six weeks and can only lay eggs if water is available.

Surprisingly enough, in that dry season there is occasionally a Anopheles coluzimosquito to see. And even more remarkable is that adult mosquitoes of that species reappear in large numbers soon after the dry season, much faster than a new generation could reach maturity. Several explanations have been suggested for their appearance: either they are engaged in long-distance migrations (and fly arriving from the wetter regions) or in aestivation (referring to the Latin summerwhich means summer).

isotopes of deuterium

Such a summer sleep is not great for animals – many species of frogs and toads do. In this way they survive the drought. of the Anopheles coluzimosquito was known that the species can spend the summer, but evidence of this from field studies has so far been limited.

To investigate how common estivation occurs in mosquito species, biologists provided mosquito larvae in two Mali villages with a “marker” in the form of deuterium isotopes, also called heavy water (D₂OR). The larvae were in a pool of deuterium-enriched water by the scientists, where they ate microorganisms enriched with deuterium. In the end, an average of about 1 in 3 mosquitoes were clearly marked.

In the seven months following the marking of the larvae, the researchers captured as many adult mosquitoes as possible, up to and including the start of the new period. All those mosquitoes should be migrated mosquitoes or summer mosquitoes, because no eggs were laid at that time and therefore no new generations emerged. (The same was true for adult mosquitoes caught during the first ten days of the new rainy season, because there was no way they could belong to the first new generation of the rainy season.)

The mosquitoes were caught with a special “suction tube” and identified before being killed. They were then divided into various parts of the body (head, thorax, abdomen, legs and wings) and the isotope ratio was determined for each thorax. As a result, it was found that at least 18 percent of the Anopheles coluzithe mosquitoes still had enriched deuterium. In other words, at least 1 in 5 mosquitoes of this species spends the drought period in aestivation. The numbers are likely even higher, as not all mosquitoes have been caught and some may have flown to other villages. The researchers also looked at other causes of high deuterium levels in mosquitoes (for example, natural variations in the presence of heavy water), but found no conclusive evidence for this.

Sterile offspring

The findings provide more information on insect survival in drought conditions, the authors conclude, and may therefore also contribute to malaria control. Introducing genetically modified male mosquitoes (which can only produce sterile offspring) over time in the middle of Anopheles coluzimosquitoes, i.e. before the dry season, female mosquitoes that during the summer would be able to produce sterile offspring only at the beginning of the new season. Sites during the summer could also be detected by mosquito repellents.

Radboudumc professor of epidemiology Teun Bousema, who is not involved in the publication, is enthusiastic about the research and approves of the results. “Inspired by previous results from the same group on aestivation, we also recently examined the presence of mosquitoes in the dry season in Burkina Faso. We see the same small spike in the number of mosquitoes that they describe as well. Their findings about summer sleep and the “awakening” from it explain this. Incidentally, our work suggests that mosquitoes that appear occasionally in the dry season do not play a clear role in the spread of malaria. “

Despite this, he “completely agrees” with the conclusions that the results could help fight back. “The article shows that we can be more creative and ambitious in the use of mosquito control and that we should not focus on the periods when mosquitoes are present in large numbers. The months when the risk of malaria is minimal and mosquitoes have to surviving in an extremely unfavorable climate offer opportunities ».