Mosquitoes hibernate during drought

Malaria mosquitoes in the western Sahel can survive dry spells by temporarily going into summer hibernation. 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 investigated how common this strategy is. Roughly 1 in 5 mosquitoes of the species Anopheles coluzzii goes into summer sleep, the researchers wrote last week in Nature Ecology & Evolution. This knowledge can be applied to combat malaria.

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

Remarkably enough, in that dry season there is now and then a Anopheles coluzziimosquito to see. And even more remarkable is that the adult mosquitoes of that species reappear in large numbers immediately after the dry season, much faster than a new generation could reach maturity. Several explanations have been suggested for their appearance: they are either engaged in long-distance migration (and flying in from wetter regions) or aestivation (referring to the Latin aestaswhich means summer).

Deuterium isotopes

Such a summer sleep is not exceptional for animals: many frog and toad species do it. In this way they survive the dry period. Of the Anopheles coluzziimosquito was known that the species can summer over, but evidence for this from field studies has been limited so far.

To investigate how common aestivation occurs in the mosquito species, the biologists provided mosquito larvae in two Malian villages with a ‘marker’ in the form of deuterium isotopes, also called heavy water (D₂O). The larvae were in a water pool enriched with deuterium by the scientists, where they ate microorganisms enriched with deuterium. Ultimately, an average of about 1 in 3 mosquitoes was clearly marked.

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

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

Infertile offspring

The findings provide more insight into the survival of insects in dry conditions, the authors conclude, and may therefore also contribute to malaria control. By introducing genetically modified male mosquitoes (which can only produce sterile offspring) in time in the midst of the Anopheles coluzziimosquitoes, i.e. before the dry season, the female mosquitoes that oversummer would only be able to produce infertile offspring at the start of the new season. Oversummer sites could also be detected by mosquito repellents.

Professor of epidemiology Teun Bousema of Radboudumc, who is not involved in the publication, is enthusiastic about the research and endorses the results. “Inspired by previous findings on aestivation of the same group, we recently also looked at the occurrence of mosquitoes in the dry season in Burkina Faso. We see the same small spike in mosquito numbers that they also describe. Their findings of summer sleep and ‘awakening’ from it explain that. Incidentally, our work does suggest that the mosquitoes that occasionally appear in the dry season have no clear role in the spread of malaria.”

Despite this, he “completely agrees” with the conclusions that the results could help combat. “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 the mosquitoes are present in large numbers. The months in which the risk of malaria is small and the mosquitoes have to survive in an extremely unfavorable climate offer opportunities.”