Tricking virgin female mosquitoes into thinking they've mated could significantly reduce the spread of malaria, according to a new study.

Researchers at Imperial College London, Harvard School of Public Health, Boston, and the University of Perugia, Italy, have been investigating Anopheles gambiae mosquitoes, the main transmitters of malaria, and have shed further light on how the male controls the post-mating behaviour of the female.

The study, in Proceedings of the National Academy of Sciences, shows how a steroid hormone called 20-hydroxyecdysone (20E), which is passed to the female mosquito during sex, not only induces her to lay her eggs, but also makes her unreceptive to other males – preventing her from mating again.

“We knew that female An. gambiae mate only once in their lifetime, and now we know why,” says lead author Dr Flaminia Catteruccia, honorary research fellow in the Department of Life Sciences at Imperial College London who is also affiliated to Harvard and the University of Perugia.

“It’s unusual in evolutionary terms that the continued survival of this species depends on a single mating event. And as successful as it’s been so far, we believe that this could make them vulnerable to new population control strategies –their Achilles heel, as it were.  If we can mimic the mechanism of this hormone, we could prevent virgin females from mating, effectively sterilising them for life. This would cause a rapid and significant reduction in their population and help reduce the malaria burden.”

Although a female mates only once, she can lay several batches of eggs throughout her 3-week lifetime thanks to a specialised compartment that stores the sperm and keeps it viable. This high reproductive rate makes An. gambiae the most efficient vector of the human malaria parasite.

Malaria is a leading cause of death in tropical and subtropical regions. The US Centres for Disease Control and Prevention estimates that malaria causes over 650,000 deaths each year, 90 per cent of them in Africa – and most of them children.   

Without an effective malaria vaccine, insecticides remain the best weapon to tackle malaria transmission, but insecticide resistance in mosquitoes is on the rise, and new methods to control mosquito populations are urgently needed.

Harvard co-author Dr Sara Mitchell is confident that applying this discovery to mosquito control strategies is achievable, and the team is currently looking to secure funding to progress the research. 

“This hormone is known to play a key role in larval development across all insects, and there are already several commercially available compounds that mimic its functions,” says Dr Mitchell, who began working on this project as a postdoctoral researcher at Imperial.

“In principle it should be possible to produce a sterilising compound based on 20E to use alongside insecticides on mosquito nets and sprays. If a virgin female survived exposure to the insecticide because she was already resistant, the sterilising compound would prevent her from reproducing. This would not only reduce mosquito populations, but also help to control the levels of insecticide resistance, as females would not pass this trait on to the next generation.”

Support for the study came from the European Research Council FP7 ERC Starting Grant project ‘Anorep,’ the Biotechnology and Biological Sciences Research Council (BBSRC), and a National Institutes of Health (NIH) grant to Dr Catteruccia.

Reference: Gabrieli et al.  'Sexual transfer of the steroid hormone 20E induces the postmating switch in Anopheles gambiae' Proceedings of the National Academy of Sciences (2014), doi: 10.1073/pnas.1410488111