Scientists have nearly wiped out the local populations of one of the world’s most invasive mosquito species on two islands in Guangzhou, China.
In a study published Wednesday in the journal Nature, an international research team combined two insect-control techniques to prevent Asian tiger mosquitoes (Aedes Albopictus) from reproducing. It’s the first time the techniques have been deployed in combination to control wild mosquito populations.
The first approach, called the sterile insect technique (SIT), involves releasing huge numbers of insects that have been sterilized using radiation. The objective is for them to mate with their wild counterparts but produce no offspring. The technique hasn’t worked well in mosquitoes, though, because irradiating males enough to sterilize them makes them less competitive as mates. And as a result, females give preference to wild males.
Using SIT on female mosquitoes isn’t a good option either. Although females require a lower dose of radiation than males to be sterilized, releasing sterilized females doesn’t affect the overall population because males will continue mating with wild females.
Enter the incompatible insect technique (IIT). Asian tiger mosquitoes are naturally infected with some strains of a bacterium called Wolbachia, and their offspring are only viable if both parents carry the same strain or strains. Scientists can interfere with reproduction by infecting lab-grown male mosquitoes with strains of Wolbachia not present in the wild populations. Unlike irradiated males, the males carrying new Wolbachia strains maintain their competitiveness as mates. And when one of these lab-grown males mates with a wild female, their eggs won’t be viable and the pair will fail to reproduce.
While IIT is generally effective at reducing mosquito reproduction, it quickly becomes ineffective if even a few females are unintentionally infected with a compatible Wolbachia strain in the lab and released. Females lay hundreds of eggs in their lifetime, and a mother can pass on her strains ofthe bacteria, quickly producing a new generation of fertile offspring that are reproductively compatible with those grown in the lab.
Combining SIT and IIT, the researchers hoped, could take advantage of each method’s benefits: Batches of lab-grown male mosquitoes could be infected with a new strain of the bacterium, keeping them competitive as mates but incapable of successfully reproducing with wild females. And then exposing the whole group to low-dose radiation could sterilize any females that were accidentally infected along with the males, without compromising the male population.
So, a team led by Zhidong Xi from Sun Yat-sen University–Michigan State University Joint Centre of Vector Control for Tropical Diseases in China, mechanically sorted Asian tiger mosquito pupae into male and female groups. They infected the male group with a strain of Wolbachia that doesn’t exist in Guangzhou’s wild population, then exposed them to low levels of radiation to sterilize any stray females that may have ended up in the mix. Then the researchers released the mosquitoes weekly during peak breeding season for two consecutive years on both islands.
This combined approach reduced the number of the local population’s hatched mosquito eggs by more than 94 percent and reduced human bite rates by over 88 percent at the two locations studied.
“That’s very impressive,” Stephen Dobson, a medical entomologist at the University of Kentucky, Lexington and the founder of MosquitoMate, a company that commercializes Wolbachia as a mosquito-control tool told Giorgia Guglielmi at Nature.
Asian tiger mosquitoes carry harmful viruses such as dengue and chikungunya, and can pose a significant public health concern. Current methods for controlling them, which include pesticides and removing standing water, are ineffective, Dobson told Guglielmi. And female mosquitoes can live as long as a month, biting humans and continuing to transmit disease throughout their life.
Though the technique has potential as a public health strategy in regions where the Asian tiger mosquito is found, it’s only a preventive measure, Zhiyong Xi, a medical entomologist at Michigan State University and lead researcher on the study told Kashmira Gander at Newsweek. "In terms of disease control, this approach may not be suitable in emergency situations during disease outbreak," Xi told Gander.
But, the potential to eliminate these pests before they spread could help stop public health crises before they start. “A new tool like what’s being described in this paper is very much needed,” Dobson told Guglielmi.