Researchers have all but obliterated populations of the world’s most invasive mosquito species — the Asian tiger mosquito (Aedes albopictus) — on two islands in the Chinese city of Guangzhou.
They reduced A. albopictus populations by up to 94% using a combination of two promising control techniques in a field trial for the first time. The two-pronged approach1, published in Nature on 17 July, integrates the sterilization of female Asian tiger mosquitoes with the infection of males using Wolbachia pipientis, a bacterium that hinders the insects’ ability to reproduce and transmit disease-causing viruses such as dengue and Zika.
This resulted in one of the most successful eradication trials of A. albopictus to date, says Peter Armbruster, a mosquito ecologist at Georgetown University in Washington DC, who wrote a commentary to accompany the study. Used in tandem with other control methods such as pesticides, the dual approach could be a very powerful tool, he says.
Problems with control
Previous studies have shown that X-ray sterilization of large numbers of male pests like screw worms (Cochliomyia hominivorax), followed by their release into target areas, can reduce the size of wild pest populations. But this is an inefficient way to control mosquitoes because even though irradiated males can still mate, they are less successful than their unaltered counterparts.
In an alternative approach, workers infect laboratory mosquitoes with strains of Wolbachia, which is found naturally in several insect species, including A. albopictus. When male mosquitoes infected with a certain combination of Wolbachia strains mate with wild females carrying a different combination, the insects can’t produce offspring.
But it’s crucial that only male mosquitoes infected with that particular combination are released into the wild, says Zhiyong Xi, a medical entomologist at Michigan State University in East Lansing, who led the study. If females with those strains are also released, they could mate and produce offspring with males carrying the same Wolbachia cocktail. Their offspring could eventually replace the local mosquito population, making future control attempts that rely on Wolbachia infection more difficult.
To prevent this from happening, facilities that rear large numbers of mosquitoes for control purposes usually separate males from females mechanically, based on size differences. But this process isn’t perfect, Xi says, so workers have to do a second, manual screening to remove female mosquitoes. It’s a tedious and time-consuming task that limits the total number of mosquitoes that can be released. So Xi and his team set out to eliminate the need for this process.
An issue of scale
Wild populations of A. albopictus are naturally infected with two strains of Wolbachia. The researchers infected wild mosquitoes with a third strain of Wolbachia to produce a laboratory colony of the insects with three bacterial variants. Then, the team exposed the colony to low levels of radiation that sterilized the females but only slightly reduced the males’ ability to mate.
During the mosquitoes’ peak breeding seasons in 2016 and 2017, the researchers released more than 160,000 of these mosquitoes per hectare each week in residential areas on two islands situated in a river in Guangzhou — the city with the highest rate of dengue transmission in China.
Their hope was that this would vastly reduce the mosquito population because wild females that mated with the altered males — and wild males that mated with sterile lab females — wouldn’t produce offspring. The team tracked population declines in adult female mosquitoes, since they’re the ones that bite people and transmit diseases. And as expected, the average numbers of wild adult females fell by 83% in 2016 and by 94% in 2017.
“That’s very impressive,” says Stephen Dobson, a medical entomologist at the University of Kentucky in Lexington, and the founder of MosquitoMate, a company that commercializes Wolbachia as a tool to control the Asian tiger mosquito.
Current strategies for controlling A. albopictus — including spraying pesticides and removing water-filled containers where the insects lay their eggs — are ineffective, Dobson says. This species lays its eggs in hidden places that can be difficult to monitor and tends to develop resistance to common insecticides, he adds. “A new tool like what’s being described in this paper is very much needed,” he says.
But scaling up the technique into an effective public-health strategy for large regions is the challenge, says Gordana Rašić, a molecular ecologist at the QIMR Berghofer Medical Research Institute in Brisbane, Australia.
Rašić says that Wolbachia-based approaches are promising strategies, and she’s hopeful that developing and testing such tools will help to reduce the incidence of mosquito-borne diseases. “We’re living in very exciting times for mosquito control,” she says.
See the related News & Views ‘A trial to tackle tiger mosquitoes’.