Asian tiger mosquitoes (Aedes albopictus) are among the world’s most invasive mosquito species and can spread dengue and Zika viruses. In a study published today (July 17) in Nature, an international team of researchers has virtually eradicated populations of the insects from two residential areas in China.
“It’s a very nice paper with potentially big implications for the whole vector control field,” says Ewa Chrostek, a researcher at the University of Liverpool in the UK who did not participate in the work. “It builds on previous work . . . but also adds additional safeguards.”
Specifically, Zhiyong Xi of Michigan State University and colleagues infected the insects with the bacterium Wolbachia to limit embryo viability and, as an added precaution, irradiated mosquitoes to induce sterility, and then released millions of male mosquitoes, which don’t bite, at their test sites. The males mated with local females, resulting in a drastic reduction in the populations of A. albopictus.
“In the past . . . people thought only about chemical insecticides for mosquito control,” Xi tells The Scientist. But that strategy is often ineffective as insect populations will often develop resistance to these agents, he says. Plus, insecticides can harm beneficial insects, but our “approach is environmentally friendly,” he says, and there is minimal risk that the population will evolve resistance.
Xi and colleagues started with Asian tiger mosquitoes harboring two strains of Wolbachia that are native to A. albopictus and added a third strain of the bacteria that is common in another mosquito species. Female mosquitoes pass Wolbachia to their offspring through the cytoplasm of their eggs, and mating with a male that carries a different strain causes the resulting embryos to stop developing before they hatch. Prior work has shown that releasing males with different Wolbachia than the strains carried by mosquitoes in the area leads to a decrease in the resident population.
The team sorted male and female pupae, irradiated them to induce sterility, and raised millions of these sterile, Wolbachia-carrying mosquitoes in the lab. The irradiation step serves as a precaution. If any females were missorted and thus made it into the group of males, they could produce live offspring with those males when released, given that they share the same set of Wolbachia.
The combination of Wolbachia-induced incompatibility and irradiation-caused sterilization has been successful before under laboratory conditions, but this study represents the first test of the combo approach in the field. The authors released nearly 200 million male mosquitoes in two test areas in Guangzhou, China, three times per week during mosquito breeding season over a two-year timeframe. They monitored mosquito levels at their study sites and also in nearby control sites. The team observed up to a 94 percent reduction in hatching eggs and in adult A. albopictus numbers in their test sites compared with the control sites, indicating that the strategy was highly effective in suppressing the mosquito populations.
This work is “promising for control of these mosquitoes in local populations,” says Igor Sharakhov, a researcher at Virginia Tech who was not involved in the study. But one open question is, “how scalable this approach would be if we want to control mosquitoes within large cities and even countries.”
Another challenge to widespread implementation is the expense of this type of mosquito control, which requires many hours of work to get the mosquitoes ready to be released. “Dr. Xi’s group—as well as my group in Kentucky—continue to rely on human production of these mosquitoes. There are some mechanical tools, but largely it’s done by human power,” says Stephen Dobson, a University of Kentucky entomologist and Xi’s former mentor who did not participate in the work.
Dobson has commercialized a similar Wolbachia-based strategy (without the irradiation step), which he sell to individual households in Kentucky via a company called Mosquito Mate. It costs about $1,500 for a summer-long plan that includes the release of some 1,000 male mosquitoes twice a week for 20 weeks, he tells The Scientist, adding that the cost is much higher than spraying insecticides and may be out of reach for places with leaner economies.
Dobson’s team is currently collaborating with Verily Life Sciences to engineer solutions that rely less on people to generate the insects, in hopes of making the treatment more affordable. And once Wolbachia-based strategies are approved by the Environmental Protection Agency (EPA), he says, the group will be able to combine them with other proven approaches, such as clearing sites of standing water where mosquito larvae grow.
“Understandably the EPA wants to see whether it works alone,” he says. “But once we get past that, I think that will also be a very interesting period where the cost can come way down.”
X. Zheng et al., “Incompatible and sterile insect techniques combined eliminate mosquitoes,” Nature, doi:10.1038/s41586-019-1407-9, 2019.