Double trouble for invasive mosquitoes

More than 90% of disease-carrying mosquitoes have been eliminated in a field trial in China by combining two techniques that have thus far been of limited value on their own.

To reduce their ability to breed, researchers infected their victims – all of them of the invasive, disease-carrying species Aedes albopictus – with bacteria, as well as irradiating them. 

Their hope is that they can reduce populations of A. albopictus, and with it the transmission of viruses such as dengue, Chikungunya and Zika. 

The international team was led by Zhidong Xi from Sun Yat-sen University–Michigan State University Joint Centre of Vector Control for Tropical Diseases in China, and included researchers from China, Australia, the US and Austria. 

Their findings are published in the journal Nature.

The researchers note in their paper that A. albopictus is “particularly challenging to control using traditional approaches”. Those approaches include sterile insect technique (SIT) and incompatible insect technique (IIT). {%recommended 8837%}

SIT involves releasing artificially reared, radiation-sterilised males into the field to mate with wild females, thereby preventing them from producing viable offspring. It has successfully suppressed other insect pests, but has not been widely used against mosquitoes because of the difficulty of irradiating males without reducing their mating competitiveness and survival. 

IIT is an attractive alternative. It involves releasing males infected with the maternally inherited endosymbiotic bacteria Wolbachia, resulting in sterile matings with field females that are not infected with the same Wolbachia strain. It has little or no effect on male mating competitiveness and survival, but the accidental release of fertile females risks causing population replacement.

“Consequently,” the authors write, “previous studies have proposed combining IIT and SIT so that any residual females that are not removed from the released males are sterilised using low-dose irradiation without affecting the males’ mating competitiveness or survival.” 

And that’s what Xi and colleagues did. Because A. albopictus is superinfected with two native Wolbachia strains, they introduced an artificial triple-Wolbachia infection, as well as sterilising their factory-reared adult males with a dose of irradiation low enough that it would not impair mating competitiveness.

They then undertook an open-release field trial in residential areas of two isolated riverine islands in Guangzhou, the city with the highest dengue transmission rate in China, and where A. albopictus is the only vector.

They subsequently reported that the number of wild mosquitos reduced annually by 83% to 94%, with no numbers detected up to six weeks following the release. 

As such, they suggest, the trial demonstrates the feasibility of area-wide application of combined IIT-SIT for mosquito vector control. 

“The combined IIT–SIT approach is environmentally friendly and cost-effective, enabling vector control in complex and inaccessible urban habitats in which implementation of standard vector control is difficult, as released males actively seek wild females, and allows release of much higher numbers of male mosquitoes in comparison to IIT alone, while simultaneously protecting against accidental female release,” they write.