A pioneering method unveiled last month to separate male and female mosquitoes could be a major step towards using the nuclear-based sterile insect technique (SIT) to control the insects that transmit diseases such as Zika, dengue and chikunguya.
SIT involves using ionizing radiation to sterilize mass-reared insects of the target pest and then releasing them into nature where they mate with wild insects, resulting in no offspring and, over time, reducing the overall insect population. SIT has been employed successfully in over 40 countries against agricultural pests such as various fruit flies, tsetse flies, screwworm and moths pests, and research for its application against the Aedes mosquitoes has intensified in the wake of the Zika crisis last year. The IAEA, in partnership with the Food and Agriculture Organization of the United Nations (FAO), is spearheading global research in the development and application of SIT, including against the Aedes mosquitoes.
The main challenge facing researchers in scaling up the use of SIT against various species of mosquitoes has been the lack of a reliable method to remove females from among the mosquitoes that are released. Eliminating females before release is crucial to the use of SIT against mosquitoes, because it is the bites by female mosquitoes that transfer diseases.
In countries where the use of SIT against Aedes mosquitoes is being tested or planned, such as Brazil, China and Mexico, a manual method is used to separate males from females. Female pupae – the stage in the life of insect between the larvae and the adult stage – are larger than male pupae, offering a way to distinguish and remove females before release. However, this method is very labour-intensive and is therefore not practical for up-scaling to the tens of millions of mosquitoes that would be required in order to use SIT at a larger scale to protect cities from disease transmission, said Rui Cardoso-Pereira, an SIT expert at the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture.
Finding alternative methods for what insiders called “sexing” of mosquitoes is the focus of an ongoing five-year coordinated research project under the auspices of the Joint FAO/IAEA Division, with the participation of experts from 13 countries.
No optical illusion
Researchers at TRAGSA, the Spanish government’s institution focusing on environmental sciences and services, have now built the prototype of a device capable of differentiating male and female mosquitoes via artificial vision and then eliminating the females with the use of laser beams. The device consists of a rotating disk to distribute the mass-reared pupae, which are than analysed using software that can distinguish the sexes based on size, explained Ignacio Plá Mora of TRAGSA’s Pest Control Department.
The preliminary results of the tests conducted have shown that 99.7% of the females were eliminated, while up to 80% of the males survived and could be released, Plá Mora said. “The results achieved are highly satisfactory compared to those obtained by the manual methods that are currently used,” he said.
While the prototype can process a million Aedes males a day, it still does not quite scale up to the level of industrial production required at a regional scale, it will be satisfactory for projects targeting individual towns or villages, particularly in countries where the labour costs associated with manual sorting of mosquito pupae is prohibitive, Cardoso-Pereira said. Further research is ongoing to perfect the method in order to eliminate fewer males and to further upscale it.
TRAGSA’s participation in the coordinated research project has helped them in the development of the new method. “When top experts in an area work together, everyone’s research accelerates,” Cardoso-Pereira said.