IAEA-based new technique and machine helping speed up sorting of tsetse fly

The tsetse is a bloodsucking insect found in sub-Saharan Africa which transmits a parasite that can be fatal to both animals and humans.

Fighting the tsetse fly using irradiation involves rearing and then releasing in the environment sterile male flies to mate with wild females producing no offspring, reducing the population over time. But sorting thousands of male flies from females every week, which is done by hand, is a tedious process. A new technique and machine – developed by the IAEA in cooperation with the Food and Agriculture Organization of the United Nations (FAO) – with an infrared, high-speed camera are helping to speed up the sorting.

The tsetse is a bloodsucking insect found in sub-Saharan Africa which transmits a parasite that can be fatal to both animals and humans. Their sorting is crucial so that only males can be sterilized, while colony females, who only produce between six and ten offspring in their lifetime, stay in the laboratory to maintain the tsetse colony needed for the sterile insect technique (SIT) programme.  

Technicians at the FAO/IAEA Insect Pest Control Laboratory (IPCL) in Seibersdorf, Austria, sort up to 7700 tsetses per week for use in SIT projects. With this new technology streamlining the sorting process, more tsetse can be sorted leading to improved disease control. Moreover, the ability to sort tsetse earlier, at their pupae state, allows for healthier sterile male tsetse to be sent to field programmes in sub-Saharan Africa as pupae are less fragile than adults.

“Sorting the insects manually is very time consuming and accounts for up to one-third of the work that we do in the insectary,” said Andrew Parker, who recently retired as the leader of the livestock pest (LP) research group at the IPCL. “Many attempts were made throughout the years to identify the sex of the tsetse pupae. We looked at differences in shape, density and weight loss, among many other factors before this new technique was developed.”

When the tsetse is a pupa – the stage before the fly becomes an adult – males and females look alike, so tsetse flies had to be separated as adults. “The previous sorting process of the tsetse was time-consuming; we would manually screen the adult tsetse flies after making the fly immobile through cooling,” said Chantel de Beer, a research entomologist and the leader of the LP research group at the IPCL. “Through this new sorting technique, we can distinguish between male and female tsetse flies earlier, meaning that the flies can be transported in the pupal stage. This allows for the safer and more streamlined transport of tsetse to FAO/IAEA field programmes using the SIT.”New sorting technique developed

In 2016 at the IPCL, Zelda Moran, a consultant, along with Parker, recorded that male tsetse pupae could be distinguished from the female when under near-infrared light. Under this light, which is invisible to the human eye, the puparium of the tsetse fly pupa becomes transparent, and it was found that structures, like the eyes and wings, would develop faster in the female fly – making it possible to sort the tsetse flies five to seven days before emergence.

Rafael Argiles, an entomologist at the FAO/IAEA and Gustavo Salvador Herranz, an engineer from the Cardenal Herrera University in Valencia, Spain, developed a near-infrared pupae sex sorter (NIRSS) prototype following the discovery by Moran and Parker in 2016. With this technique, a high-speed camera takes a video of the pupae under near-infrared light, which reveals the sex of the fly. The pupae are then automatically sorted by the machine.

The NIRSS is now currently used at both the IPCL and an insectary in Bratislava, Slovakia – helping to provide sterile male tsetse to the tsetse eradication programme in Senegal. In 2020, the IAEA trained experts from several countries, including Ethiopia and Burkina Faso, on the use of the NIRSS. They will soon receive NIRSS equipment.

“The new infrared sorting system comes at the right time to support tsetse eradication projects like the one in Senegal, which receives international deliveries of sterile male pupae twice a week,” said de Beer. “With the current restrictions in international transport imposed by the COVID-19 crisis, the length of aerial shipments of sterile insects has been extended significantly, but because these insects are transported as pupae, they survive the long journey and arrive in significantly better health to compete with wild flies for mating.”