Advancing the Sterile Insect Technique – Using Science to Stop Invasive Flies

In the Naoussa Valley in northern Greece, entomologist Vassilis Rodovitis is preparing for the release of sterile fruit flies. The area is one of the most important agricultural regions in Greece and across Europe. Here, fruit production is a cornerstone of the local economy, but invasive fruit fly species are beginning to threaten the orchards that define this landscape. We are doing the release of radiated and sterilized adults of medfly in order to see how far our flies can fly and in order to see their ability to mate with wild females and reduce the population of medfly. As global temperatures rise and as trade and mobility increase, species have begun moving beyond their native habitats, establishing themselves in new environments across the world. In the Mediterranean region, two invasive fruit fly species, Bactricera dorsalis, the oriental fruit fly, and Bactricera zonata, the peach fruit fly, are on the verge of taking hold. Until recently, neither species was a part of the local ecosystem. Their arrival calls for new methods of plant protection. To respond to this growing threat, researchers across Europe and beyond are collaborating in a project called REACT. At first glance, their approach seems unorthodox. Instead of killing insects, scientists are releasing more of them. But the logic is surprisingly simple. Instead of relying on chemical pesticides, they are releasing sterilized insects into the environment. These insects mate with the wild population but produce no offspring. Over time, the population declines and the ecosystem remains intact. This method, known as the sterile insect technique, or SIT, turns the reproductive instinct of the insect into a tool of ecological control. But turning that idea into a functioning control program requires a carefully coordinated sequence of steps. The sterile insect technique begins in the field. Traps reveal where fruit flies are present and where action is needed. In specialized rearing facilities, millions of flies are bred, separating male and female larvae before the sterilization increases the efficacy of the program. Male flies are then sterilized using carefully controlled doses of x-ray or gamma radiation. They remain able to mate but can no longer reproduce. The sterilized males are then released over target areas where they disperse and compete for females. When they mate with a wild-type female, no offspring are produced. Generation after generation, the population declines. Monitoring closes the loop, showing whether populations are falling and helping guide the next release cycle. But making SIT truly effective in the field requires far more than producing sterile insects. Researchers must determine how many insects to release, how they should be reared and fed, and how their performance can be maintained. They are refining the sterilization process, ensuring that the flies are healthy and competitive even after exposure to radiation. They are also developing more targeted release strategies designed to deploy sterile insects where and when they will have the greatest impact. At Imperial College in London, Serafima Davidova is working on a technique that could transform how fruit flies are separated before sterilization. She is injecting eggs, trying to attach a fluorescent marker to the male-specific Y chromosome so that in the future, the male pupae will glow while females will not. A small difference, but one with a potentially huge impact. Because the fewer flies that need to be bred, fed, sterilized and released, the more efficient and economically viable the technique becomes. We've injected here over 30,000 eggs. The type of injections that we're doing for the Y are site-specific, which means that we're targeting a particular region on the Y chromosome. So when we inject these eggs, we're hoping to inject the posterior end, which has obviously the germline cells, which will then develop into sperm or eggs, which hopefully are the transgenics themselves. The development of SIT, however, goes far beyond the laboratory. It depends on a detailed understanding of the flies themselves, their behavior, their fitness, and how they move through the environments they are released into. Success is not only decided in controlled conditions, but in the field. Back in the Nawasa Valley, we join researchers as they begin testing SIT in the field. They use Medfly, a species already present in the region. Sterile flies are released into the landscape, then tracked across the orchards to monitor how far they disperse and how well they survive in the wild. The goal is to understand whether these insects can successfully compete outside controlled laboratory conditions. Antonis Agostinos explains. What we do here is that we test the dispersal capacity of the flies that we have prepared in the lab, and testing some different diets to see how we will make our flies more fit out in the wild. Tomorrow we will also set up a trapping system, and the following days the traps will be collected and see what kind of flies we have in there. The more flies that you find in the traps, that means the more that they survived, at least enough to go in the traps, so this is a good thing. The worst case scenario would be to find very few marked flies in the traps, that means that they cannot disperse very well, and find them only in the traps that are very close to us. Here in the conventional one we have three adults, two adults already, so in here there is Medfly, wild Medfly. What these field trials reveal is just how delicate the balance really is. Even the most carefully prepared insects must prove themselves in the wild, because only those that survive, disperse and compete successfully will be able to reduce the population over time. But while researchers in Greece are still preparing for possible invasions, on the island of La Reunion in the Indian Ocean, the situation has already changed. Part of the Mascarine Islands east of Madagascar, La Reunion has become one of the few places within the European Union where the invasion of Bactricera dorsalis and Bactricera zonata can already be observed in the wild. Here, researcher Hélène Delatte studies how these invasive fruit flies establish themselves, spread across landscapes and compete with species that have long been part of the island's ecosystem. A long, long time ago, there was only one T. frutidae known on fruit on Reunion Island. It was Serratidis catuari, which is an endemic fruit fly of the Mascarine and present here at very low numbers. But then afterwards, successive invasions came onto the island, unfortunately on the fruits. So we had several ones with Serratidis chilisi, then Serratidis capitata, the midfly, and most recently in the year 2000, Bactricera zonata. And in 2017, we had dorsalis. Unfortunately, Bactricera zonata and dorsalis have the same, exactly same climatic niche. They like lowlands area of the islands where it's warmer, and they are also found on same fruits. The repeated invasion of species into this ecosystem has profoundly altered the fragile balance of the island. Nowhere else in the European Union can these invasion dynamics be observed so directly in the wild. La Reunion has become a living laboratory, where the spread of invasive fruit flies may be studied in real time. So we are looking at an invasion process. So we are putting traps in urban and suburban areas where no trapping has ever been done. So we are looking for how these flies are moving, and what their population dynamics within this different environment that have never been found before. In parallel, researchers in the lab are exploring these dynamics under controlled conditions. Their aim is to understand how Bactricera dorsalis behaves, and how it competes, across every stage of its life cycle. From the moment eggs are laid inside a fruit, to the development of larvae, and finally to the dispersal and feeding of adult flies. Each step reveals how this species gains an advantage. We have seen already adult fly fighting, butting their heads, or chasing one each other, between species, and dorsalis is good at this. Then you have also competition inside the fruit. When they have the same age, the same quantity, dorsalis can eat faster, have a shorter development, so be able to go out of the fruit, and maybe eat much more than the other fly. So that's on Tefritidae, but it must have also other impacts, on other insects, and maybe on birds. Together, the trials in Greece and La Reunion reveal two sides of the same challenge. From controlled experimentation, to the complexity of real world ecosystems. But in many parts of the world, this is no longer experimental. Over the past decades, SIT has been applied against a wide range of agricultural pests. From screwworms in North and Central America, to codling moths in South Africa, and fruit flies in different parts of the world. Today, SIT programs operate across large agricultural regions, helping to suppress pest populations, while reducing dependence on chemical pesticides. In South Africa, for example, SIT is used across thousands of farms to protect the citrus industry. On the island of Mauritius, researchers and rearing facilities are also preparing sterile insects, as part of ongoing efforts to strengthen future control strategies against invasive fruit flies. And in Spain, the technique has already been used for more than two decades, to control one of Europe's most persistent agricultural pests, the Mediterranean fruit fly, Serratatus capatata. In Valencia, large-scale rearing facilities now produce millions of sterile insects each week. Jaime García de Orteza oversees one of the largest of these facilities, where flies are bred for SIT programs across the region. These are the larvae that are almost fully developed, and tomorrow will be ready to start popping out from larval diet. Some have already started. When the European Union started thinking about banning the use of aerial chemical spraying, the authorities here in the Valencian region started thinking about alternatives for the growers, so they could keep their production under protection. And now with new pests coming, it will become more frequent, I guess, to see these kind of programs in Europe. Valencia stands as a compelling example of sterile insect technique deployed at scale, even within the fragmented orchard landscapes typical of European agriculture. But these programs are designed to manage pests that are already established. A very different challenge emerges when a new invasive species first appears. At that stage, the question is no longer only how to control a population, but how early an outbreak can be detected, and how quickly action can follow. For Mark Schetelig, insect biotechnologist at Eustace Liebig University Gießen and coordinator of the EU-funded project REACT, this is the central question. Well, whenever an invasion occurs, you have to find it quickly, because otherwise you cannot act appropriately. So you need rapid detection of your pests as fast as possible, and then you need local technologies to act quickly, and of course also public awareness and programs that make it possible to use your technologies quickly to eradicate them. And the REACT project develops exactly that, from the rapid detection to technologies that can be used immediately, having a plan and then really use that on the fruit flies we're working on. Together, these innovations form the basis of a rapid response system designed to detect, contain and potentially eradicate invasive populations before they spread. That process begins with detection. Researchers are developing monitoring tools capable of identifying outbreaks rapidly. In the laboratory, researchers aim to improve the separation of male and female insects and to produce stronger, more competitive sterile males for release. In the field, mobile rearing facilities and new release strategies are being tested to make SIT deployable across different regions and outbreak scenarios. The goal is not simply to control invasive species once they are established, but to intervene before establishment becomes permanent. And researchers working on invasive fruit flies know how small that window for action can be. So, unfortunately, Batrossa dorsalis is a superfly. So if dorsalis are coming into a country and stay there, only a few individuals can colonize. So we have to be more careful for sure, and to be able to eradicate very quickly the first individual that will come as soon as we see them. But even the most urgent response depends on more than scientific readiness. For SIT to work, it has to fit into existing agricultural practices and be accepted by those who are being asked to use it. In Nelspoot, home to Citrus Research International in South Africa, scientific preparedness also depends on dialogue. Here, entomologist Aruna Manrakan regularly brings together farmers and agronomists to discuss the population dynamics of Batrossera dorsalis in the region and to openly address both the benefits and limitations of the sterile insect technique. To evaluate the socio-economic and environmental impact, we first consult with stakeholders that are affected by invasive fruit flies. We assess their acceptability of the SIT technology. We try also to engage how much do they have to incur to control pests in their orchard, control fruit flies, what are their losses. So we get their perceptions, we get some values from these stakeholders. We also consult some databases just to get an idea of the production areas that are affected, production values and the losses due to fruit flies. So it's a matter of getting the stakeholder perceptions by engaging them, consulting databases and putting it all together. Once these dynamics are understood, the focus shifts from explaining to listening to farmers, where their concerns lie, what they experience and how they respond. A concern that is shared in Nawasa. Here too, engagement with farmers and agronomists is a central part of the process. Agronomist Nikos Papadopoulos brings together stakeholders from across the fruit production chain. But the situation here is different. While SIT is being prepared as a rapid response to potential invasions, the regulatory framework to counter Batrossera dorsalis and Xanata is not yet in place. For now, the focus is on awareness, on explaining the technology, sharing experiences and building understanding. So that when the moment comes and policies are adapted, action can follow without delay. We have invited stakeholders that cover the whole chain in fruit production, from producers all the way to those who are trading the fruits and to the exporting companies, to the stakeholders who are dealing with packing the fruits, all the range of this fruit production chain. And now we would like to inform the stakeholders and to give them all the examples of how we use the sterile insect techniques and also to explain a little bit more about the technology. From lab to orchard, from early detection to field deployments, researchers across the world are trying to build systems capable of responding before invasion becomes irreversible. Because the question is no longer if these species will arrive, but when. How far will they spread? How far are we willing to act before they do? In places like the Nausa Valley, preparations are already underway. Traps are being installed. Sterile flies are being tested. Farmers and researchers are learning how to respond before outbreaks spread. The challenge is not only to protect agricultural production, but to do so without overwhelming the ecosystems these landscapes depend on.