(Reuters) - At the height of the tangerine season in Croatia's Neretva river delta, two pickup trucks scour a maze of water channels carrying an odd-looking contraption: a mortar-like pipe spraying orchards with sterilized flies.
Each launch sends into the air thousands of males of the fruit fly, one of the most harmful orchard pests, in what advocates say is a prime example of how nuclear science can benefit both agriculture and the environment.
They have been bombarded with radioactive Cobalt-60 in an Israeli biotechnology plant to make them sterile in a bid to reduce the fly population and the damage they wreak on the crop.
The United Nations' atomic agency says it shows how nuclear technology - more commonly associated with energy or atom bombs - is harnessed to help make an "important contribution" in solving the world's food problems.
Scientists are developing and refining the Sterile Insect Technology (SIT) for different species - including the tsetse fly in Africa - at the Seibersdorf laboratory of the U.N.'s International Atomic Energy Agency (IAEA) outside Vienna.
"It is essentially a form of birth control," IAEA head Yukiya Amano said. "The result has been a drop of no less than 75 percent in fruit fly damage" in Neretva, where nine out of ten people are involved in the citrus fruit industry.
Twice a week, between April and November, shipments containing 5 million sterilized male flies reach the fertile valley in southern Croatia, the northernmost point in Europe where tangerines are successfully grown.
Some farmers use boats to spread the male flies in remote areas, part of project which started two years ago to combat a pest that destroyed about a third of the crop.
The sterilized males outnumber the local fruit flies and gradually take the place of normal males. They can mate, but there is no offspring. Consequently, the number of larvae, which feed on ripening tangerines, quickly declines.
FEWER FLIES, PESTICIDES
"We've had excellent results, more and more farmers want to take part. The next step will be to release the males from aircraft, which is the most efficient way," said Luka Popovic of the Croatian Centre for Agriculture and Rural Affairs.
The U.N. agency pays for shipments of flies costing about 2,300 euros each, he said. The United States helps with funding.
The flatlands of the sprawling Neretva delta are expected to produce around 60,000 tonnes of tangerines this year. Most are exported, but many countries have strict quarantine rules and ban imports of fruit suspected of carrying fruit flies.
"Every year we add new crops and the output grows by around 20 percent, so we aim to reach 150,000 tonnes in 2020," said Niko Kapovic, manager of AgroFructus, the leading local tangerine wholesaler.
Agrofructus exports 75 percent of the tangerines - mostly to Russia, the European Union and the Balkan region.
"Last year we had a big onslaught of the fruit fly and it really showed how good it was that we started this project. The treated areas had very few larvae," Kapovic said.
Jorge Hendrichs, who heads a joint pest control program of the IAEA and the U.N. Food and Agriculture Agency, said the SIT method was a win-win because it enabled farmers to fight a pest which "can easily eliminate 30 to 100 percent of the crop" and at the same time use fewer pesticides.
This is important as many importers are increasingly careful about the residue levels of insecticides in fruit, he said.
"It is a big problem for African and other exporters if they want to go to Europe. Europe is getting tougher and tougher in this respect," Hendrichs said in the Austrian capital.
GREENPEACE URGES CAUTION
For other countries - like the United States, Japan and Chile - the main thing is to prevent new pest outbreaks. "They want 100 percent assurance that the product you are exporting is 100 percent free, not one worm," he said.
The technology has been used for decades in North and South America. In Europe, a push was made in southern Spain a decade ago to overcome problems in exports to the United States.
It has helped eradicate the tsetse fly - which can cause "sleeping sickness" in humans and kill cattle - on the island of Zanzibar in east Africa and there is also a project in Ethiopia.
Andy Garner, an IAEA program coordinator, said he believed there was more demand to use nuclear applications for food production, though he acknowledged a potential "branding" issue.
If "you talk about irradiating food ... I think you do think twice before you eat that apple unless someone explains to you that it is no different than having something out of the microwave," Garner said.
Giving another example of how nuclear technology can help farmers, Amano said a plant-breeding technique involving irradiation was used to develop a barley variety grown at high altitudes which is now the leading such grain in Peru.
It has more protein, a higher yield and fetches twice the price than other barley types, boosting rural incomes, he said.
But a scientific adviser to environmental group Greenpeace - which opposes nuclear energy on safety grounds - said the use of nuclear technology in this way needed to be looked at carefully.
"We should neither view it as risk free and nor should we view it as the panacea to all food security issues," said Paul Johnston of the Greenpeace Research Laboratories at Britain's University of Exeter.
(Writing by Zoran Radosavljevic, editing by Paul Casciato)