Vasileios Bontzorlos climbs down from a tall ladder and opens his hand to reveal three brown pellets. He crushes one and skulls of mice and rats appear, tangled with tiny bones and other not particularly appetizing material. Here in the plain of Thessaly, Greece’s agricultural heartland, the fight to combat rat infestations without spreading potent poisons on the fields is underway.

Bontzorlos, a biologist and forester, speaks excitedly about the creature that helps farmers combat an enemy which, if left unchecked, can devastate entire fields, causing financial ruin. It is the barn owl, a bird that lives in low-altitude agricultural areas and feasts almost exclusively on mice, voles and rats, common pests that have been the scourge of farming for thousands of years.

A pair of barn owls eat and feed their chicks between 2,000 and 5,000 rodents a year. The parts of the meal too difficult to digest, like the skull, are naturally turned into brown pellets that are spat out and used by the owls to lay their eggs on. Barn owls have many capabilities but building nests is not one of them. They rely on nesting cavities but traditional barns are in short supply and spaces inside trees and rocks are exceedingly rare in agricultural landscapes.

Enter the wooden nest box. Erecting nest boxes on agricultural land, an idea first implemented in Malaysia, has proven effective in reducing rodent infestations in Israel, where it has been applied since the 1990s and became a national program in 2008.

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Credit: Eurydice Bersi

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“I am trying really hard to see if we are missing something, if there are any rodent infestations that go undetected, but it seems it’s working,” says Yoav Motro, rodent control specialist at the Israeli Ministry of Agriculture. “Since we started, rodenticide use has gone down by 50 per cent to 60 per cent nationally and by 100 per cent in many of the farms where nest-boxes are located, so no rodenticide use there at all.”

Tel Aviv University Professor Yoshi Leshem, a veteran ornithologist who started the initiative, says the key is to engage farmers. “The farmer is tired at the end of the day, sits at the computer and sees through a webcam that every 45 minutes, the owl brings a rodent into the nest. He says: ‘Wow, it works!’”

Putting up the boxes and maintaining them is not completely hassle-free. Motro has calculated that each box requires an average of 0.2 working days per year, meaning that a farm with five nest boxes would require a day’s work every 12 months. Barn owls hunt in a maximum radius of 3 km, so no farmer is certain that the catch comes from their own field, but when nest boxes are dispersed around an entire region, it matters little which particular owl does the work.  

There is another characteristic that makes barn owls ideal for infestation control: they lay more eggs when local rodent numbers are higher, and prey on the most common species at a given time, suppressing population peaks. Owls will never fully exterminate rats but the aim is to bring the population down significantly and turn a potentially serious threat into an occasional nuisance. 

Biological pest control goes hand-in hand with a much deeper study and understanding of the dangers to crops. “Studying the owls is the best way to know what kind of enemies exist at any given moment and where they are located,” says Bontzorlos, whose NGO Tyto (the scientific name of the barn owl), installs nest boxes on local agriculture land.

Credit: Ioannis Pangalis

”The problem with rodents is cyclical, when there is an infestation, damages can reach up to €200 million. It is a lot,” says Kostas Agorastos, head of the local administration of the Thessaly region. “We decided to adopt a natural method, in order to help agricultural production.”

Inspired by Bontzorlos’ pilot project, Thessaly is now finalizing plans to introduce hundreds of extra nest boxes, hoping to encourage more of these useful predators to reproduce in its wheat, barley and corn fields. 

Not that there are many alternatives left for rodent control. The most important advanced rodenticides – Second Generation Anticoagulant Rodenticides (SGARs) – have been banned for use in EU fields since May 2021 when authorisation of bromadiolone expired.

Numerous scientific studies have shown that SGARs are exceptionally harmful to the environment. They do not break down easily, are highly toxic and accumulate in the bodies of larger predators. Across Europe, SGARs are still applied legally in and around buildings and in open spaces, such as parks, golf courses and dikes.

“Anticoagulants fail many regulatory environmental risk assessments, nonetheless they continue to be heavily used because of the societal need for rodent control and the limited availability of safer alternatives,” wrote Nico van den Brink, toxicologist from the University of Wageningen in the Netherlands and author of a book on the subject.

The EU Commission pointed out that the use of rodenticides for buildings and open areas falls under a “different regulatory framework” than the use of the same substances in agriculture. This means there is no requirement to reduce their use under the new EU rules limiting chemical inputs in agriculture, because in theory most of them are already banned. But how does this work in practice? 

In Greece, no one contacted for this article doubted that rodenticides, even home-made ones, are occasionally still applied in farms. Agroza, a company selling SGARs for building use only, openly stated on its website that several could be used in fields (when confronted by Investigate Europe, the company promptly removed the online reference).

“Yes, there is illegal use,” says Agorastos, the Thessaly region chief. “When farmers see danger, they would use whatever they think is most effective and cheaper. There are protocols in place, but in practice they are ignored. And for us it is impossible to police hundreds of thousands of hectares. This is why we are introducing a natural alternative.” 

The Israeli experience shows there should be no expectation of immediate results. This is “a marathon rather than a sprint,” warns Yoshi Leshem. Results need decades and an adequate density of nest boxes. Meanwhile, the global rodenticide business is worth $5.2 billion and is forecast to rise to $7 billion by 2027. “I am sure they [rodenticide manufacturers] don’t like my activities. But I don’t like theirs either,” says Dr. Leshem, who has witnessed widespread poisoning of migratory birds due to pesticides.

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Credit: Alex Labhardt

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In recent years, tens of thousands of nest boxes for barn owls have been erected by conservationists in Germany, Denmark, Holland, Switzerland and other European countries. “Through the years, the attitude of the farmers is changing,” says Lausanne University professor Alexandre Roulin, who convinced Switzerland to implement a national program for the conservation of barn owls two decades ago. “They ask us for boxes, they know it works.” 

A less coordinated approach is practiced in Napa Valley, California. In the famous wine producing region, no authority guided farmers on how to install nest boxes, but they had been doing so for decades before 2015, when Mat Johnson, professor in the department of wildlife at California Polytechnic State University started measuring their effectiveness.

“We found a very significant reduction in rodent activity in vineyards with nest boxes as opposed to vineyards without boxes,” he says. “The reduction was especially large during the feeding season of the baby owls. The most exciting thing is that the relationship between the farmer and nature changes, becomes reciprocal. It is no longer just them versus the enemies of the crops. By inviting nature into their farm they can actually improve it.”