From ten meters away, the sound of a million honeybees is surprisingly soothing, like treetops whooshing in the breeze. At a daring six meters, the droning becomes ominous and insistent. At two meters away, the humming is so malevolent that the sky seems to darken. "Beautiful music, yes?" says Nikola Kezić, as if there is no doubt.
Arranging folding chairs on the lawn behind his ramshackle barn, the University of Zagreb biologist and beekeeper explains that he raises 25 colonies of up to 50,000 honeybees each. That's a million bees for his research, funded by Croatia's science ministry, in honeybee breeding and natural selection. Kezić leaves the barn doors open so the bees can come and go from the wooden boxes where they build their hives. The dusky swarm fills the doorway, nearly obscuring the hives. A rogue bee flickers insistently around Kezić's grey hair.
Following the Chernobyl nuclear power plant explosion in 1986, Kezić used his bees as biomarkers for the distribution of radioactive pollution, measuring the levels of cesium in the pollen his bees collected and the honey they produced. Now Kezić is training them to detect Croatia's estimated 250,000 landmines, installed during the war that divided Yugoslavia.
De-mining teams employ remote-controlled instruments, man-driven tanks, bomb-sniffing dogs, and even workers who pierce the earth with long sticks. But not every method works well on every type of terrain, and land is not deemed safe until it has been cleared using three different techniques. His project is still in the experimental stage, but Kezić says he is already convinced that bees can be conditioned to sniff out landmines, just as trained dogs do.
He's not the first scientist to try this. Researchers from Sandia National Laboratory and the Pentagon have also trained bees to sniff out explosives, using more sophisticated equipment than Kezić's, such as wireless transmitters that track the bees' movement.
When flowers are in bloom, worker bees drink nectar for energy and carry pollen, a protein source, back to the hives to feed their young. Their highly developed sense of smell is aided by odor receptors for a variety of molecules, and those receptors help honeybees in olfactory learning - "remembering" a scent associated with a cache of food. But in the autumn and winter, when the flowers are gone, Kezić offers them a menu of sugar water and trinitrotoluene, or TNT.
He sprinkles one gram of TNT in a large Petri dish and covers it with soil. Then he places a smaller dish containing the sugar solution on top, leaving it surrounded by margins of TNT-rich soil. To get to the sweet stuff, a bee must first pass over the soil permeated with molecules of gradually evaporating TNT, a cream-colored powder with the tangy, organic scent of rocks.
Kezić erects a tent in the field next to his barn, scatters the stacked dishes around the grass, and lets loose a colony. "Within four or five days, they are conditioned to associate the smell of TNT with the sugar solution," he says. To test the bees, he then replaces the stacked pairs of Petri dishes with single dishes containing only TNT. The conditioned bees head for them, sniffing around for something to drink, he says.
He drilled his bees on a practice minefield at the Center for Testing, Development, and Training (CTRO), which the Croatian Mine Action Center founded to find new landmine detection methods. The results were promising. The next step, for which he as yet has no funding, would generate data. A thermal infrared camera, operated by colleagues at the university's department of surveying, would help count how many bees "signaled" at dishes containing TNT compared to dishes without the explosive, he says.
Nikola Pavković, the CTRO director, emphasizes that funding to develop new detection methods remains scarce in Croatia, but any technique that would increase the certainty that a field had been cleared would be welcome.