Searching for landmines is a painstaking and risky business, but scientists in the United States are getting help from some unusual helpers.

They are trying to discover if bees can be used to detect minefields.

"We chose bees because bees are probably nature's premiere samplers," said Jerry Bromenshenk from the University of Montana.

"They go out from their boxes each day, make tens of thousands of trips always returning home, canvas the area around and bring us back samples of everything that they encounter out there."

There are an estimated 120 million landmines. In many cases there are no maps showing where they lie buried, and thousands of acres of fertile land remain uncultivated because farmers are afraid to step into the fields.

Currently there is no effective way to screen a large area for landmines. But the mines leave chemical marks, they leak explosives into the soil, which are dissolved by water and consumed up by plants.

Sensing explosives

Millions of landmines across the world

Bees are sensitive to a wide variety of chemicals in the environment. So if explosive residues are in the soil or the plants themselves, the bees are likely to pick them up.

"Obviously for their own needs, they bring back water to cool their hive and to drink," said Dr Bromenshenk.

"They bring back pollen for protein for their young. They bring back nectar from which they make honey. And they themselves get dirty."

Dr Bromenshenk is working with chemists from Sandia National Laboratories in New Mexico to find out how chemicals from landmines are taken up by plants that are visited by bees.

Early stages

Dr Phil Rodacy: Determine whether plants absorb explosives

Susan Bender and Phil Rodacy are growing several different types of plants in a greenhouse. Dr Rodacy says that they have contaminated the soil with TNT, which is taken up by the plants.

"What we're trying to do is determine whether or not plants are taking enough explosives out of the soil and putting it into either the pollen or into some of the moisture that is respirated by the plants, so the bees can pick that material up and carry it back to the hive so we can detect it," said Mr Rodacy.

Susan Bender emphasises that the project is still in the early stages. But they hope to identify a plant that takes the chemicals into its roots, stems and leaves. She imagines that a mined area would be seeded with a TNT-absorbing plant. This would maximise a bee's chance of picking up the chemicals.

Susan Bender: Trying to find traces of explosives on bees

"Nitro-aromatic compounds (chemicals many explosives are made from) are extremely sticky, so we know that it will attach itself to water molecules, to soil, to anything it can," she said.

"So therefore as the bee goes out to forage and they bring back these particulates into the hive. We're hoping to find trace residues of the nitro-aromatic compounds, or the explosives, on the bees."

If all goes well, bees could be used to carry out a chemical survey to test whether there is a minefield in a particular area. But Dr Bromenshenk is also hoping that the bees could help to pinpoint the location of individual mines.

"Assuming that the bees can detect the smell of TNT, or some other explosive residue, there are mechanisms by which you can train bees to seek things, and if you train them to seek it then you also have to know where they've gone," he said.

Bee training

Jerry Bromenshenk on how to train a bee

Dr Bromenshenk is confident bees can be trained to sniff out mines.

"You would do it in very much the same thing that you would train your dog, or the Navy trains seals. You do it on a reward basis and the real reward for a bee is honey," he said.

"We use sugar syrup. What we do is we spike the feeder with the odour that we want them to associate with the reward. Once you've got the trained colony for that particular chemical, you can move that colony to a field where you want to find something.

"You set that colony out and the bees would fly out seeking those odours. Then you would have to track the bees to see where they're going to find it."

The tracking could be done by fitting the bees with tiny radio transmitters but Dr Bromenshenk admits that bee training has its limits.

Once the bees have located a spot of TNT contaminated soil, you have to bring in humans to find individual mines and dig them out. But by covering vast areas of land, bees may soon make the work of humans easier and safer.