He and his colleagues have analysed thousands of samples from stricken hives in many countries.

"We started in 2000 with the hypothesis that it was pesticides, but soon ruled it out," he told Reuters in an interview on Wednesday.

Pesticide traces were present only in a tiny proportion of samples and bee colonies were also dying in areas many miles from cultivated land, he said.

They then ruled out the varroa mite, which is easy to see and which was not present in most of the affected hives.

For a long time Higes and his colleagues thought a parasite called nosema apis, common in wet weather, was killing the bees.

"We saw the spores, but the symptoms were very different and it was happening in dry weather too."

Then he decided to sequence the parasite's DNA and discovered it was an Asian variant, nosema ceranae. Asian honeybees are less vulnerable to it, but it can kill European bees in a matter of days in laboratory conditions.

"Nosema ceranae is far more dangerous and lives in heat and cold. A hive can become infected in two months and the whole colony can collapse in six to 18 months," said Higes, whose team has published a number of papers on the subject.

"We've no doubt at all it's nosema ceranae and we think 50 percent of Spanish hives are infected," he said.

Spain, with 2.3 million hives, is home to a quarter of the European Union's bees.

His team have also identified this parasite in bees from Austria, Slovenia and other parts of Eastern Europe and assume it has invaded from Asia over a number of years.

Now it seems to have crossed the Atlantic and is present in Canada and Argentina, he said. The Spanish researchers have not tested samples from the United States, where bees have also gone missing.

Treatment for nosema ceranae is effective and cheap -- 1 euro (US$1.4) a hive twice a year -- but beekeepers first have to be convinced the parasite is the problem.

Another theory points a finger at mobile phone aerials, but Higes notes bees use the angle of the sun to navigate and not electromagnetic frequencies.

Other elements, such as drought or misapplied treatments, may play a part in lowering bees' resistance, but Higes is convinced the Asian parasite is the chief assassin.