Wuerzburg, Germany  - The collapse of bee populations, which is worrying farmers in many nations, has multiple causes including a loss of wild countryside which used to offer the insects a varied diet, a German biology professor said Wednesday.

Some German apiarists lost their entire stock of bees in the past winter.

"It's almost a natural disaster," said Juergen Tautz of the University of Wuerzburg.

The national average of losses in Germany had been 30 per cent although the winter was mild. Under natural conditions, a failure of about 10 per cent of swarms was normal in a German winter.

The direct cause of most bee deaths is the varroa mite, which infects bees, but the indirect causes for the insects' lack of immunity to parasite and illnesses were highly varied, Tautz said, blaming breeding programmes that had created docile, weak bees.

"Nowadays it only takes one tenth as many mites to wipe out a swarm as it took a decade ago," he said.

He suggested the deaths of the bees had also come about through modern farming, with greater use of insecticides and huge crops eliminating variety in the bee diet. "The bees' imbalanced diet makes them weak and susceptible to illness," he said.

Tautz counselled farmers to leave the sides of roads overgrown and to cut back pesticide use if they wanted to do bees a favour. (dpa)