Bumblebees appear to be contracting deadly diseases from their domesticated honeybee cousins, according to a study that looked at 26 UK sites. The wild bees probably pick up infections from flowers that have been visited by honeybees.

The diseases are economically significant because the wild insects pollinate a large proportion of the world's crops.

"Wild populations of bumblebees appear to be in significant decline across Europe, North America, South America and also in Asia," said Mark Brown of the University of London. He said his study, which is published in Nature, confirmed that a major source of the decline was "the spillover of parasites and pathogens and disease" from managed honeybee hives.

Smaller studies have shown disease going back and forth between the two kinds of bee, but Brown said his is the first to look at the problem on a national scale. The study tracked nearly 750 bees at 26 sites throughout the UK. He and his colleagues also did lab work on captive bees to show disease spread.

What the study shows is that "the spillover for bees is turning into [a] boilover," said University of Illinois entomology professor May Berenbaum, who wasn't part of the study.

Study co-author Matthias Furst of the University of London said the team's research does not definitively prove the diseases are passed from honeybees to bumblebees, rather than the other way round. But the evidence points in that direction because virus levels and infection rates are higher in the honeybees, he said.

Bumblebees probably pick up diseases when they go to flowers after infected honeybees have visited them, or when they invade honeybee hives and steal nectar, he added. Bumblebees can be nearly twice as big as honeybees, can sting multiple times and don't produce surplus honey.

The latest research shows bumblebees are more severely affected by the diseases, Brown said. In general, the average wild bumblebee lives 21 days, but the infected ones live closer to 15 days. And while honeybee hives have tens of thousands of workers and can afford to lose some, bumblebee hives only have hundreds at the most.

Studies suggest that bumblebees provide $3bn (£1.8bn) worth of flower pollination annually in the US alone, while honeybees provide closer to $20bn (£12bn), Berenbaum said.

The new study did not look at colony collapse disorder, a mystery disease that affects North America more than elsewhere. Other diseases and parasites kill even more honeybees than colony collapse disorder.