The abundance of four common species of bumblebees in the U.S. has dropped by 96 per cent in just the past few decades, according to the most comprehensive national census of the insects. Scientists said the alarming decline, which could have devastating implications for the pollination of both wild and farmed plants, was likely to be a result of disease and inbreeding.

Bees in general pollinate some 90 per cent of the world’s commercial plants, including most fruits, vegetables and nuts. Coffee, soya beans and cotton are all dependent on pollination by bees to increase yields.

But the insects, along with other crucial pollinators such as moths and hoverflies, have been in serious decline around the world since the last few decades of the 20th century. It is unclear why, but scientists think it is from a combination of new diseases, changing habitats around cities, and increasing use of pesticides.

Sydney Cameron, an entomologist at the University of Illinois, led a team on a three year study of the changing distribution, genetic diversity and pathogens in eight species of bumblebees in the U.S.

By comparing his results with those in museum records of bee populations, he showed that the relative abundance of four of the sampled species (Bombus occidentalis, B pensylvanicus, B affinis and B terricola) had declined by up to 96 per cent and that their geographic ranges had contracted by 23 per cent to 87 per cent, some within just the past two decades.

Cameron’s findings reflect similar studies across the world. According to the Centre for Ecology and Hydrology in the U.K., three of the 25 British species of bumblebees are already extinct and half of the remainder have shown serious declines, often up to 70 per cent, since around the 1970s. Last year, scientists inaugurated a GBP10m programme, called the Insect Pollinators Initiative, to look at the reasons behind the devastation in the insect population.

Cameron’s team also showed that declining species of bees had higher infection levels of a pathogen called Nosema bombi and lower genetic diversity compared with the four species of bees that were not in decline — B bifarius, B vosnesenskii, B impatiens and B bimaculatus.

The N bombi pathogen is commonly found in bumblebees throughout Europe but until now has been largely unstudied in north America. The infection reduces the lifespan of individual bees and also results in smaller colony sizes.

The reduction in genetic diversity seen in the declining bees means that they are less able to fight off any new pathogens or resist pollution or predators. “Higher pathogen prevalence and reduced genetic diversity are, thus, realistic predictors of these alarming patterns of decline in north America, although cause and effect remain uncertain,” Cameron wrote on Monday (3JAN) in Proceedings of the National Academy of Sciences.

Insects such as bees, moths and hoverflies pollinate around a third of the agricultural crops grown worldwide. If all of the U.K.’s insect pollinators were wiped out, the drop in crop production would cost the U.K. economy up to GBP440m a year.

It is estimated that a third of everything we eat depends upon pollination by bees, which means they contribute some GBP26bn to the global economy.