Amid mounting worries about disappearing bees, one hardy, ginger-fringed bumblebee is bucking the trend and spreading north.

The tree bumblebee (Bombus hypnorum) has been seen in east Scotland for the first time, providing evidence this foreign bee is establishing itself north of the Border.

Tree bumblebees arrived in the south of England from France in 2001 and have since gradually colonised the country, being first detected in the west of Scotland last summer.

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Now they have been seen in Edinburgh and East Lothian, and are expected by experts to move to Fife and then across the Tay to help pollinate Scotland's famous raspberries and other soft fruit crops.

A tree bumblebee was spotted and photographed earlier this month by countryside ranger Jen Edwards in her back garden in East Linton. She also heard of a second sighting close to Traprain Law, near Haddington.

"We are tremendously excited by these appearances and are sure more sightings will shortly be confirmed," she told the Sunday Herald. The council has introduced grazing animals at sites across East Lothian in an attempt to reduce gorse, encourage wild flowers and boost wildlife.

Conservationists are encouraging people to look out for tree bumblebees to help map their progress. The bees have a black head, a fuzzy ginger-brown thorax, a black abdomen and a white tail, and often nest in bird boxes.

Stuart Roberts, chairman of the UK Bees, Wasps and Ants Recording Society, runs the tree bumblebee mapping project. "It is thrilling to note that the tree bumblebee appears to be consolidating its presence in Scotland after its arrival in 2013," he said. "The tree bumblebee will be a welcome newcomer in the raspberry growing areas in the Carse of Gowrie, as it is a major visitor to raspberry flowers and a first-class pollinator."

One of the reasons tree bumblebees are successfully expanding is that they are thought to be more resistant to parasite infections than other species. A study by the University of London suggests they could reproduce while infected, unlike other bees.

Since they arrived in England, tree bumblebees have spread over 4500 square miles a year - about half the size of Wales. There are some concerns, though, that they might displace other bees.

"We still don't know whether there could be any negative impacts if the bumblebees compete for food or nesting sites," said Professor Mark Brown, from the School of Biological Sciences at Royal Holloway, part of the University of London. "Further research should focus on how our native bees are affected."

But Dr Richard Comont, data monitoring officer from the Bumblebee Conservation Trust, stressed there was no evidence so far that the tree bumblebee was having a detrimental impact on native species. "It's really nice to have a species arrive here on its own, rather than being transported by humans, and to have a bumblebee doing well and spreading out, rather than declining," he said.

The public can report sighting of tree bumblebees here: