Two-thirds of the country's total bee species have been recorded in Norfolk in the past four decades.

Of the 240 species found in Britain, 171 have been identified in the county, said Tim Strudwick, who wrote a definitive account in the Norfolk & Norwich Naturalists' Society's official journal in 2011.

Since then three more species have been added to the county list, which includes 18 out of the 24 British bumblebees – all positively identified since 1969.

Mr Strudwick, who is a county recorder and manager of the RSPB's Yare reserve, said Norfolk was sixth or seventh in the county league table for all bee species.

Although there has been a long history of recording bees dating from the later years of the 19th century, Norfolk's first definitive list of bee species was only published in 1905 by John Bridgman.

Of the county's bumblebees, one species was lost in the 1920s and three have not been recorded since the 1960s. However, there have been new arrivals including the tree bumblebee, which was recorded in 2008 at Earlham Cemetery. It was fairly widespread in 2011 with 63 recorded.

The bumblebee has often been described as 'the sound of summer,' said Nick Owens, who is the author of a 72-page booklet, Bumblebees of Norfolk, published last year, with county recorder, David Richmond, of Reepham.

Mr Owens had become fascinated by bumblebees after planting lavender cuttings at his Weybourne home in the 1980s, which rapidly attracted more than 100 bumblebees. He has since recorded 14 species visiting his garden in more than 30 years of watching.

As a volunteer with the Bumblebee Conservation Trust's bee walk project since 2008, he has recorded species along a 1km set route from his garden to Kelling Heath at least twice a month.

His observations suggest that numbers of bumblebees were very poor in both the last two years. Mr Owens said that the results were much better this year, with about two to three times more counted in the second half of July compared to the previous four years. 'Bumblebee numbers have been rather like butterfly numbers in recent years with a bounce back this year after two poor years,' he added.

Mr Strudwick, who said that solitary bee species had not been as fully recorded in recent decades, suggested that some species might have been more widespread – partly because most surveys have been confined to the Brecks, Broads and the Norwich area. While the weather has had an impact on populations, it had been quite a mixed picture. 'Bees are very seasonal. Last year when we had a lovely early spring, the early bees did very well and then the late, spring/ summer bees did really badly. It wasn't a good summer for them. This year, it was the opposite and early spring bees did not do well,' he said.

The recent weather has been generally good for summer bees. 'They have a very good three or four weeks of hot dry weather and have bred very well, so next year we'll see the benefit of that – and it is a similar picture with butterflies. Certain bees are only around for four or five weeks so it is quite critical to have good weather in that period,' he added.

'There's always been good and bad weather and most species have survived millions of years of all sorts of extremes. It is the steady changes in the environment of things which are less obvious – those are of biggest concern.

'It is good to see quite a lot of bees increasing at the moment. It is good to see new species coming because some of the bumblebees have had dreadful declines. There are still plenty of bees that were much more widespread in the general countryside than they were a long time ago,' said Mr Strudwick, who became interested in bees in 2005.

Since his 2011 provisional county list, three more bee species have been identified, which have spread from the south-east counties. He spotted a little bee in Breckland last year, which feeds on ragwort and lives in old beetle holes and tree stumps and Mr Owens recorded another not seen since Victorian times.

At the same time, a combination of the evolving climate and farming had also brought another new species to Norfolk. A species of solitary grey mining bee, Andrena cineraria, has spread into Norfolk partly because of the popularity of oilseed rape.

Increasingly, solitary bee species have been recognised as especially efficient pollinators of oilseed rape and other crops.