Bumblebees are Scotland's most important pollinators in May and June.

The steady hum of bees busying in my wild cherry blossom is one of my favourite sounds as I stroll beside my burn. Although there are 24 bumblebee species in the UK, only six appear in Scottish gardens, including Bombus hortorum, the garden bumblebee, and Bombus pratorum, the early bumblebee.

Ninety-seven per cent of wildflower-rich grassland and hedges has been lost since the 1940s, and bumblebee numbers have plummeted as a consequence. Two species have even become extinct in the UK. On the plus side, however, a species new to these islands has been found.

The tree bumblebee, Bombus hyponorum, was first recorded in the south of England in 2001 and has been steadily moving north. The Bees, Wasps and Ants Recording Society has been monitoring its progress, noting that tree bumblebees reached Scotland in 2013 with many more sightings last year, mainly in the Lothians and the south-west.

The tree bumblebee is particularly important because it is one of the first species to get started in the spring. As soon as she emerges, the queen sets about building up reserves by sucking nectar through her tiny straw-like tongue.

Finding a suitable nest site is her next priority. Most bees nest in tussocks, holes in the ground or beneath sheds, but Bombus hyponorum is a high flyer. You can see her searching along fences, house walls and eaves. Bird boxes are also highly favoured. Clive Hill, a beekeeping expert, writer and member of the Bumblebee Conservation Trust, knows of a pair of blue tits that were evicted to make room for a new tenant. Hill reckons bird boxes help increase these bumblebee numbers, a glimmer of hope that's just been echoed by research findings.

Last month, Katherine Baldock of Bristol University and colleagues published a paper in The Proceedings of the Royal Society, subtitled The Importance of Urban Gardens for Flower-visiting Insects. This is the first study comparing pollinator numbers and species richness between rural and urban areas. The scientists selected 32 sites, including places in Glasgow, Edinburgh and Dundee.

Baldock reported: "Our findings suggest urban areas can contain high bee species richness. If high-quality urban areas are able to support good populations of insect pollinators, they could act as important source areas, refuges and corridors of favourable habitat in a hostile [area] such as intensive agricultural landscapes." Lots of small or even tiny gardens, when aggregated together, can have a hugely positive impact on wildlife populations.

Some garden plants provide very little nectar and pollen for pollinators. These include bedding plants and double-flowered specimens, with little or no scent. The extra whorls of petals have been bred to replace the male and female parts of the flower.

Despite this, overall, the scientists found gardens supported more individual species than intensively managed farms. They also demonstrated that some non-native plant species can outperform native species. They may provide more energy-rich nectar which queens and worker bees need for themselves and the precious larvae. The pollen can also contain high-quality protein, essential for larval growth.

This potential wealth of plants and insects in a garden was borne out by botanical enthusiast Jennifer Owen. She recently published the results of a 30-year study of her own garden. Her research came to the same conclusion. She noted that she had identified almost 2,700 species: 474 plants, nearly 2,000 insects and 138 other invertebrates. There's overwhelming evidence that how we manage our gardens matters.

Bees enjoy peace and quiet, so they build their nests where they won't be disturbed. Research has also shown they prefer their own homemade nests and spurn the deluxe establishments you might be tempted to buy. Keep an eye out for their nests, arm yourself with a cup of coffee and enjoy watching them at work.