There are fewer flowers and so less pollen about.

BBC: Making a bee-line for city roofs

"We know that honeybees can fly over 10km from their hive to a patch of flowers to collect food (nectar and pollen) and then fly home - a round trip of over 20km, " says Dr Raine.

BBC: Mapping the flight of the bumblebee

"All over the city, flowers are now blooming, providing nectar and pollen for all sorts of important bee species and other pollinators, " she said.

BBC: Gloucester named first 'bee guardian' city in UK

Pollen from male plants must be dusted by hand onto the flowers of female plants.

FORBES: Child Labor