I think that Michel is right, a lot of it is down to harmful pesticides, which kill off bugs that might harm crops, but are also seriously harming the bee population. Other problems include climate change and the introduction of species from other parts of the world (known as ‘alien species’) which are harming local bee populations.
In terms of what we can do to help, I think that buying regional and especially organic food probably helps, as these tend to be less chemically-intensive farming processes, and so less harmful to bees!
The bee population is declining for a number of reasons. Firstly their habitats in woodlands and hedge grows have been reduced. Secondly bees need a varied diet to survive and there is a huge decline in the number of wild flowers due to intensive farming. Climate change has also been attributed as a factor. Finally pesticides and insecticides have been killing them. To help stop the decline we need to stop destroying their habitats. A lot of councils are now dedicating areas to become wild flower meadows, I know there is a beautiful one in the regional park in Ballincollig (near my parents house). We need to look at using pesticides etc that are not harmful to bees and finally governments need to help tackle climate change ?