Most German beekeepers think that insecticides and other chemicals in combination with a decreasing food supply are the most serious threat for the survival of the bees and bumblebees.

The monocultures of our industrial agriculture must appear the honey bees like 'green deserts' without any usable food sources (= flowers). In addition many garden owners prefer trimmed grass together with some conifers and cherry laurel plants instead to give rich flowering plants any chances and let some corners in their gardens grow wild with stones and deadwood as useful hiding spots for many insects.

For the ones who want to help our insects:

• plants useful for bees
• plants useful for bumblebees

Most conservationist organizations like for example the NABU don't consider other factors like the climate change or bee diseases as main causes: "Den Klimawandel oder besonders kalte oder warme Winter können wir ausschließen. Vieles deutet darauf hin, dass wir es mit einer weit reichenden Vergiftung der Insekten in unserer Umwelt zu tun haben." Means they exclude the climate change respectively especially warm or cold winters as most important factor and instead of that focus on the pollution of the environment.
One reason why I tend to agree are regions in China where bees are already completely extincted because of the extensive use of chemicals (which happened long time ago when the climate change was still not as advanced as nowadays). In these regions humans have to polinate the fruit trees manually.