…unless actions are taken to slow the decline of domesticated honeybees and augment
[increase] their populations with wild bees, many fruits and vegetables may disappear from
the food supply, said claire kremen, a conservation biologist at princeton university in
new jersey.…
the honeybee decline, which is affecting domesticated and wild bee populations
around the world, is mostly the result of diseases spread as a result of mites and other
parasites as well as the spraying of crops with pesticides, scientists say.…
source: “bee declines may spell end of some fruits, vegetables,”
national geographic news, october 5, 2005
the decrease in the honeybee population is partly due to
(1) the use of pesticides (3) the use of fertilizers
(2) a decrease in prey (4) a decrease in fruits and vegetables

I would also have to say: 1) the use of pesticides

I did a powerpoint presentation on the decrease of honeybees for biology in college. I cannot find the presentation I did however, I do remember some of the points I made.

Varroa Mites aka Varroa destructors (parasite) are also contributing to the honey bee population decreasing. They latch onto the honeybees and feed off of them. They also feed off of the larva's (baby version of a honeybee).  

Another issue could be that GMOs. Plants DNAs have been altered so that they can fight disease  and kill unwanted bugs.

Another issue is population ecology, which deals with density independent and dependent factors. Density-independent, deals with habitat destruction by natural disasters or humans.  Dependent factors- overcrowding of an area.

Hopefully this helped and good luck.